Chapter 19: Cardiac Implantable Electronic Devices

INTRODUCTION

The very first cardiac implantable electronic devices (CIED) were single-chamber, ventricular pacemakers with epicardial leads that offered only asynchronous pacing. Essentially only fixed or intermittent complete AV block was treatable. Patients had Stokes-Adams attacks and life-threatening asystolic episodes. Transvenous leads led to less invasive implantation, and then dual-chamber pacing allowed restoration of AV synchrony for AV block patients and treatment of sinus bradycardia with AV pacing. In the 1980s, implantable defibrillators were inserted, initially for patients with recurrent sudden cardiac arrest (SCA, see Chapter 15). These followed a similar evolution from epicardial to transvenous leads and from single to dual chamber as technology developed. Indications expanded due to such progress and to clinical trials. More recently, biventricular pacing in select patients with LBBB has been used for cardiac resynchronization therapy (CRT). His bundle pacing has reemerged with the focus on more physiologic pacing. While transvenous leads offered easier implantation than epicardial ones, long-term risks include intravascular infection, and their removal in the event of either intravascular or pocket infection or failure, poses significant risks. Thus, miniaturized, leadless pacemakers and subcutaneous defibrillators have been developed.

The indications for pacing have recently been updated,¹ and the reader is referred to a discussion of the causes of bradycardia in Chapter 10 and indications for ICD in Chapter 15. In brief, pacing to treat symptomatic bradycardia is needed either for a failure of impulse generation in the sinus node or failure of AV conduction. Sinus node dysfunction is defined, in part, as a resting rate less than 50 bpm, diurnal pauses exceeding 3 seconds, and/or failure to augment heart rate sufficiently with exercise (chronotropic incompetence). Recognition of reversible causes for bradycardia, or normal variants (such as bradycardia or pauses due to high vagal tone in athletes) and avoiding pacing in these cases constitutes 1 of the main challenges.¹ The other main task is associating symptoms to the bradycardia, especially for sinus node dysfunction, because it is rarely a life-threatening condition. AV block, on the other hand, requires a consideration of pacing in some potentially asymptomatic scenarios, such as alternating bundle branch block, Mobitz II block, or for prolonged HV interval (detectable at EP study; see Chapters 10 and 16).

Several types of patients present challenges from an indication perspective. With sinus node dysfunction, it can be difficult firmly to link symptoms with significant bradycardia. One should be wary of being asked to implant the patient before clarifying the time of day of a tracing and/or any attendant symptoms. Prolonged monitoring, exercise testing, and a thorough history sometimes resolve the uncertainty. If the situation remains unclear, these patients can be followed closely before deciding on device implantation. With regard to the chronotropic incompetence (CI) subtype of sinus node dysfunction, it is again important to elicit symptoms, although they can be subtle due to the patient’s adjustment to their limitations. Appropriate age and sex references are ...