CHAPTER 89: PACEMAKERS AND DEFIBRILLATORS

Cardiac implantable electronic devices (CIEDs), notably pacemakers and implantable cardioverter-defibrillators (ICDs), represent dramatic technological advances that have become essential in addressing the problems of arrhythmia and heart failure (HF) in modern cardiovascular patients. Although similar in design, these technologies represent two distinct paradigms of treatment. Where conventional single or dual-chamber pacemakers are used to address the fundamental problem of bradyarrythmia, ICDs are designed to automatically detect and treat tachyarrhythmias, specifically hemodynamically unstable ventricular tachycardia (VT) or ventricular fibrillation (VF). In the past two decades, a third paradigm has also emerged—cardiac resynchronization therapy (CRT)—in which biventricular (BiV) pacing is used to improve left ventricular (LV) pump function in patients with intraventricular conduction delay and systolic HF. Along with developments in fundamental approaches to pacing and defibrillation, recent years have also brought about exciting advances in design, with the development of leadless pacing and subcutaneous defibrillators. The purpose of this chapter is to provide an overview on pacing, defibrillation, and the indications for specific types of devices, and to review seminal clinical trials supporting their use in clinical practice.

Although experimental use of short-term direct myocardial pacing for cardiac arrest had been described in 1932 by Dr. Albert Hyman,¹ Dr. Paul Zoll is credited with spurring widespread interest in cardiac pacing after his description of successful resuscitation through external subcutaneous electrical stimulation in 1952.² Pacing soon became indispensable to the nascent field of cardiac surgery when, in 1957, Dr. C. Walton Lillehei applied a direct myocardial electrode to the heart of a child undergoing a ventricular septal defect repair who developed complete atrioventricular (AV) block intraoperatively. He attached the electrode to an externalized transistor pacemaker developed by Earl Bakken.³ The first description of a technique for transvenous pacing in humans came only 1 year later.⁴ Subsequently, the field has undergone an explosion of development and miniaturization, although has remained astonishingly faithful in concept to these initial pioneering descriptions (Fig. 89–1). It was not until close to three decades of the seminal work of Dr. Michel Mirowski and Dr. Morton Mower that an “automatic” implantable defibrillator was used in humans for the termination of malignant ventricular arrhythmias.⁵ Fast forward more than a decade later to the first case report of BiV pacing in the treatment of dilated cardiomyopathy.⁶ Both pacing and defibrillation CIED systems share similar components, consisting of an impulse generator—the “battery” that stores electrical charge—and a lead or leads that are used to deliver electrical energy to either pace or defibrillate myocardial tissue. Although with similarities in overall design, it has been the process of developing and refining indications in large trials that has led to the adaptation of these devices for the use of specific clinical needs and that informs their continuing evolution today. An overview of key indications and seminal trials will now be given prior to discussion of the mechanics of implant, management of complications, and troubleshooting.

Pacemakers

Indications for Pacing

Cardiac pacing was initially developed to address the problem of bradyarrhythmia, particularly caused by AV block and sinus node dysfunction (SND). These continue to constitute the primary indications for consideration for pacing, although pacing for neurocardiogenic syncope and special situations (eg, postcardiac transplant, neuromuscular disease, sleep apnea, or infiltrative disease) has grown. Pacing for the termination of arrhythmia (or anti-tachycardia pacing [ATP]) is almost exclusively employed in combination pacemaker-defibrillator devices in the modern setting. The clinical guidelines for consideration of pacing were recently outlined by a collaboration between the American College of Cardiology Foundation (ACCF), American Heart Association (AHA), and Heart Rhythm Society (HRS).⁷

Pacing in SND

Class I recommendations for pacing for SND hinge primarily on the presence of symptoms, along with class II consideration for pacing in patients with heart rate < 40 bpm while awake, with (class IIa) or without (class IIb) clear symptomology associated with bradycardia (Table 89–1). SND may be caused by either intrinsic disease or sinoatrial exit block, and also includes sinus pause after cessation of atrial tachyarrhythmia (Fig. 89–2).

Pacing in AV Block

Indications for pacing in acquired AV block attempt to discriminate between sites of block in the cardiac conduction system. Broadly, block above the AV node is considered lower risk than below the node (ie, infranodal block) (Fig. 89–3) because of the relative instability of escape rhythms emanating from below the AV junction. This is particularly salient in patients with stigmata of other His-Purkinje disease, such as bundle branch block (BBB) or fascicular block. In addition, AV block at any level may contribute to the risk of ventricular arrhythmia through a pause-dependent mechanism. The indications for pacing in acquired AV block take into account the site of block and, similar to recommendations for SND, suggest pacing in patients with symptomatic bradycardia < 40 bpm (including HF), for those who require medical therapy and are at risk for bradycardia, and in those at risk for significant pauses (> 3 seconds). In contrast to indications for SND, however, pacing is recommended even in asymptomatic patients if infranodal block (ie, intra-His or infra-His) is suspected (Table 89–2). Patients with alternating BBB (ie, right BBB [RBBB] alternating with left BBB [LBBB], or RBBB alternating with associated left anterior fascicular block and left posterior fascicular block), or with chronic bifascicular block associated and type 2 second-degree or advanced AV block, also meet a class I indication for pacing, irrespective of symptoms. AV block is also frequently a sequel of ST-segment elevation myocardial infarction (MI), and is associated with poorer prognosis overall. Generally, patients with inferior MI demonstrate nodal AV block that is transient, whereas anterior MI may cause direct infranodal and fascicular ischemia leading to higher degree of persistent AV block. An electrophysiology (EP) study may be necessary in differentiating the site of block in patients where the site of block is uncertain (class I indication; level of evidence [LOE] B).

Pacing in Hypersensitive Carotid Sinus Syndrome or for Neurocardiogenic Syncope

Pacing in these conditions is generally reserved for patients in whom a cardioinhibitory reflex (as opposed to vasodepressor response) predominates with respect to their syndromes. These include patients in whom carotid sinus stimulation induces ventricular asystole for > 3 seconds and is associated with syncope (class I indication; LOE C). The indications are downgraded in patients with a hypersensitive cardioinhibitory response without clear, provocative events (class IIa; LOE C), and for those with significantly symptomatic neurocardiogenic syncope and bradycardia documented spontaneously (as with event monitoring or implantable loop monitor) or during tilt table testing (class IIb; LOE B).

Pacing in Specific Conditions

Specific conditions that merit indication for pacing include patients who have undergone cardiac transplant, patients with neuromuscular diseases such as myotonic dystrophy and Emery-Dreifuss syndrome, sleep apnea, cardiac sarcoidosis, and patients with hypertrophic cardiomyopathy (HCM) and evidence of dynamic left ventricular outflow tract (LVOT) obstruction. In addition to standard indications, patients postcardiac transplant may benefit from pacing even for relative bradycardia if it is prolonged or recurrent (class IIb; LOE C), and asymptomatic patients with neuromuscular disease with prolonged His-Ventricular (HV) interval or abnormal resting electrocardiogram (ECG) may also benefit from pacing. Pacing indications for nocturnal bradyarrhythmia in patients with sleep apnea that remains persistent despite continuous positive airway pressure have not been established.^7,8 With respect to cardiac sarcoidosis, a recent expert consensus statement of the HRS suggested that, in addition to the accepted guidelines, there is a class IIa indication for device implantation even in patients with transient AV block owing to the unpredictable nature of the disease and its variable response to immunosuppression.⁹ After initial exuberance for the role of dual-chamber pacing to alter septal contraction timing and reduce LVOT gradients for obstructive HCM, subsequent randomized crossover studies have not shown consistent benefit. Permanent pacing is now reserved only for medically refractory patients with obstructive HCM in whom septal reductive therapy is not possible (class IIb; LOE B).¹⁰

Seminal Pacemaker Trials

Beginning in the late 1950s, the original indications for pacing consisted of treatment for malignant bradyarrhythmias. Given that the benefits of cardiac pacing were imminent and lifesaving (eg, pacing to prevent ventricular asystole), use of pacing versus sham was not studied for these indications in controlled trials. With the development of dual-chamber and multisite pacing systems, there has been randomized investigation with respect to pacemaker device type and mode selection. The evidence base to guide mode selection was recently reviewed in an expert consensus jointly released by the HRS and the ACCF¹¹ along with a systematic review commissioned by the National Institute for Health Research.¹² Observations regarding the seminal trials supporting these recommendations, as well as emerging technology in so-called “leadless” pacing, will briefly be reviewed here.

Pacing Mode Selection for SND

There have been four major randomized trials comparing single or dual-chamber pacing for patients with bradycardia secondary to isolated sinoatrial dysfunction in the absence of AV block.^13,14,15,16 There are multiple possible approaches to pacing for these patients, including single-chamber atrial-only pacing, single-chamber ventricular-only pacing, or dual-chamber atrial and ventricular pacing (Fig. 89–4). The first large study seeking to answer this question was conducted by Andersen and associates at a single center in Denmark and has been referred to subsequently as the Danish Study.¹⁶ In the study, 225 consecutive patients with sick-sinus syndrome were randomized to single-chamber atrial or single-chamber ventricular pacing. Mortality was not significantly different between the two groups at 40 months, but was significantly lower in patients receiving atrial-only pacing at 5.5 years (hazard ratio [HR] 0.66, P = .045).¹⁷ Incidence of atrial fibrillation (AF) was also significantly less common in patients who were atrially paced and, interestingly, the rate of thromboembolic events—specifically stroke or peripheral arterial embolism—was lower in patients receiving single-chamber atrial pacing (17% vs 5%, P = .0083). In order to enroll, patients were required to demonstrate a PQ interval of less than 220 ms and atrial pacing with 1:1 AV conduction was required at a rate of at least 100 bpm. With these criteria, a low rate of progression of AV block was noted (0.6% per year).¹⁸ Single-chamber atrial versus dual-chamber atrial and ventricular pacing was next studied in another randomized study of 177 patients by Nielsen et al.¹⁵ In contrast to ventricular-only pacing, there was no difference in this study with respect to morality, thromboembolism, or HF, but dual-chamber pacing was associated with increased left atrial size and reduced LV fractional shortening versus atrial-only pacing.

The matter of whether atrial-only or dual-chamber pacing is superior in SND was evaluated most comprehensively in DANPACE. Investigators at 20 European centers randomized 1415 patients with isolated SND and no AV block or BBB to single or dual-chamber rate-adaptive pacemakers.¹³ The primary outcome of all-cause death was no different between the two groups, nor was there a difference with respect to secondary end points of incidence of stroke or HF. Surprisingly, and in stark contrast to prior randomized studies of atrial versus ventricular-only or dual-chamber pacing, atrial rate-adaptive pacing was associated with an increased risk of paroxysmal AF (HR 1.27, P = .024) in DANPACE, although permanent AF was not different. The mechanism for this difference is unclear; patients were programmed to paced AV delays of ≤ 220 ms, which may have promoted better AV synchrony. Perhaps more importantly, however, the reoperation rate was significantly higher in this multicenter investigation of single-chamber devices (HR 1.99, P < .0001), and was driven by the need for change of pacing mode as a result of progression of heart block (at a mean rate of 1.7% per year, close to three times the rate of the Danish study). Any pacemaker reoperation is associated with a risk of infection or extraction, and these considerations factored into the class I recommendation for consideration of dual-chamber over single-chamber atrial pacing in SND.¹¹

Importantly, the largest randomized pacemaker study for isolated SND was the Mode Selection Trial (MOST), although it did not look at atrial-only pacing systems. The trial randomized a total of 2010 patients to either dual-chamber (atrial and ventricular) or single-chamber (ventricular-only) pacing.¹⁴ Neither the primary end point of death from any cause or nonfatal stroke, nor the secondary end point—a composite of death, stroke, or HF hospitalization—differed between patients assigned to dual-chamber versus ventricular-only pacing. Incidence of AF was less common with dual-chamber pacing (HR 0.79, P = .008), HF scores were better (P < .0001), as were quality of life metrics. Importantly, close to one-third of patients assigned to ventricular pacing crossed-over to dual-chamber pacing. This was driven by incidence of the pacemaker syndrome in half of all crossovers. Notable here is that MOST had a strict definition for pacemaker syndrome, requiring documentation of both symptoms of dyspnea or syncope along with demonstration of ventriculoatrial (VA) conduction or blood pressure (BP) reduction of 20 mm Hg or more during ventricular-only pacing. In addition, there was signal regarding the deleterious effect of right ventricular (RV) pacing, as cumulative percentage of RV pacing (cum%VP) was associated with an increased risk of HF or AF in patients with both dual-chamber (HR 2.99, P = .024 for cum%VP > 40) and single-chamber (HR 2.56, P = .0007, cum%VP > 80) devices.¹⁹ These findings were later further validated in the SAVE-PACe trial, which randomized 1065 patients with SND to dual-chamber devices programmed to deliver conventional AV pacing or to implement device-based algorithms to minimize ventricular pacing.²⁰ Conventional pacing systems with ~99% ventricular pacing versus those designed to minimize pacing (mean 9.1%) were associated with 40% increased risk of persistent AF (P = .0009), but no differences in mortality.

Pacing Mode Selection for AV Block

There have been three large randomized trials investigating the role of pacing mode in patients with AV block.^21,22,23 Two of these trials, the Pacemaker Selection in the Elderly (PASE) study and the Canadian Trial of Physiologic Pacing (CTOPP), enrolled patients with SND and AV block. The largest and most recent, the United Kingdom Pacing and Cardiovascular Events (UKPACE) study, was focused on the question of how best to pace patients with AV block only.

PASE was a single-blind, randomized study of ventricular versus dual-chamber pacing in patients over the age of 65 years.²¹ Of 407 patients studied, 201 demonstrated AV block (of which 59% had third-degree AV block). Importantly, all patients received dual-chamber devices and randomization was implemented based on programming only. Perhaps because of this, there was a high-degree of crossover in the trial (26%), attributed to the pacemaker syndrome (driven by symptoms of fatigue and dyspnea on exertion). There was no significant difference with respect to incidence of all-cause death, stroke, or HF hospitalization. The primary end point of the study, an assessment of health-related quality of life, reached significance only in patients with SND receiving dual-chamber programming. There was no difference among patients with AV block. Interestingly, incidence of AF was higher in patients with SND assigned to ventricular-only pacing, but not in those with AV block.

The CTOPP investigators enrolled 2568 patients (with AV block present in 60%) at 32 Canadian centers and randomized them to a “physiologic pacing system” (either atrial-only pacing or dual-chamber) versus a ventricular-only pacemaker. The primary outcome, a composite of cardiovascular death and stroke, was not different the two groups. In subgroup analysis, and in contrast to PASE, there appeared to be greater benefit for physiologic pacing among patients with AV block, although this did not reach significance (P = .29). In addition, rates of crossover resulting from pacemaker syndrome were also considerably lower than PASE, with 99.2% of patients assigned to ventricular-pacing only maintaining that mode. Incidence of AF, however, appeared lower among patients with physiologic pacing (5.3% vs 6.6%, P = .05).

UKPACE was heralded as the definitive trial to guide mode selection among patients with AV block. In the trial, 2021 patients > 70 years of age undergoing implant for high-grade AV block were randomly assigned to a single-chamber ventricular pacemaker or a dual-chamber pacemaker. Over a median follow-up of 4.6 years, there was no difference in all-cause mortality. There was also no difference with respect to the secondary outcomes of rate of AF, HF, and a composite of stroke, transient ischemic attack (TIA), or other thromboembolism at 3 years. Surprisingly, there was an increase in incidence of AF among patients assigned to dual-chamber pacing in the initial 18 months, which may have reflected a systematic ascertainment bias, as dual-chamber devices could more readily detect AF. Similar to PASE, rate of crossover resulting from pacemaker syndrome at the end of the study was low (3.1%). Interestingly, median percentage of ventricular pacing was higher in patients assigned to dual-chamber pacing (99% vs 94%). There is speculation that—given that AV block may be intermittent—continuous RV pacing may have been unnecessary and diminished the possible benefits of a dual-chamber pacing system. Regardless, it has been these findings that leave open a class I recommendation that ventricular-only pacing is an acceptable alternative in patients with AV block who have specific clinical situations (including but not limited to sedentary patients, those with significant medical comorbidities, or those with vascular access limitations) that make them less likely to benefit from dual-chamber systems (LOE B).¹¹

Leadless Pacing

There has been a resurgence of interest in ventricular-only pacing because of the development of integrated pulse generator and sensing/pacing systems that are fully self-contained (and are therefore “leadless”). These devices overcome some of the limitations of the conventional transvenous lead-based systems, including the early risk of acute complications such as pneumothorax, upper extremity thrombosis, or frozen shoulder, as well as mitigate longer-term risks such as lead-associated infection, fracture, or need for extraction. Indeed, some of have argued that the lead is the “Achilles’ heel” of a pacing system.²⁴ Two fully integrated pulse generator and sensing/pacing systems have now been studied in human trials: the Nanostim leadless cardiac pacemaker (LCP, St. Jude Medical) and the Micra transcatheter pacing system (TCP, Medtronic) (Figs. 89–5 and 89–6). Both devices rely on a catheter-based deployment tool and are delivered to the RV for ventricular-only pacing.

The feasibility of LCP was tested in the LEADLESS trial, a prospective, nonrandomized single-arm multicenter study of safety and clinical performance in 33 subjects.²⁵ Indications for implant included permanent AF with slow response or AV block, normal sinus rhythm patients with high-degree block and low level of physical activity or short lifespan, or sinus bradycardia with infrequent pauses or unexplained syncope with abnormal findings at EP study. The overall complication-free rate acutely was 94%,²⁵ with no pacemaker-related adverse events, stable device sensing, and pacing thresholds between 3 and 12 months in follow-up.²⁶ Interim analysis of the LEADLESS II study was also recently reported.²⁷ Similar to LEADLESS, the study was a prospective, nonrandomized, multicenter analysis of 526 patients receiving LCP with a primary outcome of efficacy and safety. Pacemaker implant success was achieved in 95.8% of patients, with an average procedural time of 28.6±17.8 minutes and an average hospital stay of 1.1±1.7 days. Successful implant failure resulted predominantly from inadequately sensed R waves. Serious adverse events were observed in 6.7% over a 6-month period, and included device dislodgement (1.7%), cardiac perforation (1.3%), and vascular complications (1.3%).

Early results of the Micra TCP trial have also been reported.^28,29 A prospective, nonrandomized investigation conducted at 56 centers, criteria for entry were broader than in LEADLESS, including all class I or class II guideline-based indications for pacing. A total of 725 patients underwent implant. The primary indication for enrollment was persistent or permanent atrial tachyarrhythmia (64%), SND (17.5%), and AV block (14.8%). Implant success was achieved in 99.2% and freedom from major complication was 96% at 6 months. Cardiac perforation or effusion (1.6%) and vascular access complication (0.7%) were the most common. No radiographic device dislodgements were noted, but three patients required system revision as a result of elevated pacing threshold, symptoms of pacemaker syndrome, or intermittent loss of capture.

With relatively short implant times, low rate of dislodgement, and projected longevity lasting 10 to14 years, there is growing enthusiasm with respect to the possible role of fully self-contained CIEDs for pacing. Although they do not expand the indication for pacing, they may be associated with fewer adverse events, particularly those resulting from late infection or extraction, as compared with conventional transvenous systems. Further randomized study will help clarify whether their role is that of niche player or whether they represent a new paradigm in the delivery of care.³⁰

Implantable Cardioverter-Defibrillators

Indications for ICDs

Defibrillation to treat VF was first reported in the surgical setting in 1947.³¹ Less than a decade later, Zoll demonstrated that trans-chest countershock using paddles was a viable means for defibrillation for routine clinical practice.³² Development of an internal, automatic defibrillator was first described and successfully tested by Dr. Mirowski and Dr. Mower in canines by 1970.³³ Misgivings regarding the safety profile or appropriate patient population for the device delayed human clinical investigation.³⁴ Indeed, in the initial pilot study, subjects were required to have survived two episodes of cardiac arrest on dual antiarrhythmic therapy in order to enroll.⁵ Indications for ICDs have expanded since this period of initial work, and can broadly be categorized as (1) applying to patients who have survived prior cardiac arrest, sustained VT/VF, or syncope caused by ventricular arrhythmia (ie, secondary prevention); versus (2) individuals at risk, but who have not yet had a documented episode of sustained VT/VF, arrhythmic syncope, or cardiac arrest (ie, primary prevention).⁷ A recent HRS/ACC/AHA expert consensus statement also comments on use of ICDs in patients who are not included or not well represented in clinical trials.³⁵ Both sets of secondary and primary prevention recommendations are made with the caveat that patients maintained goal-directed medical therapy and have a reasonable expectation of survival for greater than 1 year with good functional status.

Secondary Prevention

The fundamental principle guiding ICD therapy in secondary prevention is that patients who have survived sudden cardiac death (SCD) are at increased risk for subsequent events and, therefore, will usually meet the indications for an ICD (Table 89–3). ICD therapy is not recommended in secondary prevention, however, among patients with completely reversible causes for malignant arrhythmia (eg, electrolyte abnormalities, drugs, trauma, or VT amenable to cure with ablation) or those within 48 hours after a diagnosis of MI. Data regarding the 48-hour window of increased risk of VT/VF after MI was initially obtained from early trials of thrombolysis in which ventricular arrhythmia after MI was a marker of increased risk, particularly when occurring greater than 2 days after admission.^36,37 The increased mortality attributed to early VT/VF in these studies was incurred during the index hospitalization. In more recent trials of percutaneous coronary intervention (PCI), it was observed that the majority of VT/VF episodes occurred within 48 hours, frequently during or immediately after PCI.^38,39,40 Although both early (< 48 hours) and late VT/VF is associated with increased risk, patients with later VT/VF episodes sustained greater mortality in follow-up for both ST-segment elevation MI and non-ST-segment elevation MI populations.

Primary Prevention

The indications to implant ICDs for primary prevention among patients with chronic ischemic heart disease (IHD) or nonischemic cardiomyopathy (NICM) are based on large randomized trials that have demonstrated reduction in mortality with ICD therapy. There are also recommendations for ICDs in select clinical situations that have been informed by limited prospective study and consensus opinion. These include inherited arrhythmia syndromes, HCM, arrhythmogenic right ventricular cardiomyopathy (ARVC), nonhospitalized patients awaiting cardiac transplant, sarcoidosis, giant cell myocarditis, and Chagas disease (Table 89–4).

The primary tool for risk assessment in clinical practice is assessment of left ventricular ejection fraction (LVEF), based on the landmark trials which led to the approval of ICD therapy for SCD prevention. The guidelines acknowledge that the determination of LVEF lacks a “gold standard” and there is variation among commonly used techniques for LVEF assessment (including echocardiography, nuclear study, or cardiac magnetic resonance [CMR]). The writing committee recommended the clinician use the LVEF determination that “they feel is the most clinically accurate and appropriate.”⁷ In patients who are New York Heart Association (NYHA) class II or III, ICD therapy is indicated for patients with LVEF ≤ 35% resulting from IHD or NICM; for patients who are NYHA functional class I, an LVEF ≤ 30% is recommended for patients with IHD (and may be considered [class IIb] for NICM); among patients with a history of IHD and with inducible VT or VF, the LVEF criterion is relaxed to ≤ 40%, irrespective of etiology of LV systolic dysfunction. The waiting period of 40 days after acute MI or 90 days after revascularization draws in part from large-scale trials assessing efficacy of ICD therapy in these clinical settings and will be discussed further below.

Special Situations

With regard to special situations, there have been increasing data regarding use of defibrillators among patients with inherited arrhythmia syndromes that have informed specific recommendations for these patients.⁴¹ Given that syncope may portend malignant ventricular arrhythmias, there is a class IIa recommendation for primary prevention ICD among long QT syndrome patients who experience recurrent syncopal events while on β-blocker therapy, among patients with Brugada syndrome with a type 1 ECG pattern and arrhythmic syncope, or among lamin A/C mutation-positive patients with progressive cardiac conduction disease (PCCD) with LV dysfunction or nonsustained VT (NSVT). The recommendation is upgraded to class I for catecholaminergic polymorphic VT (CPVT) patients who demonstrate polymorphic VT or bidirectional VT on medical therapy. There are also more modest class IIb indications to consider ICD for patients with Brugada syndrome who develop VF during programmed electrical stimulation, among asymptomatic patients with short QT syndrome and a family history of SCD, among asymptomatic patients with a high-risk early repolarization (ER) pattern or symptomatic family members of ER syndrome patients with inferolateral ER, and in first-degree relatives of patients with idiopathic VF who experience unexplained syncope.

Specific recommendations have also been elaborated for patients with HCM.¹⁰ In this group, there is a class IIa recommendation to proceed with ICD in patients with a family history of SCD in a first-degree relative or with LV wall thickness ≥30 mm or recent unexplained syncope. There is also class IIa recommendation to consider ICD in patients with NSVT and SCD risk factors, including LVOT obstruction (resting gradient ≥ 30 mm Hg), the presence of late gadolinium enhancement (LGE) on CMR, LV apical aneurysm, or malignant genetic mutations. In patients with HCM and only NSVT or abnormal BP response during exercise without SCD risk factors, the recommendation is downgraded to class IIb.

Seminal ICD Trials

Secondary Prevention

There have been three pivotal trials that led to the approval of ICDs for secondary prevention: Antiarrhythmics Versus Implantable Defibrillators (AVID),⁴² the Canada Implantable Defibrillator Study (CIDS),⁴³ and the Cardiac Arrest Survival in Hamburg (CASH) trial.⁴⁴ AVID was the largest and was conducted first—enrolling 1016 patients who had either survived near-fatal VF or undergone cardioversion for sustained VT with syncope, or with sustained VT and an LVEF of ≤ 40% with symptoms of hemodynamic compromise from VT such as near-syncope, congestive HF, or angina. Patients were assigned to either ICD or antiarrhythmic therapy (amiodarone or sotalol). The trial was stopped early (before its original goal of 1200 patients) because of significance of the primary outcome—overall survival—which was improved in patients randomized to ICDs at 1 year (89.3% vs 82.3%), 2 years (81.6% vs 74.7%), and 3 years (75.4% vs 64.1%) (unadjusted estimates, P< .02). Incidence of tachycardia therapy with either ATP or shocks was common at 3 years (85% for VF patients and 69% for VT patients). Importantly, however, in subgroup analysis, it was found that patients with LVEF > 35% or nonischemic arrhythmia were less likely to benefit from ICD versus antiarrhythmic therapy.

CIDS and CASH were both smaller than AVID. Eligible patients for CIDS included those who were > 72 hours from MI and electrolyte imbalance and demonstrated documented VF, out-of-hospital arrest requiring defibrillation or cardioversion, sustained VT with syncope, sustained VT with presyncope in patients with LVEF ≤ 35%, or unmonitored syncope with subsequent documentation of spontaneous VT ≥ 10 seconds or sustained VT (≥ 30 seconds) induced at EP study. A total of 659 patients were randomized to either ICD or amiodarone. A nonsignificant 20% relative risk reduction was found in all-cause death and 33% reduction in arrhythmic mortality. Similarly in CASH, 288 patients with resuscitated from cardiac arrest from documented sustained VT or VF were randomized to amiodarone and β-blockade versus ICD, and a 23% nonsignificant reduction in all-cause mortality was noted. Meta-analyses combining the data from AVID, CIDS, and CASH have shown that, when taken together, the benefit of ICDs in secondary prevention does reach significance and is associated with 28% relative risk reduction of overall mortality, driven by a 50% reduction in arrhythmic death.^45,46

Two additional trials bear mentioning with respect to secondary prevention. The first Multicenter Automatic Defibrillator Implantation (MADIT-I) trial evaluated 196 patients with LV systolic dysfunction (LVEF ≤ 35%), prior MI, and either documented asymptomatic NSVT and inducible nonsuppressible sustained VT or VF at EP study.⁴⁷ Coronary artery bypass grafting (CABG) within the past 2 months or PCI within 3 months were exclusion criteria. Although distinct from secondary prevention in that patients enrolled in MADIT-I had no history of arrhythmic syncope or sustained VT, all patients demonstrated inducible and nonsuppressible (by procainamide or another antiarrhythmic) VT at EP study. These patients were allocated to either ICD or conventional medical therapy, and the trial revealed an impressive reduction in mortality (HR 0.46, P = .009). Also relevant to secondary prevention patients is the Multicenter Unsustained Tachycardia Trial (MUSTT) in which 704 patients with coronary artery disease (> 4 days from the most recent MI or revascularization procedure), LVEF ≤ 40%, and asymptomatic NSVT (lasting for ≥ 3 beats) underwent EP study.⁴⁸ Patients with sustained VT or VF at EP study were then randomized to either antiarrhythmic therapy with either ICD or EP-guided antiarrhythmic therapy versus no antiarrhythmic therapy. The trial showed that, although patients with EP-guided therapy demonstrated lower risk of arrhythmia, the risk of arrhythmic death was lowered only in patients assigned to receive an ICD (relative risk [RR] 0.24 versus those who received EP-guided antiarrhythmic therapy, P < .001). Perhaps more than any other secondary-prevention trial, MUSTT makes a compelling argument for ICD among patients with coronary artery disease and inducible VT or VF, above and beyond medical therapy.

Primary Prevention

Evidence supporting the use of ICDs in the primary prevention of SCD was comprehensively reviewed in a recent report commissioned by the Agency for Healthcare Research and Quality (AHRQ).⁴⁹ Their meta-analysis of 14 studies showed a significant reduction in all-cause mortality (HR 0.69, 95% CI 0.6-0.79) and an even stronger case for reducing SCD (HR 0.37, 95% CI 0.26-0.52). The seminal trials that led to the approval of ICDs for current indications include MADIT-II,⁵⁰ the Sudden Cardiac Death-Heart Failure (SCD-HeFT) trial,⁵¹ and the Defibrillators in Non-Ischemic Cardiomyopathy Treatment Evaluation (DEFINITE) study.⁵² In MADIT-II, 1232 patients with prior MI and LVEF ≤ 30% were randomly assigned (in a 3:2 ratio) to ICD or conventional medical therapy. Patients with MI within the past month or coronary revascularization within the prior 3 months were excluded. The trial was stopped early because of the superiority of ICDs. The primary outcome was all-cause death, and was significantly reduced for patients assigned to ICD versus usual care (HR 0.69, P = .016). SCD-HeFT was larger and more inclusive in enrollment than MADIT-II, including NYHA class II or II patients with stable HF and LVEF ≤ 35% from ischemic or nonischemic causes, and randomized subjects to placebo, amiodarone, or single-chamber ICD. The primary outcome was also death from any cause and was significantly reduced for patients assigned to ICD (HR 0.77, P = .007), and was similar for patients with ischemic or nonischemic HF. Finally, DEFINITE enrolled 458 patients exclusively with nonischemic dilated cardiomyopathy, LVEF < 36%, and presence of ambient arrhythmias (NSVT 3 to 15 beats at a rate ≥ 120 bpm) or at least 10 premature ventricular contractions (PVCs) per hour on 24-hour Holter. Patients were randomized to either medical therapy or medical therapy supplemented with single-chamber ICD. The primary end point of all-cause death was better for patients assigned to ICD, but did not reach significance (HR 0.65, P = .08). A secondary end point of reduction in SCD, however, was significantly better for patients assigned to ICD (HR 0.20, P = .006).

Arguably even more informative than trials demonstrating benefit among primary prevention patients have been three large negative studies which have constrained use in patients after revascularization. These include the Coronary Artery Bypass Graft-Patch (CABG-Patch),⁵³ Defibrillator in Acute Myocardial Infarction Trial (DINAMIT),⁵⁴ and the Immediate Risk-Stratification Improves Survival (IRIS) study.⁵⁵ In CABG-Patch, patients scheduled for elective coronary bypass surgery were screened for age < 80 years, LVEF < 36%, and abnormalities on signal-averaged electrocardiogram; 1055 patients were enrolled and finally 900 assigned to either epicardial ICD or control (no ICD). The study was terminated early because of futility (HR 1.07 for all-cause death, P = .64) and was attributed to a perceived low rate of SCD among CABG patients, although this could not be ascertained with certainty because the majority of ICDs used lacked the ability to store electrograms. In addition, postoperative infection was significantly more common among patients assigned to ICD.

DINAMIT evaluated another high-risk group, patients between 6 and 40 days post MI. A total of 674 patients with LVEF ≤ 35% and evidence of reduced heart-rate variability or elevated average heart rate > 80 bpm on Holter monitoring were randomly assigned to ICD or no-ICD. The primary end point of all-cause mortality was not improved by ICD (HR 1.08, P = .66), whereas the risk of arrhythmic death was significantly reduced (HR 0.42, P = .009). Concerning, however, was that the risk nonarrhythmic death was significantly greater among patients assigned to ICD (HR 1.75, P = .02). Some commentators speculated that inappropriate device therapies might be harmful and have led to this difference.^56,57 The larger IRIS trial was meant to tackle the disappointing findings of DINAMIT head-on, enrolling 898 patients 5 to 31 days post MI with LVEF ≤ 40% and either high resting heart rate (≥ 90 bpm) or at least three beats of NSVT at a rate of ≥ 150 bpm. Patients were randomized to ICD or medical therapy alone. Results were in line with what had been found in DINAMIT—overall mortality was not reduced (HR 1.04, P = .78), SCD was reduced in patients assigned to ICD (HR 0.55, P = .049), but nonarrhythmic death was substantially greater for patients assigned to ICD (HR 1.92, P = .001). In part, it was the findings of DINAMIT and IRIS that primed investigators to the possible harm of ICDs, setting the stage for more contemporary trials of device programming to improve outcome (discussed further below).

Subcutaneous ICD

Defibrillator systems have evolved from devices requiring an open-chest surgical approach to modern defibrillators that can be placed transvenously. The trials discussed above were conducted exclusively with either an epicardial or transvenous system. Indeed, as noted above in discussion regarding the rationale behind leadless pacing systems, the lead itself may be the weakest link in defibrillation systems—associated with greater risk of periprocedural complication and susceptible to later bacterial seeding and infection, insulations break, or fracture. Although subcutaneous arrays have been utilized to lower defibrillation threshold for a number of years, they have been used in accompaniment with transvenous or epicardial sensing leads for sensing. An entirely subcutaneous system was first conceptualized and demonstrated in canines in 1970,^58,59 but the idea largely went unnoticed. It was not until 2001 that the concept was pursued rigorously in human investigation (Fig. 89–7). The first study evaluated 78 patients in whom different configurations of a temporary subcutaneous ICD (S-ICD) were evaluated.⁶⁰ The best configuration (parasternal electrode and left lateral thoracic pulse generator) was then studied in 49 patients to determine appropriate defibrillation threshold, and then evaluated in longer-term use in a pilot study of 6 patients followed by a nonrandomized trial of 55 patients. Initial results were promising, with the detection and treatment of 12 spontaneous episodes of arrhythmia.

A number of small cohort studies have now published safety and efficacy data regarding the S-ICD, which has been overall favorable.^{61,62,63,64,65} The largest data set was pooled from two large registries: the S-ICD System IDE Clinical Investigation (IDE) study and the Boston Scientific Post Market S-ICD Registry (EFFORTLESS) trial, reporting on the results of 882 patients who were followed for close to 2 years.⁶⁶ Spontaneous VT or VF was detected in 111 discrete events in 59 patients with a first-shock success of 90.1% and 98.2% successful with five available shocks. The 3-year inappropriate shock rate was 13.1% and reduced with dual-zone programming. Device-related complications occurred in 9.6%, with device-related infection requiring revision or removal being the most common. Importantly, given that the device is subcutaneous, no bacteremia or device-related endocarditis occurred, and the complication rate was reduced with operator experience.⁶⁷ The ongoing Prospective, Randomized Comparison of Subcutaneous and Transvenous Implantable Cardioverter-Defibrillator Therapy (PRAETORIAN; NCT01296022) trial seeks to compare the efficacy and safety of S-ICD versus conventional transvenous ICD systems head-to-head and will help answer questions regarding which type of system to implant for a primary prevention patient going forward.⁶⁸

Wearable Cardioverter-Defibrillator

Use of a wearable cardioverter-defibrillator (WCD) (Fig. 89–8) to terminate VF in humans was first described in 1998,⁶⁹ and has emerged as a viable option to “bridge” patients during a period of increased risk prior to eventual ICD implantation or to a point at which ICD implantation is no longer indicated. There are limited observational data available regarding the efficacy of WCDs leading to its general approval.^70,71 An ongoing study, Vest Prevention of Early Sudden Death Trial/Prediction of ICD Therapies Study (VEST/PREDICTS; NCT01446965) seeks to address the question of whether WCDs in patients with low LVEF (≤ 35%) after MI might benefit from additional protection with randomization to WCDs in addition to goal-directed medical therapy. PREDICTS will follow VEST as part of an ongoing observational study after 2 months (the completion of VEST). The largest real-world data regarding use comes from the WEARIT-II Registry, in which 2000 patients (40% ischemic, 46% nonischemic, and 12% with congenital or inherited arrhythmia) received WCDs for a median duration of 90 days.⁷² Sustained VT/VF was detected in 2.1% of patients, with therapy delivery required for 1.1% (and the remainder spontaneously terminating). Risk was higher for patients with ischemic cardiomyopathy or congenital/inherited disease than NICM. Similar findings were also seen in another large single-center cohort study.⁷³ The WCD, therefore, may be an option to cover the “gap” highlighted by DINAMIT and IRIS in patients at risk after MI. Importantly, rates of inappropriate therapy were low (0.5%). At the end of WCD use in WEARIT-II, less than half (42%) required ICD, with improvement in LVEF being the most common reason for deferral.

Cardiac Resynchronization Therapy

Indications for CRT

During the past two decades, CRT has emerged as a safe and efficacious device-based therapy for heart failure patients with severe systolic dysfunction and evidence of intraventricular conduction delay. CRT is the use of a pacemaker or defibrillator with three electrical leads to coordinate myocardial contraction. Two leads are endocardial, placed in the right atrium and right ventricle, and a third lead is traditionally placed in a tributary of the coronary sinus overlying the epicardial surface of the left ventricle (Fig. 89–9). CRT exerts its physiological impact via synchronizing ventricular contraction, leading to improved LV filling, reduced mitral regurgitation, increased pumping efficiency, and improved cellular bioenergetics. Indeed, in marked contrast to traditional pacing systems, the goal in CRT is to pace every single beat. Multiple prospective randomized studies have shown that CRT yields long-term clinical benefits, including improved quality of life, increased exercise capacity, reduced heart failure hospitalization, and decreased all-cause mortality. Since the initial development of CRT as a modality, indications for therapy have been tailored to those with evidence of left-sided intraventricular conduction delay, with the strongest indication for patients in sinus rhythm with wide QRS secondary to LBBB (Table 89–5).

Seminal CRT Trials

Conventional Indications

Since its first description in 1994, there has been an explosion of research directed towards elaborating the impact of CRT on ventricular performance, as well as elucidating its clinical impact. A recent comprehensive review commissioned by the AHRQ provides evidence regarding the efficacy and safety of CRT among Medicare beneficiaries.⁷⁴ Among the first controlled clinical trials of CRT was the Multisite Stimulation in Cardiomyopathy (MUSTIC) trial,⁷⁵ in which 67 patients with severe heart failure (LVEF ≤ 35% and LV end-diastolic diameter [LVEDD] > 60 mm), sinus rhythm, and QRS interval ≥ 150 ms were implanted with CRT devices. The patients then underwent a single-blind, randomized, crossover study in which the study compared responses during 3-month periods of inactive pacing (or ventricular pacing only at heart rates < 40 bpm) versus aorto-BiV pacing for 3 months. Each patient acted as her or his own control, and the primary end point was 6-minute walking distance (6MWD). This distance was 23% greater with BiV pacing (P < .001); in addition, quality of life score as assessed by the Minnesota questionnaire (P < .001) and peak oxygen uptake were improved and two-thirds fewer hospitalizations occurred for those who received BiV pacing in the first 3 months (P < .05). These results were impressive and led the way for further trials exploring clinical and echocardiographic end points after the CRT.

Later, two seminal trials led to the initial approval for widespread use of CRT: the Comparison of Medical Therapy, Pacing, and Defibrillation on Heart failure (COMPANION) study⁷⁶ and the Cardiac Resynchronization-Heart Failure (CARE-HF) study.⁷⁷ In COMPANION, 1520 patients with NYHA class III or ambulatory class IV HF from ischemic cardiomyopathy or NICM, LVEF ≤ 35%, and QRS ≥ 120 ms, were assigned to optimal pharmacologic therapy (OPT), OPT plus CRT pacemaker (CRT-P), or OPT plus a combined CRT-defibrillator (CRT-D). The primary end point was a composite of time to death or any-cause hospitalization and was significantly reduced by CRT-P (HR 0.81, P = .014) and CRT-D (HR 0.80, P = .01). The secondary end point of all-cause death was reduced by 24% in CRT-P patients in a trend that neared significance (P = .059) and was significant for CRT-D patients (36% reduction, P = .003). The CARE-HF study randomized 813 patients to medical therapy or medical therapy and CRT-P. The primary end point was a composite of death and cardiovascular hospitalization, and was significantly reduced by CRT-P (HR 0.63, P < .001). The secondary end point of mortality alone was also significantly reduced (HR 0.64, P < .002). CRT-P patients demonstrated better quality of life, less severe symptoms of HF, improved LVEF, reduced mitral regurgitation, and reduced N-terminal pro-brain natriuretic peptide (NT-proBNP).

The Resynchronization Reverses Remodeling in Systolic Left Ventricular Dysfunction (REVERSE),⁷⁸ the Multi-Center Automatic Defibrillator Implantation Trial-CRT (MADIT-CRT),⁷⁹ and the Resynchronization-Defibrillation for Ambulatory Heart Failure (RAFT) trials led to an expansion of the indication of CRT to less symptomatic patients.⁸⁰ Briefly, REVERSE evaluated 610 patients with NYHA class I or II HF with QRS ≥ 120 ms and LVEF ≤ 40%. Patients were randomized to CRT-ON or CRT-OFF. The primary end point was an HF clinical composite, which did not reach significance, but with greater improvement in LV remodeling and significantly reduced time to first HF hospitalization. Overall, the trial was felt to be underpowered. The larger MADIT-CRT study compared CRT-D with ICD only in 1820 patients with QRS ≥ 130 ms, LVEF ≤ 30%, and NYHA class I (ischemic only) or II (ischemic or nonischemic) symptoms. The trial was terminated early because of efficacy (HR 0.66 for primary end point in CRT-D patients, P < .001), driven by a reduction in HF hospitalization. The annual mortality was low, at approximately 3% annually, and was not significantly different in the two groups. Improvement in LV volumes and LVEF was also noted. A few months later, the similarly sized (1798 patients) RAFT study evaluated CRT-D versus ICD in patients with LVEF ≤ 30%, NYHA class II or II HF, and a QRS duration ≥ 120 ms. Results were quite similar, with reduction in the primary end point of all-cause death or HF hospitalization with BiV pacing (HR 0.75, P < .0001). Mortality reduction was also achieved in this slightly more-advanced HF population (HR 0.75, P = .003). Together, MADIT-CRT and RAFT led to a change of the guidelines, with approval for CRT for less symptomatic HF patients and earlier in the pathophysiologic disease process.

Narrow QRS

Although initially developed to address the problem of electrical conduction delay (as evidenced by a wide QRS), there was initial enthusiasm that CRT might have a role in patients with narrower QRS and echocardiographic evidence of mechanical dyssychrony. An initial randomized controlled trial, the Cardiac Resynchronization Therapy in Patients with Heart Failure and Narrow QRS (RethinQ),⁸¹ showed no difference in a primary end point comparing increase in peak oxygen consumption, although patients with a QRS width of 120 to 130 ms did benefit. The largest and most comprehensive study evaluating the question of CRT in patients with narrow QRS with respect to hard clinical end points was the Echocardiography Guided Cardiac Resynchronization therapy (EchoCRT) trial.⁸² Eligible patients demonstrated NYHA class III or IV HF, LVEF ≤ 35%, LVEDD ≥ 55 mm, QRS duration ≤ 130 ms, and echocardiographic evidence of LV dyssynchrony (ie, opposing-wall delay of ≥ 80 ms or speckle-tracking anteroseptal-to-posterior wall delay of ≥ 130 ms). Patients were randomized to either CRT-on or CRT-off. The trial was terminated early because of futility, with no benefit found at a mean of 19.4 months. There was also a worrisome signal of increased mortality in CRT-on patients (HR 1.81, P = .02). Persistent or worsened dyssychrony was a marker of worse clinical outcomes.⁸³

RV-Paced Patients

By contrast, the use of CRT in patients with paced QRS complexes (even if the underlying native is narrow) has been shown to be successful at improving outcomes in select populations if pacing percentage is high. Putatively this is because RV pacing is analogous to a LBBB-like activation. The Biventricular versus Right Ventricular Pacing in Heart Failure Patients with Atrioventricular Block (BLOCK HF) trial evaluated whether BiV pacing would be superior to RV pacing in patients with high-degree AV block, NYHA class I to III symptoms, and LVEF ≤ 50%.⁸⁴ The primary outcome was a composite of time to death for any cause, HF visit requiring intravenous therapy, or increase in LV end-systolic volume index by ≥ 15%. 691 patients were randomized and the primary outcome was reduced significantly (HR 0.74, 95% CI 0.60-0.90), and this effect was also seen when examining clinical end points of time to all-cause death and HF treatment alone (HR 0.73, 95% CI 0.57-0.92). Recent guidelines have taken BLOCK HF into account in consideration for BiV upgrade for patients with chronic RV pacing. The results of the larger Biventricular Pacing for Atrioventricular Block to Prevent Cardiac Desynchronization (BioPace) trial⁸⁵ looked at a similar population, and preliminary results were presented at the European Society of Cardiology Congress in 2014. In it, any patient with high-degree AV block, or with first-degree block or slow AF with an expected high burden of pacing, could be eligible.⁸⁶ Patients with normal LVEF and no HF could also enroll, and a total of 1810 patients were randomized. Average ventricular pacing was > 85% in both groups at 1 month and the average LVEF in the study was 55%. Although there was a trend toward benefit for mortality and HF hospitalization reduction, it did not reach significance (HR 0.87, P = .08). Interestingly, there was no significant difference when stratified by LVEF greater or less than 50%. Implant failure was 14.8% among patients assigned to BiV devices and may have also confounded results. Even as fundamental indications expand, implementation of technology continues to be a moving target, particularly with respect to the LV lead in patients with CRT. Further discussion will be provided below to provide a perspective on pacing and defibrillation systems, their fundamental components and implant, as a general background to understanding indications and use.

Impulse Generator

A transvenous pacing system (used in both pacemakers and ICDs) consists of an implanted pulse generator and the leads through which it delivers electrical stimuli in the various chambers of the heart. The pulse generator houses complex electronic circuitry and the battery, or the power source. The pulse generator contains an output circuit, sensing circuit, timing circuit, and telemetry coil that sends and receives programming instructions and diagnostic information. Most current devices also have another circuit for the rate-adaptive sensor. In ICDs, the pulse generator also includes a capacitor to store and deliver the energy used in high-voltage shocks.

Lithium-iodine batteries are the most commonly employed batteries used for pacemakers, whereas ICDs utilize slightly different battery chemistries (eg, lithium-silver vanadium oxide, lithium-manganese dioxide, and lithium hybrids). The advantages of the lithium-iodine battery is that it has a high-energy density, long shelf-life, low battery life loss caused by internal self-discharge, and predictable characteristics that allow early warning of battery depletion. Most single-chamber pacemakers have expected battery longevity of 7 to 12 years, whereas dual-chamber pacemakers have an expected longevity of 6 to 10 years. Most pacemakers generate 2.8 V at the beginning of life. When the voltage nears the end of life (2.1-2.4 V), several elective replacement indicators in the pacemaker are activated. The common indicators of elective replacement can be found during routine device interrogation, including:

• Percent or fixed decrease in pacing rate on magnet application or free running rate

• Change to a simpler pacing mode (DDDR to VVI; VVIR to VOO)

• Reduced battery voltage

• Elevated battery impedance

• Restricted programmability

Once the elective replacement indicator (ERI) is activated, the pacemaker should generally be replaced within 1-3 months, or earlier if the patient is pacemaker dependent. When the battery reaches the end-of-service (EOS, < 2.1 V), the pacemaker changes to the simplest mode (VOO), fails to communicate or reprogram, fails to pace or sense, and may function erratically. If the pacemaker reaches EOS, it should be replaced immediately.

Pacing and Defibrillation Leads

Permanent pacing leads have five major components: (1) electrodes, (2) conductors, (3) insulation, (4) connector pin, and (5) fixation mechanism. Pacing leads are identified as unipolar or bipolar based on the number of electrodes in contact with the myocardium, although all leads utilize a circuit with cathodal and anodal terminals. For example, unipolar leads carry a single conductor to a single electrode present at the lead tip (cathode), which is in contact with the myocardial tissue, and the surface of the pacemaker acts as the anodal terminal. Bipolar lead systems, on the other hand, house two conductors in the lead body leading to a cathodal tip electrode and an anodal ring electrode, 10 to 20 mm proximal to the tip electrode. Most modern pacing systems allow for bipolar leads to be “programmed unipolar” based on clinical scenario, such as to increase the field-of-view for sensing or possibly reduce the pacing output needed to capture the myocardium at a particular location. Downsides to the unipolar configuration are greater susceptibility to myopotential oversensing, far-field sensing, cross-talk (atrial stimulus sensed by ventricular channel or vice-versa), and skeletal muscle stimulation than the bipolar configuration.

Contemporary electrodes have a small tip surface area with a porous or roughened surface and steroid-eluting capability that reduces stimulation thresholds, decreases current drain, and improves sensing. The electrodes are connected to the connecter pin at the proximal end of the leads by the conductor wires, which are usually made of a nickel-composite alloy. The two primary materials utilized for insulation for leads are silicone rubber and polyurethane. Silicone demonstrates excellent biostability, flexibility, and resistance to thermal injury (as from electrocautery). Polyurethane insulation, on the other hand, has the advantage of having higher tear strength, lower friction coefficient, and smaller diameter.

Defibrillation leads are similar in design to pacing leads, but with a multilumen design. In addition to conductors for pacing and sensing, defibrillator leads carry conducting cables for high-voltage output used during defibrillation. These are typically contained in distinct inner channels with a secondary insulator, usually composed of fluoropolymers (polytetrafluoroethylene [PTFE] or ethylene tetrafluoroethylene [ETFE]).

The most commonly used transvenous endocardial leads have either a passive or an active fixation mechanism to avoid early dislodgement. Passive fixation mechanisms are limited to pacing leads only and include tines, fins, or wings that are entrapped within the trabeculae of the right-sided heart chambers and are subsequently covered by fibrous tissue. Contemporary active fixation leads are isodiametric, and use an extendable/retractable or exposed helix which is deployed in screw-like fashion into the myocardium. Active fixation allows for more precise positioning of the pacing lead in locations other than the right atrial appendage or RV apex. They are also associated with reduced rates of acute dislodgement rates and greater ease in extraction, but slightly increased risk of perforation when compared with passive-fix leads.

Transvenous Device Implantation

Approach

Transvenous pacemaker and defibrillator device implantations are usually performed under conscious sedation and local anesthesia targeting the cephalic, axillary, or subclavian veins. Cephalic or axillary vein access avoids the potential complication of compression damage to leads inserted via medial subclavian puncture by the costoclavicular ligament (also referred to as “subclavian crush”). The pulse generator is usually placed in the upper pectoral region subcutaneously, and occasionally the pacemaker is implanted behind the pectoral muscle or behind the breast via an inframammary approach, especially in young women. In patients who do not requiring pacing, an entirely subcutaneous defibrillation system (the S-ICD) may be used, which is usually positioned in the left mid-axillary line between the fifth and sixth intercostal spaces (see further below).

Pacing Site

Atrial Lead

The atrial lead is generally placed in the right atrial appendage; however, in patients with prior cardiopulmonary bypass (requiring an right atrial cannula), the atrial septum or the lateral wall is often utilized. It is preferable to avoid the atrial free wall with active fixation leads, because this has the potential to cause delayed pericardial effusion and/or cardiac tamponade. There has been interest in evaluating variable atrial pacing strategies to reduce the incidence of AF, including atrial septal, coronary sinus (CS) ostium, Bachmann’s bundle, and biatrial pacing. No clear benefit for AF suppression has been demonstrated in randomized trials in patients with SND,^87,88,89 although there may be utility for Bachmann’s bundle pacing in patients with documented interatrial conduction system delay,⁹⁰ or for biatrial pacing or Bachmann’s bundle pacing patients after CABG surgery.^91,92

Right Ventricular Lead

The RV lead is typically placed in the RV apex. However, attempts to maintain the normal sequence of ventricular activation by high septal pacing near the His-bundle and the RV outflow tract (RVOT), especially in patients with cardiomyopathies, have suggested hemodynamic benefits by improving cardiac output compared with RV apical pacing. A randomized crossover trial of patients with congestive HF, LV dysfunction (LVEF < 42%), and chronic AF pacing from RVOT or dual-site RV pacing (RVOT + RV apex) did not consistently improve quality of life or other clinical outcomes compared with RV apical pacing.⁹³ On the other hand, direct His-bundle pacing is a viable approach for permanent cardiac pacing in patients with dilated cardiomyopathy, which appears to better preserve LV function compared with RV apical pacing,^94,95 and with results comparable to BiV pacing.⁹⁶ Indeed, recent data suggest that in patients with at least moderate pacing percentage (> 40% ventricular pacing), heart failure hospitalization can be significantly reduced in patients with His-bundle pacing relative to RV pacing.⁹⁷ Although provocative, these findings need to be reproduced and verified in randomized study prior to effecting a change in the routine approach to RV lead placement.

Left Ventricular Lead

In patients receiving BiV pacing systems, an additional lead is placed for LV pacing. The LV lead is typically targeted to a posterior or lateral tributary of the coronary sinus on the epicardial surface of the heart and requires use of specialized delivery systems. Direct epicardial LV lead placement pacing via thoracotomy may need to be considered in occasional patients with inadequate venous access, phrenic nerve stimulation, mechanical prosthetic tricuspid valve, or in patients with significant right-to-left shunting. Endocardial LV lead position may have theoretical advantage with respect to ventricular activation, but has been associated with an increased risk of stroke in an early study.⁹⁸ LV lead localization to improve outcome in CRT has been an area of active investigation. There are emerging data that targeting the LV lead to locations of either mechanical or electrical delay is associated with better outcome. Clinical trials have demonstrated that LV lead localization to areas either concordant or adjacent to mechanical delay, as assessed by echocardiographic strain mapping, is associated with better reverse LV remodeling and reduced HF hospitalization.^99,100 LV lead locations associated with longer LV lead electrical delay (LVLED) or surface QRS-to-sensed-LV time (QLV) have also been associated with better acute and longer-term success,^101,102,103 even in patients with nonideal anatomical location.^104,105,106 Particularly as methods for LV lead delivery become more refined, intraprocedural assessment of optimal location will be a valuable means to improve CRT response. Recently, quadripolar LV leads (see example of quadripolar LV lead in Fig. 89–9) have made it possible to interrogate greater potential regions of the left ventricle for pacing as well as develop strategies for “multisite” pacing in which sequential or simultaneous LV bipoles are utilized to increase LV recruitment to improve resynchronization and reduce dyssynchrony.^107,108,109 This may be particularly beneficial in non-LBBB patients in whom regions of electrical delay are difficult to predict from standard surface ECG analysis.¹¹⁰

Pacing Threshold and Sensing

Atrial and ventricular leads are placed into the appropriate chambers and sutured in place after ensuring adequate pacing and sensing thresholds. The basic premise in obtaining acute pacing and sensing thresholds during implant is that these thresholds may degenerate over time, and adequate safety margins must be maintained to ensure safe long-term pacing and sensing. It is essential to understand the strength-duration curve (Fig. 89–10). The strength-duration curve for stimulation is the quantity of voltage required to stimulate the heart at a series of pulse widths. As shown in Fig. 89–10, increasing the pulse width beyond 0.6 ms usually does not decrease the voltage threshold. Ideally, at implant an atrial pacing threshold of < 1.5 V and ventricular threshold of < 1 V should be obtained. The threshold commonly increases over the next 2 to 4 weeks, reaches a peak, and then decreases to a chronic threshold level after 6 to 8 weeks. With steroid-eluting leads, the acute rise in threshold is ameliorated and the chronic threshold is significantly lower than in nonsteroid-eluting leads. During initial programming, the pacing output is programmed at three to five times the threshold voltage with a pulse width of 0.4 to 0.5 ms. At the 2- to 3-month follow-up visit, the output is decreased to no less than twice the threshold to maintain an adequate safety margin and prevent battery drain. Some pacemakers have algorithms that enable the device to confirm capture. These algorithms may determine pacing threshold at programmed time intervals or on a beat-by-beat basis. Using algorithms to automatically check pacing-capture thresholds, these devices adjust pacing voltages to just above the pacing threshold to reduce current drain and prolong battery longevity. Automatic capture algorithms are available for atrial, RV, and LV leads.

Sensing is usually measured as the peak-to-peak or base-to-peak amplitude of the intracardiac electrogram in millivolts. The ventricular electrograms should measure at least 5 mV and frequently measure in excess of 10 to 20 mV. Ventricular sensitivity (the level in mV that the intracardiac electrogram must exceed to be sensed by the device) is generally programmed between 2 to 3 mV so that an adequate safety margin exists for sensing intrinsic ventricular depolarization without the risk of oversensing T waves or other artifacts. Atrial electrograms are smaller in amplitude than ventricular electrograms; however, a minimum atrial electrogram of 1 to 2 mV should be obtained. In patients with paroxysmal AF or atrial flutter, the atrial electrogram during tachycardia might be smaller than during sinus rhythm. The atrial sensitivity is usually programmed at 0.5 mV; however, the sensitivity may have to be adjusted depending on the size of the far-field ventricular electrogram and associated atrial arrhythmias. Newer pacemakers also incorporate algorithms that automatically adjust sensitivity to the size of the atrial or ventricular signal.

Impedance

Lead impedance is the resistance to the flow of current from the generator to the myocardial tissue through the lead. Although there is a wide variability of normal lead impedances (250-1200 Ω), chronic lead impedances should not vary widely between outpatient follow-up visits. A fractured lead exhibits markedly elevated lead impedance. Insulation breaks manifest by reduced lead impedances. Lead fractures or insulation breaks often are intermittent problems. Therefore, normal lead impedances and pacing and sensing thresholds do not rule out these problems. The leads can be stressed by having the patient change position and do various provocative arm movements (eg, isometric exercise) to facilitate diagnosis of lead-related problems that are not otherwise observed. Many pacemakers intermittently monitor lead impedance and alert the physician to measured impedances that are out of the typical range.

Leadless Pacemaker Implant

In contrast to traditional transvenous system implantation, deploying leadless cardiac pacing systems does not require the creation of a pre-pectoral pocket. Femoral venous access is obtained, preferably on the right side, and a delivery catheter is used to position the LCP or TCP into the appropriate RV position. The LCP utilizes an active helix fixation mechanism in an apicoseptal position whereas the TCP employs self-expanding nitinol tines delivered into the RV apex. Both devices are tethered to the delivery catheter and “tug-tested” before final release.

There has been also growing interest regarding endocardial LV lead positioning. This has been achieved either through use of transseptal access and use of active helix leads as well as with the development of leadless LV pacing. Traditional leads have been associated with a need for a higher degree of therapeutic anticoagulation and increased risk of stroke. On the other hand, the Wireless Stimulation Endocardiallly for CRT (WiSE-CRT) study utilized a system with a small LV lead receiver electrode powered by a subcutaneous ultrasound pulse generator.¹¹¹ This technique has generated interest, but was associated with increased risk of periprocedural complication resulting from pericardial effusion in early case series.¹¹² Continued research with a modified delivery system is under way.¹¹³

Subcutaneous Defibrillator Implant

Prior to implantation, all patients who are candidates for S-ICD are screened with a three-lead surface ECG and the use of a template tool provided by the manufacturer. A patient “passes” only if a single lead consistently falls within a designated area in order to minimize the risk of T-wave oversensing in the S-ICD device.¹¹⁴ This is particularly problematic for patients with HCM or congenital heart disease,^115,116,117 although a new software upgrade has included an algorithm that appears to reduce T-wave oversensing by 40%.¹¹⁸ Template evaluation during rest and exercise testing has been advocated by some as a means to best identify a vector associated with lower risk.¹¹⁹

At the time of implantation, multiple approaches have been used. Fluoroscopy is not required, although limited pre-procedural fluoroscopy may be useful in marking the patient. The implant procedure relies on use of anatomical landmarks utilizing either a three-incision (one for pulse generator, one for B-electrode, and one for A-electrode) or two-incision technique (which requires tunneling A-electrode from B-electrode incision).¹²⁰ Special attention paid to posterior placement of the pulse generator as well as accurate localization of the B electrode at the base of the heart (Fig. 89–11). Device testing is presently recommended for S-ICD systems at the time of implantation, although as experience increases with the S-ICD and more data are acquired regarding its first shock success rate, this may well change as it has already for transvenous ICD systems.¹²¹

Pacemaker Nomenclature

The North American Society of Pacing and Electrophysiology and the British Pacing and Electrophysiology Group established a five-letter pacemaker code to describe basic pacemaker mode and function (Table 89–6).^103,122 The first letter represents the chamber being paced and the second the chamber being sensed: A for atrium, V for ventricle, and D for both atrium and ventricle. The third position describes the response of the device to a sensed event: I for inhibition, T for triggered, and D for both inhibition and triggering. The device can either inhibit (I) pacing output from one or both of its leads, or it can trigger (T) pacing after the sensed event. In a DDD pacemaker, a sensed atrial event inhibits the atrial pacing channel and triggers ventricular pacing after a programmable AV delay. The fourth position refers to the programmability of the device: R for rate-responsive pacing. The fifth position refers to antitachycardia function for pacemakers only; with the evolution of implantable defibrillators, this position is rarely used.

Pacing Modes

VVI Mode

As the code indicates, the ventricle is the chamber sensed and paced, with inhibition of ventricular pacing in response to a sensed ventricular event. A sensed or paced ventricular event initiates two timing cycles:

1. A refractory period that is programmable (ventricular refractory period) during which no ventricular sensing occurs to prevent inappropriate sensing of T waves. Any ventricular event occurring during this interval will also not reset the timing cycle.

2. A lower-rate interval (LRI), which corresponds to the programmed pacing rate. If there is no sensed ventricular event after the end of the ventricular refractory period and this interval expires, the pacemaker initiates a paced ventricular event. If a ventricular event is sensed after the ventricular refractory period (VRP), the pacemaker resets the timing cycle to begin a new LRI and VRP (Fig. 89–12).

3. VVI pacing is the most commonly used pacing mode. Although it is a simple pacing mode and offers protection against bradyarrhythmias, loss of AV synchrony is not well tolerated in many patients and may lead to pacemaker syndrome (discussed later in this chapter). In patients with chronic AF, this is an ideal pacing mode, especially if rate responsiveness is available.

AAI Mode

Similar to VVI mode, sensing and pacing occur in the atrium, with the atrial pacing channel inhibited in response to a sensed atrial activity. An atrial refractory period (ARP) and an LRI are initiated in response to a sensed or paced atrial event. AAIR pacing, with rate-responsiveness, is an excellent pacing mode in patients with SND and normal AV conduction. Patients with SND may develop AV block, which may be a source of concern when using AAI pacing. However, with careful selection of patients, including normal PR intervals, absence of BBB, and AV Wenckebach phenomenon occurring at atrial pacing rates of more than 120 bpm, the risk of development of second- or third-degree AV block may quite low (< 0.6% per year in the Danish study).¹⁸

VOO/AOO Modes

In these asynchronous pacing modes, there is no sensing. The chamber is paced asynchronously at the lower rate limit. The timing cycle consists of only an LRI and will not be reset by any intrinsic activity. Because there is no sensing, there are no refractory periods. This mode is rarely used except during procedures using constant electrocautery to avoid inhibition of pacing caused by sensing of the high-frequency impulses of electrocautery. When a magnet is applied over the pulse generator, the pacemaker switches to an asynchronous mode (VVI to VOO, DDD to DOO). The obvious disadvantage of this pacing mode is the possibility of asynchronous pacing during the vulnerable period and initiation of either AF or VF (although this is rare in patients in the absence of ischemia or prolonged QT). Importantly, magnet application to an ICD usually only disables tachyarrhythmia therapies (shocks and antitachycardia pacing), but does not switch to an asynchronous mode.

VDD Mode

In this mode the pacemaker paces only the ventricle, senses in both the atrium and the ventricle, and responds to atrial sensing with ventricular pacing. The ventricular channel is also inhibited by spontaneous ventricular activity. A sensed atrial event will start an AV interval followed by ventricular pacing. If spontaneous AV conduction occurs before the termination of the AV interval, ventricular pacing is inhibited. If there is no spontaneous atrial activity, ventricular pacing occurs at the lower-rate limit as in a VVI pacemaker. However, because there is no atrial pacing, this mode should not be used in patients with SND. In patients with normal sinus node function and AV block, this is an excellent mode choice.

DDD Mode

A DDD pacemaker senses and paces in both the atrium and the ventricle, and the response to sensing involves both inhibition and triggered output. The DDD pacemaker uses numerous timing cycles. Most modern pacemaker timing cycles are ventricular-based (with some modifications) and are explained as follows:

• The LRI starts with a sensed or paced ventricular event and ends with a paced ventricular event, and consists of two portions.

• The ventricular event initiates an atrial escape interval (AEI) or VA interval at the end of which atrial pacing is initiated.

  • The atrial paced or sensed event initiates the AV interval, at the end of which ventricular pacing is initiated. So, LRI = VA interval + AV interval.

• A sensed atrial event occurring before the completion of the VA interval terminates this interval and starts the AV interval.

• A sensed ventricular event occurring before the completion of the AV interval will terminate this interval and the LRI and reinitiate the LRI.

• A sensed or paced ventricular event will also initiate the VRP to avoid T-wave oversensing and simultaneously initiates the postventricular atrial refractory period (PVARP).

  • Thus, the PVARP helps prevent the sensing and tracking of any retrograde P waves. This refractory period is programmable and is essential to the prevention of pacemaker-mediated, endless-loop tachycardia, which is an arrhythmia in which the dual-chamber pacemaker serves as the antegrade limb and the patient’s retrograde conduction as the retrograde limb.

• The AV interval and the PVARP together constitute the total atrial refractory period (TARP), during which the atrial channel remains refractory and will not be tracked.

  • This in essence determines the upper rate interval (URI) or the maximal tracking rate interval (MTRI) in a DDD pacemaker.

In patients with DDD pacemakers, four different rhythm scenarios are possible, and many patients exhibit more than one scenario:

1. Normal sinus rhythm with no atrial or ventricular pacing: Here the patient’s rate is faster than the programmed lower rate of the pacemaker, and the native PR interval is shorter than the programmed AV interval, as a result of which both the atrial and ventricular channels are inhibited (Fig. 89–13).

2. Atrial sensed, ventricular pacing: The patient’s atrial rate is faster than the lower-rate limit, and the AV conduction interval is longer than the programmed AV interval, or there is no AV conduction, resulting in sensing of P waves and triggering of the ventricular channel after the programmed AV interval. In this situation, the patient is ventricularly paced at the patient’s sinus rate until the upper rate limit (see Fig. 89–13).

3. Atrial paced and ventricular sensed: In this situation the patient’s atrial rate is slower than the programmed lower rate and the patient’s AV conduction interval shorter than the programmed AV interval. Here the patient is atrially paced at the lower-rate limit (see Fig. 89–13).

4. Atrial and ventricular pacing: Here the sinus rate is slower than the lower-rate limit, and AV conduction is longer or absent, resulting in atrial and ventricular pacing at the lower-rate limit (see Fig. 89–13).

Atrioventricular Interval

The AV interval after a sensed atrial event is usually programmed to a shorter value than the AV interval after a paced atrial event, and is termed the differential AV interval. Some DDD pacemakers also have the added capability of shortening the AV interval with increase in heart rates; this is called dynamic, or rate adaptive, AV interval. Another available feature increasingly incorporated into modern pacemakers is AV hysteresis in which the device periodically increases the AV interval to allow native AV conduction and intrinsic ventricular activation.

The paced AV interval is considered as a single interval with two subportions. The initial interval is the blanking period (12-50 ms, programmable) during which the ventricular channel is blanked to avoid ventricular sensing of the atrial pacing artifact. If a spontaneous ventricular event occurs in this period, it will not be sensed. The second portion of the AV interval is the cross-talk sensing window during which a ventricular event is sensed if it occurs and leads to ventricular safety pacing with a shorter AV interval of 100 to 110 ms. This is to avoid cross-talk, or inhibition of the ventricular channel by sensing of the atrial pacing artifact (Fig. 89–14).

Upper Rate Behavior

In VDD and DDD modes, in addition to the programmed lower rate limit, there is an upper rate limit beyond which ventricular tracking of atrial events will not occur. When the atrial rate exceeds the programmed upper rate limit, the pacemaker will exhibit either Wenckebach or 2:1 AV block behavior. When the patient with complete heart block exercises to a sinus rate beyond the upper rate limit of the pacemaker, the P wave that is sensed is followed by ventricular pacing with prolongation of the AV interval beyond the programmed value to avoid violating the upper rate limit for pacing. When one subsequent P wave falls in the PVARP, it will not be tracked, resulting in pacemaker Wenckebach behavior (Fig. 89–15). When the atrial rate increases further, such that every other P-wave falls in the TARP, a 2:1 pacemaker AV block pattern occurs. The TARP should be programmed shorter than the URI to prevent sudden development of a 2:1 pacemaker AV block.

In young patients with complete heart block, the upper rate of the pacemaker should be programmed to faster rates corrected for the patient’s age to prevent Wenckebach behavior of the pacemaker during exercise. Programming dynamic AV interval and dynamic PVARP allows the TARP to be shorter at higher pacing rates and avoid sudden slowing of ventricular pacing rates. Another option is rate-responsive features, where a separately programmable sensor rate allows the pacemaker to continue to pace at the sensor-driven rate during exercise.

Mode Switch

When pacemaker patients with AV block (in DDD or VDD mode) develop atrial tachyarrhythmias (atrial tachycardia, atrial flutter, or AF), many atrial events are sensed and ventricular tracking occurs up to the programmed upper rate of the device. The surface ECG typically shows an irregular ventricular paced rhythm at rates close to the upper rate limit of the device. Occasionally in patients with slower atrial flutter, every other atrial electrogram may fall in the PVARP, resulting in a 2:1 AV block. In patients with intact conduction, the ventricular rates are determined by the patient’s intrinsic AV conduction.

In patients with persistent atrial tachyarrhythmias, reprogramming the pacemaker to DDI(R) or VVI(R) mode avoids rapid ventricular tracking of the atrial arrhythmias. Automatic mode switching is a programmable option in all current-generation pacemakers for patients with paroxysmal atrial tachyarrhythmias (Fig. 89–16). When the atrial rate exceeds the programmed mode switch rate, the device automatically changes its mode to either the VVI or DDI, in which ventricular tracking of atrial-sensed events do not occur. Current devices can also provide a complete history of mode-switch episodes with regard to the duration and frequency of these episodes and may also provide electrograms to confirm their nature. During follow-up of these devices, it is important to monitor for mode-switch episodes and confirm by reviewing the electrograms that they truly represent atrial tachyarrhythmias. Arrhythmia logs from the device provide a very efficient way of assessing the effects of various drug therapies in patients with AF.

Pacing Modes to Reduce Right Ventricular Pacing

RV pacing leads to asynchronous activation and can cause deterioration of LV function through complex effects on regional ventricular wall strain and loading conditions. Most pacemakers now use a variety of different AV interval algorithms to search for intrinsic conduction and avoid unnecessary ventricular pacing. ECGs obtained during such pacemaker behavior may cause concern for the interpreting physicians unaware of these pacemaker functions. Unlike traditional mode switch from DDD to VVI or DDI mode during atrial tachyarrhythmias, newer pacemakers can switch pacing mode from AAI(R) to DDD(R) or VVI back-up. Algorithms used by various vendors to minimize RV-only pacing include the Medtronic Managed Ventricular Pacing (MVP) mode, Boston Scientific RHYTHMIQ, St. Jude Ventricular Intrinsic Preference (VIP), Biotronik Vp Suppression, and Sorin SafeR.

The general approach to minimize ventricular pacing is to maintain the AAI mode and lengthen the AV interval to allow for intrinsic conduction. Some ventricular beats may be “dropped” during various algorithms, which falsely leads concern for pacemaker malfunction, but actually represents normal pacemaker behavior (Fig. 89–17).

Rate-Responsive Pacing

Rate-responsive pacing refers to the ability of the pacemaker to increase its lower rate in response to physiologic stimuli. Simple VVI and AAI pacemakers do not have the ability to increase their pacing rates in response to exercise. In patients with normal sinus node function and DDD pacemakers, the ventricular pacing rate increases in response to an increase in sinus rate and is physiologic. However, in the presence of SND, the pacing rate does not increase commensurate with the increase in physiologic need. Rate-responsive pacemakers provide the ability to increase pacing rates through special sensors incorporated in the pacing system that monitor various physiologic processes. Based on information from the sensors, the lower rate of the pacemakers constantly varies up to the upper sensor rate. At any point in time, the sensor rate overrides the programmed lower rate of the pacemaker. Rate-responsive pacing is available in almost all current pacemakers in VVIR, AAIR, DDIR, and DDDR modes. In patients with SND, rate-responsive pacing in AAIR, DDIR, or DDDR mode is preferable to non–sensor-driven pacing. Also, in patients with chronic AF and heart block, VVIR pacing is preferable to VVI pacing. In patients with severe coronary artery disease, it may be necessary to limit the programmed upper rate limit. The ideal mode for pacing is decided based on information regarding sinus node function, AV nodal function, atrial arrhythmias, heart rate response to exercise, and patient-specific clinical factors.

Rate-Responsive Sensors

An ideal rate-responsive pacing system should provide rate response appropriate for the metabolic demand during a wide range of activities. The optimal sensor should provide the following:

• Proportionate increase in heart rate to match increase in metabolic demand

• Appropriate rapid change in heart rate with exercise onset

• High sensitivity and specificity

• Appropriate slowing of heart rate after exercise

Sensors available or under development can be classified using a physiologic or technical classification system. In physiologic classification, the sensors are classified according to the physiologic level at which they sense (Table 89–7).

Activity Sensors

Activity-Based Sensors

Using piezoelectric crystals or accelerometers to detect vibration are the most widely used sensors. Although the piezoelectric crystal senses vibration associated with up-and-down motion, the accelerometer detects anteroposterior motion of the body. Accelerometer-based sensors provide adequate and quick increases in heart rate during treadmill exercise as compared with piezoelectric crystals. Although both sensors have been shown to provide excellent rate-responsiveness, direct manual pressure, walking down stairs, horseback riding, or riding on a bumpy street can cause an inappropriate increase in heart rate with the piezoelectric crystal-based sensor. These activity-based sensors do not provide adequate heart rate response with emotional stress or isometric exercises. Currently, the accelerometer is the most widely used rate response sensor.

Minute-Ventilation Sensor

The specificity and sensitivity of minute-ventilation sensing pacemakers for changes in metabolic workload are excellent. Current pulses are emitted from the pacemaker can, and the proximal ring electrode of the RV lead (or right atrial lead) and transthoracic impedances are measured. Measurement of phasic impedance changes provides respiratory rate and tidal volume information, thus determining the minute ventilation. The minute ventilation changes are used to calculate the pacing rate and have good correlation with exercise. Minute-ventilation sensing pacemakers can inappropriately increase pacing rates with hyperventilation, coughing, and mechanical ventilation and in patients with chronic obstructive pulmonary disease exacerbations.

Dual Sensors

Most of the currently available sensors have an excellent track record, but they may occasionally respond to nonphysiologic stimuli. A multisensor rate-responsive pacemaker can improve specificity by having one sensor verify the other sensor (eg, sensor cross-checking). A combination of two sensors can better simulate the normal sinus node response. Pacemakers with dual sensors can provide patients with rapid responses during the start of exercise and then use a more physiologic sensor (minute ventilation) to provide more proportional heart rate responses during steady state (eg, sensor blending).

Contractility Sensors

Attempts have been made to gauge cardiac contractility, which from first principles, would provide valuable data regarding physiologic status, including possible response to emotional or psychic stimuli. Closed loop stimulation (CLS) is a proprietary algorithm in which RV contractility is assessed by measuring multiple impedance values during the QRS in order to generate a reference waveform, which is then compared beat-to-beat in order to determine whether pacing rate should be increased. CLS appears to compare favorably to simple accelerometer based rate-adaptive pacing.¹²³ Another presently available CRT technology utilizes a pressure transducer as part of a specialized atrial lead that is used to detect the amplitude of heart sounds as a proxy LV maximal pressure generation and overall contractility. AV and VV intervals are then automatically adjusted by the device. Early data suggest this method may correlate well with other noninvasive methods of contractility assessment and randomized trial evaluating clinical outcomes are under way.¹²⁴

Implantable Cardioverter-Defibrillator Programming

Pacing Programming

Although initial ICDs were not developed with pacing capabilities, modern transvenous devices were combined with pacemaker functionality and there was initial enthusiasm regarding the role of dual-chamber pacing to facilitate optimal medical therapy and improve hemodynamics. The Dual Chamber and VVI Implantable Defibrillator (DAVID) trial was designed to test the hypothesis that DDD rate-adaptive pacing at 70 bpm (DDDR 70) was superior to ventricular back-up pacing at 40 bpm (VVI 40).¹²⁵ A total of 506 patients with LVEF ≤ 40% were enrolled who had no indications for antibradycardia pacing. Surprisingly, but in line with studies from patients receiving conventional pacemakers, patients with less RV pacing were significantly less likely to die or to be hospitalized for new or worsened HF (61% less; P ≤ .03). In large part because of DAVID, most patients receiving primary prevention ICDs are programmed to back-up ventricular pacing at low rates to minimize unnecessary RV pacing.¹²⁶

Anti-tachycardia Pacing and Shock Programming

More recently, there has been evolving investigation regarding the possible risks of ICD therapies with respect to delivery of ATP or shocks. The prevalence of “inappropriate” use (ie, device therapy for supraventricular arrhythmia) is common and has been associated with harm in large randomized controlled trials,^127,128,129 particularly in younger patients.¹³⁰ In addition, there is increasing recognition of therapy that, although appropriately directed toward VT or VF, may be avoidable. In addition to optimization of medical therapy or selective application of catheter ablation, device programming to reduce avoidable therapies and mitigate harms has been of growing interest. These were nicely summarized in a recent expert consensus document of the HRS.¹³¹ There are three commonly used programming domains that can be manipulated to determine ICD therapies:

1. Duration and rate criteria for detection

2. Use of single or multizone programming

3. Use of supraventricular tachycardia (SVT)-VT discriminators

Multiple cohort studies^132,133 and three large randomized studies^134,135,136 support the use of high-rate cutoffs (between 185 and 200 bpm) and tachycardia duration of 30 intervals or at least 6 to 12 seconds before completing detection and initiating therapies for primary prevention patients. For secondary prevention patients, rate cutoff should be tailored to VT thresholds, if known. Discrimination algorithms (ie, SVT-VT discriminators) can also be safely used at rates up to 200 bpm. In general, there has been migration away from early treatment in order to allow for spontaneous termination of VT or VF. This strategy also appears effective for devices programmed for shock-only therapies, such as the S-ICD.

Patients undergoing a new CIED system implantation are generally monitored in the hospital for a day in anticipation of any untoward complications. The various complications associated with transvenous CIEDs are outlined in Table 89–8.

Pneumothorax

At the time of implantation, subclavian or axillary venous access can rarely result in pneumothorax, hemothorax, or hemopneumothorax. This can occur from inadvertent puncture and laceration of the subclavian vein or the subclavian artery or the lung. The use of venograms to locate the venous system in difficult cases and the routine use of axillary venous access have reduced these complications significantly. Occasionally, air embolism may occur during cephalic vein or subclavian vein cannulation. The use of safe sheaths with one-way valve mechanism minimizes this risk.

Cardiac Perforation

Cardiac perforation resulting in pericardial effusion and occasionally cardiac tamponade is a rare, but potentially life-threatening, complication of pacemaker lead insertion. This may be recognized at the time of lead insertion by fluoroscopic position of the lead, RBBB morphology of the paced QRS complex, diaphragmatic stimulation, or hypotension resulting from cardiac tamponade. If identified at the time of implantation, lead withdrawal and repositioning is usually not associated with tamponade. In patients with chest pain, friction rub, and a small pericardial effusion, serial echocardiograms may be performed to assess for hemodynamic deterioration. Cardiac perforation and pericarditis may also develop as a late complication and occasionally lead to tamponade weeks to months after the pacemaker implantation, especially with active fixation leads.

Hematomas

Hematomas occurring at the pacemaker pocket site can vary from a small ecchymosis to large and tense swelling. Most small hematomas can be managed conservatively with cold compresses and withdrawal of antiplatelet or antithrombotic agents. Occasionally, large hematomas that compromise the suture line or skin integrity may have to be surgically evacuated. In patients who require therapeutic anticoagulation, heparin should be delayed for at least 24 to 48 hours after implantation to avoid bleeding complications. A recent large randomized trial has confirmed that the risk of hematomas is lower if the procedure is performed while warfarin is continued at therapeutic levels.¹³⁷ There is a relative paucity of data regarding management of direct oral anticoagulants (DOACs) at the time of device implantation. Most operators will interrupt therapy at the time of implantation,¹³⁸ with discontinuation at least 24 hours before procedure for dabigatran and 36 hours or longer for rivaroxaban.¹³⁹ With the recent availability of reversal agents for DOACs, however, practice may change going forward.

Venous Occlusion

Venous occlusion may result from acute subclavian or axillary vein thrombosis and lead to ipsilateral arm edema and thrombosis. Venous thrombosis is generally treated with heparin and 3 to 6 months of warfarin. Rarely, invasive surgical interventions may be required. Close to one-third of patients undergoing pacemaker implantation may develop partial to complete venous occlusion over time and may remain asymptomatic because of the development of venous collaterals.¹⁴⁰ Upper extremity deep vein thrombosis, when symptomatic, can be managed with anticoagulation in most cases.¹⁴¹ Superior vena cava syndrome is a rare complication of transvenous devices, and may require device explant and endovascular stenting for definitive treatment.¹⁴²

Infection

The use of prophylactic antibiotics and pocket irrigation with antibiotic solutions has decreased the rate of acute infections after pacemaker implantations to < 1% to 2% in most series. Periprocedural antibiotics has been validated in double-blinded clinical trials, and cefazolin is a commonly used agent.¹⁴³ Although early infections are generally caused by Staphylococcus aureus and may be aggressive, late infections associated with CIEDs may be associated with coagulase-negative staphylococci, and their course tends to be indolent.¹⁴⁴ The signs of infection include local inflammation and abscess formation, erosion of the pacer, and fever with positive blood cultures without an identifiable focus of infection. Occasionally the infected pacemaker tends to erode through the skin. Transesophageal echocardiography helps determine whether vegetations are present on the pacemaker leads. Incisional or superficial infection with no bacteremia or involvement of the CIED can usually be managed with oral antibiotic therapy. With that said, complete system removal if advocated for source control in patients with definite evidence of CIED system involvement (subcutaneous, transvenous, or epicardial) or lead-associated vegetation or valvular endocarditis without definite involvement of the leads (class I indication).¹⁴⁴ Patients with bacteremia but no localizing evidence of generator-site or clear lead involvement are a challenging group. In patients with relapsing or occult staphylococcal bacteremia, system explant is recommended (class I) and may also be reasonable for gram-negative bacteremia (class IIa).

Lead Dislodgement

Leads may dislodge from the initial implant site in the first few days to few weeks after implantation. Active and passive fixation mechanisms of leads help prevent this complication. Atrial lead dislodgement is slightly more common than dislodgment of ventricular leads. Although passive fixation leads are stable in the atrial appendage, active fixation leads are necessary to prevent dislodgment in patients with prior cardiac surgery. Lead dislodgment may result in an increase in pacing thresholds, failure to capture, or failure to sense. Lead dislodgment may be radiographically visible or it may be a microdislodgement, where there is no radiographic change in position, but there is significant increase in pacing threshold and/or decline in the electrogram amplitude.

Diaphragmatic/Phrenic Nerve Stimulation

Diaphragmatic/phrenic nerve stimulation may lead to significant discomfort. This phenomenon is most commonly observed in patients with LV coronary vein branch lead placement for BiV stimulation. During implant, high-output pacing at maximal voltage and pulse width should be tested routinely to avoid diaphragmatic stimulation. With the advent of quadripolar LV leads, multiple pacing vectors are possible and allow the physician to avoid repeat surgery by reprogramming the stimulation vector.

Twiddler Syndrome

First described in 1968,¹⁴⁵ Twiddler syndrome is a term applied to patients who intentionally or unintentionally manipulate their pulse generator, causing twisting of the entire pacemaker system (Fig. 89–18). This leads to lead dislodgment or lead fracture. Elderly age and obesity, along with a capacious pocket, have been identified as risk factors. Submuscular reimplantation or use of reinforced tie-downs and use of an antibacterial pouch may be used prevent recurrence.¹⁴⁶

Pacemaker Syndrome

The constellation of neurologic and cardiovascular signs and symptoms resulting from deleterious hemodynamics induced by ventricular pacing has been termed pacemaker syndrome. The basis for pacemaker syndrome is not only loss of AV synchrony, but also the presence of VA conduction. Atrial contraction against closed AV valves leads to increases in jugular and pulmonary venous pressure, causing cough and malaise in patients with intact cardiac function and congestive HF in other patients with structural heart disease. The symptoms may vary from mild to severe and the onset of symptoms range from acute to chronic.

The exact incidence of pacemaker syndrome is unknown, but severe manifestations are expected to occur in 5% to 7% of ventricularly paced patients. Milder symptoms or significant drops in systolic BP and cardiac output during ventricular pacing occur in > 20% of patients. In the Pacemaker Selection in the Elderly (PASE) trial, 26% of patients in the ventricular pacing arm crossed over to the DDD pacing mode because of symptoms caused by pacemaker syndrome.²¹ The management of pacemaker syndrome usually requires restoration of AV synchrony. In many patients, an upgrade to a dual-chamber pacer is indicated. In some patients with intact sinus and AV conduction, lowering the pacing rate in VVI mode and using the hysteresis mode may promote sinus rhythm, lessening the symptoms associated with pacemaker syndrome.

Pacemaker-Mediated Arrhythmias

Pacemaker-mediated tachycardia (PMT), or endless-loop tachycardia, is a well-recognized arrhythmia mediated by the pacemaker in patients with atrial-sensed ventricular pacing systems. In patients with intact VA conduction, a PVC may result in retrograde conduction to the atria, which, if outside the PVARP, is sensed and followed by ventricular pacing after the programmed AV interval. The paced ventricular event will again be followed by VA conduction, resulting in endless-loop tachycardia. PMT can be prevented by programming the PVARP to be longer than the native VA conduction time. However, if the VA interval is very long, the PVARP cannot be lengthened, as this will limit the upper rate. Modern pacemakers have several options to prevent or terminate PMT. One such feature is automatic extension of PVARP after a sensed premature ventricular beat to prevent tracking of retrogradely conducted P waves. If PMT is established (atrial-sensed ventricular pacing close to the upper rate limit with stable VA intervals), the PVARP is automatically extended to a longer interval of 500 ms for one cycle, usually terminating the tachycardia (Fig. 89–19).

Lead Failure

Transvenous leads are subject to long-term complications. The insulation of the leads may break, leading to problems with oversensing (caused by electrical noise), undersensing, and failure to capture (caused by current leak). This problem often manifests intermittently and may be difficult to detect during a routine pacer check. The patient may complain of pectoral muscle stimulation caused by current leakage around an insulation break. An abnormally low impedance with demonstrable lead malfunction is diagnostic for insulation break. Leads may also fracture over time. Early lead fractures lead to increased impedances associated with failure to capture, oversensing, and undersensing. Lead damage can occur at the site of venous access (subclavian vein) in the costoclavicular space, causing the crush syndrome.

CIED system malfunction can be secondary to circuitry failure or to lead dysfunction. Not uncommonly, what appears to be device malfunction may actually represent normal functioning of the pacemaker: A pacing artifact may be delivered in the middle of the normal QRS and may represent pseudofusion (caused by late sensing) and not undersensing. Unexplained longer pauses after sensed, but not paced, complexes might suggest oversensing, but it may represent normal function caused by hysteresis. Major electrocardiographic abnormalities of pacemaker system malfunction are broadly categorized into the following: failure to capture, failure to output, undersensing, and oversensing.

Failure to Capture

The loss of pacemaker capture occurs when there is a visible pacing stimulus and no atrial or ventricular depolarization. The differential diagnosis of failure to capture is outlined in Table 89–9. Failure to capture may be intermittent or persistent. Lead dislodgment can cause obvious failure to capture. An increase in the pacing threshold above the programmed pacing output can occur as a result of the rise of the threshold within a few weeks after lead placement because of drug therapy, electrolytes, MI, or ischemia. Fracture of the lead, insulation breaks, and loose set screws are mechanical problems that can cause failure to capture (Fig. 89–20). Finally, battery depletion may cause the pacing output to decline sufficiently such that pacing failure occurs. Loss of capture requires a check of the pacing threshold and of pacing lead impedance and a chest radiograph. For instance, if the problem is an elevated pacing threshold, pacing outputs must be increased. Abnormal lead impedances may confirm a lead failure and the need for lead replacement. Functional noncapture occurs when a stimulus falls during the physiologic refractory period of a native depolarization. It may be secondary to undersensing or as a function of the pacing mode (AOO, VOO).

Failure to Output

Another pacing system malfunction is the absence of pacing stimuli and hence no capture. In bipolar systems the pacing artifact is diminutive, especially if isoelectric in some surface leads. It is important to record multiple leads simultaneously. Failure to output is often caused by oversensing and inhibition of pacing output, and is less commonly caused by circuitry failure (Table 89–10). Oversensing may be caused by T waves, P waves, or far-field R waves and may cause inhibition of pacing output. In unipolar systems, oversensing of myopotentials is common. A loose set screw may cause noise and oversensing, leading to inhibition of pacing output. Electromagnetic signals from arc welding and electrocautery may be sensed by the pacemaker. Occasionally, the ventricular channel may be inhibited by oversensing the atrial channel output (cross-talk). If the absence of pacing output is caused by oversensing, a magnet application changes the pacemaker to an asynchronous mode to eliminate the pauses. In generator component malfunction, there is no delivery of pacing output by the device and magnet application does not restore pacing output. Systematic analysis of the pacing parameters, evaluation of lead impedances at rest, and provocative isometric exercises and chest radiograph may help identify the cause.

Oversensing

In a single-chamber pacemaker or defibrillator, oversensing leads to inhibition of the pacing channel and causes inappropriate pauses. However, in dual-chamber devices, oversensing elicits either inappropriate inhibition or triggering, depending on the channel in which oversensing occurs and the programmed pacing mode. Oversensing in the ventricular channel in the DDD or DDI mode results in inhibition of both atrial and ventricular outputs and resetting of timing cycles. In patients with complete heart block, this may result in ventricular asystole (Fig. 89–21). Oversensing in the atrial channel can lead to inappropriate triggering of ventricular output. The various etiologies of oversensing were discussed in the previous section. One example of oversensing peculiar to dual-chamber pacemakers is cross-talk. The atrial channel output can be sensed in the ventricular channel as a far-field signal and inhibits the ventricular pacing output. In patients with complete heart block, this can result in ventricular asystole. Although far more common with unipolar systems, cross-talk can also occur in bipolar pacing systems. To prevent this phenomenon, there is a programmable ventricular blanking period (51-150 ms) after atrial pacing, during which the ventricular channel is refractory. Additionally, there is a cross-talk–sensing window after the blanking period, during which a ventricular event, if sensed, will lead to ventricular pacing with a shorter AV interval. Oversensing as a result of lead fracture, insulation break, or other electrode problems will usually be random and erratic. With early lead problems, the malfunction is typically intermittent and may be exacerbated by certain body positions or motions. In later stages, the combination of oversensing, undersensing, and failure to capture is almost always diagnostic of a lead-related problem. Programming to an asynchronous mode may temporarily control this problem while awaiting lead replacement, which should be carried out as promptly as possible.

Undersensing

An inadequate intracardiac signal can lead to undersensing. The intracardiac electrograms can deteriorate because of inflammation or scar formation at the tissue lead interface. Additionally, some drugs, electrolyte abnormalities, infarction, ischemia, lead fracture, or insulation breaks can all lead to undersensing. Cardioversion or defibrillation can also cause attenuation of intracardiac electrograms. Usually, undersensing is a greater problem in the atrium than the ventricle. The atrial electrograms are typically significantly lower in amplitude during AF than they are during sinus rhythm. The optimal solution is to program an enhanced sensitivity (decreased sensing level, eg, from 1.5 to 0.55 mV). With bipolar systems, the programmed sensitivity can usually be reduced to as low as 0.18 mV in the atrium in some devices, without oversensing of myopotentials or other extraneous signals. Other etiologies for undersensing occur when intrinsic atrial or ventricular complexes fall within one of the programmed refractory periods. Undersensing can also result when a pacemaker is inadvertently programmed to an asynchronous mode (occasionally occurring with battery depletion or pacemaker generator reset).

Electromagnetic Interference

Electromagnetic interference (EMI) is defined as any nonphysiologic electrical signal that interferes with pacemaker function. EMI can originate from a variety of sources both within the hospital environment and outside. EMI can result in the inhibition of the pacemaker, inappropriate triggering, noise reversion, resetting of the pacemaker parameters and, occasionally, damage to the circuitry or electrode-myocardial interface; unipolar pacemakers are more susceptible to EMI than bipolar pacemakers. EMI has become an increasingly important problem because sources of interference are ubiquitous in the hospital and workplace.

In the hospital environment, the most common sources of EMI include electrocautery and defibrillation. Electrocautery can cause inhibition of the pacemaker, and in a pacemaker-dependent patient, this may result in severe bradycardia or asystole. Specific perioperative management of a device includes the following¹⁴⁷:

• Identify the type of the device and its manufacturer; identifying the manufacturer is essential.

• Identify whether the patient is dependent on the device for antibradycardia pacing; this may be obtained from the history of syncope, complete heart block, or AV node ablation requiring pacemaker placement. Interrogating the device may help identify whether the patient has a stable underlying escape rhythm.

• Determine whether EMI is likely to occur during the procedure. The device should be reprogrammed to asynchronous mode (VOO or DOO) or triggered mode (VVT), especially if the patient is dependent. Rate-adaptive pacing function should be turned off. Antitachyarrhythmia functions should be suspended (if an ICD). The surgeon should be advised to use a bipolar electrocautery (if possible) to minimize EMI.

• Place a magnet over the pacemaker to convert this device to function in an asynchronous mode (VVI to VOO or DDD to DOO) for as long as the magnet remains over the device. Removing the magnet returns the device to its original mode. In patients with ICDs, placing a magnet disables its antitachycardia therapies. However, this will not alter its pacemaker function. Magnets should be used when reprogramming options are not immediately available. All operating room personnel, including the surgeon, anesthesiologist, and nursing staff, must be aware of the presence of the device, the potential for EMI encountered in the operating room, and corrective techniques. An external defibrillator with transcutaneous pacing capabilities should be readily available.

• Intraoperatively, electrocardiographically monitor all patients to assess for device inhibition and with peripheral pulse evaluation (plethysmography or arterial pressure, etc.) in case of electrocautery-induced artifact on telemetry. Individuals performing the procedure should be advised to avoid electrocautery in the immediate field of the device generator and leads. Electrocautery should be used in short, intermittent, and irregular bursts.

• If the patient requires electrical cardioversion, deliver the shock as far as possible from the pulse generator and place the defibrillation pads on an axis perpendicular to that of the leads and pulse generator (to avoid transient undersensing or pacing inhibition).

• Interrogate devices after the procedure to assess for threshold changes and to reprogram the device to its original function if necessary.

Other potential sources of EMI in the hospital include extracorporeal lithotripsy, radiation therapy, and magnetic resonance imaging (MRI). Traditionally, MRI has been avoided in patients with pacemakers because of concerns for rapid pacing triggered by the pulsing of the magnetic field, device reprogramming, reed switch closure, heating at the myocardial-lead interface, or mechanical movement of the device. MRI “safe” (or conditional) pacemakers, ICDs, and now CRT devices are available, however. In addition, there is growing robust evidence regarding the use of protocols to image patients with CIEDs that are not conditionally approved (although abandoned leads are an absolute contraindication).^148,149 Approaches to minimize the risk of MRI to patients consist of programming the device to asynchronous mode, reducing the pacemaker output, limiting the MRI to areas remote from the device, and monitoring BP and ECG during the MRI scanning.

Lithotripsy can also result in inappropriate inhibition or oversensing, especially in rate-responsive pacemakers with piezoelectric crystals. The pacemaker should generally be programmed to VVI or VOO mode with the rate-responsive feature turned off before lithotripsy. Radiation therapy should be avoided directly over the field of the pacemaker. If shielding the pacemaker and limiting the field of radiation cannot be performed with safety, repositioning of the pacemaker before radiotherapy should be considered. Other possible EMI sources in the hospital include therapeutic diathermy, radiofrequency catheter ablation, transcutaneous electric nerve stimulation (TENS) units, and spinal cord stimulators.

Daily sources of EMI include digital cellular phones, electronic article surveillance devices, metal detectors, and electric razors, and work or industrial environment sources include high-voltage power lines, transformers, welding, and electric motors. Activated digital cell phones should not be placed in the breast pocket ipsilateral to the pacemaker, and the phone should be held to the ear contralateral to the pacemaker. Electronic surveillance devices or antitheft devices present in most stores and libraries can cause transient asynchronous pacing, atrial oversensing with tracking, and ventricular oversensing and inhibition. Patients with pacemakers should quickly walk through these devices and avoid lingering near them. Common household appliances such as electric can openers, stereos, televisions, video recorders, power tools, electric blankets, electric shavers, microwave ovens, and electric lawnmowers do not cause any pacemaker interference.

As the number of implantable cardiac devices has increased, device alerts and advisories have become a part of routine clinical practice. Recent lead recalls that have captured national and international attention has been the Medtronic Sprint Fidelis lead and the St. Jude Riata lead, both with implications regarding device efficacy and thus prompting discussion regarding extraction. When a physician is faced with the management of a patient with an implanted device that has been the subject of a recall or advisory, the major concern facing the clinician is how to manage the patient and whether the device must be replaced. Compared with other medical devices, these devices are unique because they are often implanted for life-threatening problems for which device failure may be causally linked to death or poor outcome. A review by Maisel and colleagues of device manufacturer performance reports issued between 1990 and 2002 found that > 17,000 devices were explanted during this timeframe for confirmed malfunction.¹⁵⁰ The explanted devices were evenly split among pacemakers and ICDs, but when comparing the number of implants in each category, ICDs appeared to be four times more likely to malfunction and require explantation. A meta-analysis of device registries found that pacemaker reliability has improved markedly and consistently over the past 20 years.¹⁵¹

There are a wide variety of malfunctions that have been reported with pacemakers and ICDs. These include both hardware component malfunction and software problems. In general, hardware malfunction tends to be far more common than software malfunction and is also more problematic for physicians and patients. Among hardware problems, battery/capacitor and electrical issues account for more than half of the reported malfunctions. Gould and Krahn reported the results from a multicenter Canadian study of the consequences of various ICD advisories.¹⁵² From approximately 18% (533 of 2915 patients) of devices electively replaced because of advisories, complications were observed in 8.1% of cases within 2.7 months after ICD generator replacement (5.8% were major complications requiring reoperation, including two deaths). By contrast, there were only three (0.1%) device malfunctions from the devices that were included in advisories, but no clinical complications from these malfunctions. In some centers, very low rates of complications have been observed, emphasizing the importance of individualizing patient management and knowing an individual center’s risk of complications.¹⁵³

The HRS has proposed a series of recommendations for physicians and industry.¹⁵⁴ Device manufacturers and the US Food and Drug Administration (FDA) notify physicians and patients after determining a device problem by an advisory; however, decisions about management are left to physicians. All patients with devices affected by recall, advisories, or alerts should be seen in the office for immediate device interrogation and discussion of the implications of the notification to the patient and their immediate and long-term device management and clinical status. The patient’s indication for device implantation and relative device dependence should be discussed, and well as potential consequences of device failure. The decision regarding which patients affected by the advisory/recall should undergo device replacement may be difficult for both the patient and the physician. Pacemaker dependence, ICDs implanted for secondary prevention, and high likelihood of device failure are some of the factors that lead to consideration for device replacement. Alternatively, patients may be scheduled for more frequent in-clinic follow-up or by transtelephonic or computer-based remote monitoring based on a careful review of the company, FDA, and any outside physician panel recommendations. Patients should be instructed to present to the device clinic or use remote follow-up whenever any symptoms (eg, syncope, presyncope, palpitations, angina, etc.) or other symptoms that suggest possible device malfunction are experienced. Device self-monitoring capabilities and the ability to do intensive remote follow-up allow for more reliable device follow-up in the future. Remote automatic checks with wireless or Internet-based physician notification schemes may improve monitoring of device performance, but currently office interrogations remain the most commonly used modality for follow-up. Diagnostic tools are also being developed to help detect potential hardware problems that could lead to malfunction.¹⁵⁵

Lead Extraction

Particularly as use of CIEDs has risen, the need for device and lead removal, because of infection, lead fracture, or recall has become an area of investigation for which specialized tools and methods have been developed. In particular, the field of transvenous lead extraction (TLE) has evolved beyond methods of simple extraction, to include the development of tools such as specialized locking stylets, snares, and mechanical, rotating threaded tip, electrosurgical, telescoping, and laser excimer sheaths.¹⁵⁶ In 2009, the HRS released an expert consensus statement on facilities, training, indications, and patient management for TLE.¹⁵⁷ The statement comments on the importance of having a multidisciplinary team in order to care for patients, including a primary operator properly trained in lead extraction, an immediately available cardiothoracic surgeon for backup during cases, anesthesia support, “scrubbed” and “nonscrubbed” assisting personnel (eg, nurses, technicians, as well as other physicians), and trained echocardiography support. When performed at high-volume centers, lead extraction can be associated with high acute procedural success, include complete lead removal 96.5% of the time, and have a 97.7% overall clinical success rate.¹⁵⁸ Overall in-hospital mortality approaches 2%, however, and risk factors for major adverse events include length of time leads have been implanted (particularly > 10 years), type of lead (with dual-coil defibrillator leads associated with higher risk), lower body mass index, presence of chronic kidney disease or diabetes, LVEF, and extraction for infection, among others.^158,159 Taken together, the decision on whether or not to pursue extraction should be made after careful assessment of risks and benefits in conjunction with the patient and family members.

The indications for CIED use in the care of the cardiovascular patient are rapidly expanding. In addition to the treatment of bradyarrhythmia and tachyarrhythmia (pacemakers and ICDs), devices are being employed in the treatment of symptomatic heart failure to reduce hospitalization (CRT devices). Furthermore, complementary to their role in therapy, devices also provide diagnostic data through evaluation of real-time heart rate, patient activity, and arrhythmia burden. Indeed, device-based diagnostics have already been correlated with HF hospitalization and mortality.^160,161 In concert with wider networks of patient management such as telemonitoring and remote patient care, the future use of CIEDs will be invaluable in providing additional insight into patients’ dynamic disease state, and as a means to help tailor and direct real-time care. There is also growing interest in providing device-based data stored online to patients to help facilitate greater engagement in healthcare as well as improve the patient-physician interaction. These areas of active investigation are expanding the fundamental paradigms of these tools, from devices that go well beyond arrhythmia or HF care, to overall cardiovascular disease management and patient-centered fitness.

We would like to thank Dr. Vijayaraman and Dr. Ellenbogen, who contributed to the previous version of this chapter in the 13th edition.