Chapter 11. Supraventricular Tachycardias

• Heart rate greater than 100 bpm.
• Rhythm is supraventricular in origin.

General Considerations

Supraventricular tachycardias (SVTs) are rapid rhythm disturbances originating from the atria or the atrioventricular (AV) node. In the absence of a bundle branch block, there is intact conduction to the ventricles via the right and left bundles leading to a narrow and normal-appearing QRS. Therefore, these arrhythmias are also often called narrow complex tachycardias. Since many of the SVTs are episodic, many clinicians also refer to this group of arrhythmias as paroxysmal SVTs. Radiofrequency ablation has become an important therapeutic option in the management of SVTs because of its ability to cure these arrhythmias safely. Table 11–1 outlines the pharmacologic therapy for SVTs.

Pathophysiology & Etiology

Arrhythmias occur as a result of three main mechanisms: reentry, which is most common; enhanced or abnormal automaticity; and triggered activity.

Reentrant arrhythmias sustain themselves by repetitively following a revolving pathway comprising two limbs, one that takes the impulse away from and one that carries it back to the site of origin. For reentry to exist, an area of slow conduction must occur, and each limb must have a different refractory period (see the discussion on AV nodal reentrant tachycardia). In this situation, pacing (by inducing refractoriness in one limb of the circuit) can typically initiate a reentrant tachycardia. Once established, pacing can also terminate the tachycardia by interfering with impulse propagation in one of the limbs.

The second mechanism, automaticity, refers to spontaneous and, often, repetitive firing from a single focus, which may either be ectopic or may originate in the sinus node. It should be noted that automaticity is an intrinsic property of all myocardial cells. This mechanism comprises two subcategories. Enhanced automaticity is defined as a focus that fires spontaneously and may originate in the sinus node, subsidiary pacemakers in the atrium including the Eustachian ridge, Bachmann bundle, coronary sinus and AV valves, the AV node, His-Purkinje system, and the ventricles. Abnormal automaticity is usually secondary to a disease process causing alterations in ionic flow that produces a lower (ie, more positive) resting diastolic membrane potential. Threshold potential is therefore more easily attained, thereby increasing the probability of a sustained arrhythmia.

The third mechanism, triggered arrhythmias, depends on oscillations in the membrane potential that closely follow an action potential. In the absence of a new external electrical stimulus, these oscillations, or after-depolarizations, cause new action potentials to develop. Thus, each new action potential results from the previous action potential. These arrhythmias can be produced by early or late after-depolarization, depending on the timing of the first after-depolarization relative to the preceding action potential (the one that spawned the triggered activity). In early after-depolarizations, membrane repolarization is incomplete, which allows an action potential to be initiated by a subthreshold stimulus. This type is often associated with electrolyte disturbance and may be the mechanism responsible for arrhythmogenesis related to the prolonged QT syndrome and torsades de pointes caused by quinidine. With delayed after-depolarization, membrane repolarization is complete, but an abnormal intracellular calcium load causes spontaneous depolarization. The reason for the high calcium levels is unclear, but it can be related to inhibition of the sodium pump by drugs such as digoxin. In either type of arrhythmia, the process may be repetitive and lead to a sustained tachycardia.

General Diagnostic Approach

A systematic approach to interpreting the 12-lead electrocardiogram (ECG) will allow accurate determination of the type of SVT in most cases (Figure 11–1). The first step is to determine whether the rhythm is regular or irregular. If it is irregular, the rhythm is likely either atrial fibrillation, atrial flutter with variable conduction, or multifocal atrial tachycardia (MAT). The appearance of the P waves or lack of P waves will usually distinguish between these three entities. In atrial fibrillation, there is chaotic atrial activity. In atrial flutter, P waves are seen at rate of 240–320 bpm. In MAT, there are P waves preceding each QRS complex, and there are at least three different P-wave morphologies.

If the SVT is regular, there are several main types of SVT to consider. The SVT could be sinus tachycardia, sinus node reentry, atrial flutter, atrial tachycardia, AV nodal reentrant tachycardia (AVNRT), junctional tachycardia, or atrioventricular reciprocating tachycardia (AVRT). The type of regular SVT can be usually identified by examining four aspects of the 12-lead ECG: onset and termination, heart rate, P wave morphology, and R-P relationship. Sinus tachycardia and junctional tachycardia typically have very gradual onset, whereas the other SVTs usually start and stop more suddenly. Rate can also be helpful since sinus tachycardia cannot typically go over 220-age bpm and the heart rate in atrial flutter is often a multiple of 300. P-wave morphology can be helpful since retrograde P waves (negative in the inferior leads: II, III, and aVF) favor AVNRT and junctional tachycardia. Finally, R-P relationship refers to the distance from the R wave to the next P wave during tachycardia. If this distance is longer than the P-R interval, the SVT is termed “long R-P,” whereas if this distance is short, it is termed “short R-P” (Figure 11–2).

Sinus Tachycardia

Essentials of Diagnosis

• Onset and termination: gradual.
• Heart rate: 100 to (220 – age) bpm.
• P wave: identical to normal sinus rhythm P wave.
• R-P relationship: long.

General Considerations

When the sinus node fires at a rate of more than 100 bpm, the rhythm is, with one exception, considered sinus tachycardia (see later section, Sinus Node Reentry). The onset and termination of sinus tachycardia are invariably gradual. The range for heart rate in sinus tachycardia is 100 to (220 – age) bpm; faster rates usually imply a different cause. Confirming that the tachycardic P waves are identical in morphology and axis to the normal sinus rhythm P waves is essential to the diagnosis. Like normal sinus rhythm, the R-P relationship in sinus tachycardia is typically long R-P, unless the patient has a very long P-R interval, which can usually be seen on the baseline ECG.

Sinus tachycardia is usually a physiologic response, activated when the body requires a higher heart rate to meet metabolic demands or maintain blood pressure. Common causes are exercise, hypotension, hypoxemia, heart failure, sepsis, fever, hyperthyroidism, fluid depletion, and blood loss.

The heart rate achieved is proportional to the intensity of the stimulus, but the rapidity with which the heart rate increases and decreases is a function of how quickly the stimulus is applied and withdrawn.

Treatment

Vagal maneuvers slow the tachycardia gradually but only while being performed; when the vagal stimulus is removed, the heart rate gradually returns to where it started.

Attempting to slow the heart rate pharmacologically can be detrimental because it counteracts the compensatory mechanism provided by the tachycardia. Therefore, management is usually focused on treating the underlying cause of the sinus tachycardia.

Sinus Node Reentry

Essentials of Diagnosis

• Onset and termination: sudden.
• Heart rate: 100–160 bpm.
• P wave: identical to normal sinus rhythm P wave
• R-P relationship: long.

General Considerations

This uncommon rhythm accounts for less than 5% of SVTs. It uses the sinus node or perinodal tissue as a critical part of the reentrant circuit, producing P waves identical to those seen during normal sinus rhythm. The heart rate usually falls between 100 and 160 bpm. Like sinus tachycardia, the R-P relationship is typically long R-P. Unlike sinus tachycardia, sinus node reentry is initiated by an ectopic beat rather than a physiologic stimulus and possesses the characteristics typical of a reentrant circuit. It therefore begins and ends abruptly and responds to vagal maneuvers and pharmacologic interventions by terminating rather than slowing.

Treatment

The arrhythmia can be terminated quickly with intravenous adenosine, verapamil, or diltiazem, or via carotid massage. Long-term treatment uses β-blockers and calcium channel blockers. The largest reported series of patients treated with catheter ablation described success in all 10 patients. No complications were reported. Other smaller series found similar efficacy.

Essentials of Diagnosis

• Onset and termination: sudden.
• Heart rate: usually a multiple of 300.
• P waves: flutter waves at 250–340 bpm.
• R-P relationship: undefined due to flutter waves.
• Prominent neck vein pulsations of about 300/min.

General Considerations

Atrial flutter is usually associated with organic heart disease and is second in frequency only to atrial fibrillation in post–coronary bypass surgery patients, with an incidence of up to 33%. With a typical atrial rate of 300 bpm (range 250–340 bpm), atrial flutter usually produces a “sawtooth” appearance (F waves). As is the case with atrial fibrillation (see Chapter 12), the ventricular rate depends on conduction through the AV node. Unlike atrial fibrillation, the ventricular impulses are transmitted at some integer fraction of the atrial rate. In rare circumstances, 1:1 conduction may occur. Fixed 2:1, 3:1, or 4:1 block is the usual scenario. However, variable block can also occur, leading to one of the three types of irregular SVTs (see Figure 11–1). If flutter is suspected but F waves are not clearly visible, vagal maneuvers or pharmacologic agents, such as adenosine, can help unmask the flutter waves by enhancing the degree of AV block.

Pathophysiology

Atrial flutter occurs in a variety of forms; the most common is isthmus-dependent counterclockwise atrial flutter; followed by the isthmus-dependent clockwise atrial flutter; and then the atypical, nonisthmus-dependent variety. The counterclockwise flutter is recognized electrocardiographically by negative F waves in leads II, III, and aVF and positive F waves in lead V1. The single reentrant wavefront proceeds up the interatrial septum, across the roof of the right atrium, down the lateral wall, and across the inferior wall. Clockwise flutter, on the other hand, has positive F waves in leads II, III, and aVF and negative F waves in lead V1. The reentrant circuit in this case moves in the reverse direction. In both of these types of atrial flutter, the atrial rates range between 250 and 340 bpm.

Clinical Findings

Symptoms attributable to atrial flutter are secondary to the ventricular response in addition to any underlying cardiac diseases. Dizziness, palpitations, angina-type chest pain, dyspnea, weakness, fatigue, and, occasionally, syncope may be the presenting symptoms. In those patients with poor left ventricular function, overt congestive heart failure may ensue.

Clinical evaluation is similar to that described for atrial fibrillation (see Chapter 12), but underlying heart disease is detected more often with atrial flutter than with fibrillation.

Prevention

Several antiarrhythmic agents can prevent recurrences of atrial flutter. It appears that both class Ia and Ic agents are effective. Class III agents, such as sotalol and amiodarone, can also work very well. Dofetilide, a newer class III agent, which blocks the rapid form of the delayed rectifier current, Ikr, has also been found effective in converting to and maintenance of sinus rhythm. Its administration requires initiation in the hospital and an ECG-monitored setting. Drugs that are contraindicated with its use include verapamil, ketoconazole, cimetidine, trimethoprim, prochlorperazine, megestrol, and hydrochlorothiazide. With regard to safety, dofetilide has an overall proarrhythmic event rate of approximately 0.9%, which is less than the 3.3% seen in patients with congestive heart failure or the 2.5% in patients with previous ventricular tachycardia.

It should be emphasized that an AV nodal blocking agent should be started before initiating a class I drug. If the AV node is unblocked, a type I agent could facilitate conduction of atrial flutter by improving nodal conduction or by slowing the flutter rate and paradoxically increasing the ventricular response.

Treatment

Conversion

Once the diagnosis of atrial flutter is made, assessment of the patient's status will dictate whether to perform cardioversion immediately. Immediate cardioversion can be accomplished with synchronized direct-current (DC) cardioversion, rapid atrial pacing to interrupt the macroreentrant circuit, or intravenous infusion of an antiarrhythmic agent. For DC cardioversion, as little as 25 J may be all that is required; however, at least 50 J is recommended to avoid extra shocks, and 100 J will terminate almost all episodes of atrial flutter. The major drawback with DC cardioversion is the need to administer sedation.

Rapid atrial pacing is another method that may terminate the arrhythmia. Pacing is best performed in the right atrium at a rate faster than the flutter rate, which allows the circuit to be entered by the pacing impulse. If the extrinsic pacing rate exceeds the rate that can be sustained through the zone of slow conduction, the flutter wavefront can be interrupted and will no longer be present when the pacing is stopped. If the patient has a pacemaker or implantable cardiac defibrillator with an atrial lead, pace termination can be done painlessly via the device. An alternative method uses a swallowed transesophageal electrode. Because of the interposed tissue, a high current is often necessary to capture and pace the atrium reliably, which may cause significant discomfort to the patient. Of note, overdrive pacing may precipitate atrial fibrillation, which usually terminates spontaneously after several minutes. Should the atrial fibrillation persist, however, control of the ventricular response rate is typically easier when compared with atrial flutter.

Finally, rapid pharmacologic cardioversion can be considered with intravenous agents such as ibutilide. Ibutilide is a unique class III antiarrhythmic agent with a rate of conversion of approximately 60% in patients with atrial flutter of less than 45 days duration. Cardioversion can be expected within 30 minutes of administration. The major complication with this agent is the development of torsades de pointes, which can occur in up to 12.5% of patients, with 1.7% requiring cardioversion for sustained polymorphic ventricular tachycardia. These occur primarily within the first hour after administration. For this reason, patients given ibutilide are typically kept on monitor for several hours after administration. Procainamide is another intravenous agent that can be given to pharmacologically convert atrial flutter.

Rate Control

In general, controlling the ventricular rate in atrial flutter is more difficult than in atrial fibrillation. β-Blockers and calcium channel blockers are moderately effective in controlling the rate. Digoxin is less helpful since it only weakly blocks AV node conduction. Intravenous amiodarone, which has some β-blocking effect, has been shown to be at least as efficacious as digoxin.

Catheter Ablation and Other Modalities

The reentrant circuit in typical atrial flutter has been successfully mapped and includes an area of slow conduction called the isthmus, which is bound by the tricuspid annulus, the inferior vena cava, and the os of the coronary sinus. Ablation in the isthmus region interrupts the reentrant circuit and has been shown to be highly efficacious (90–100%) in permanently eliminating atrial flutter. In cost-effective analysis, ablation appears to be the preferred approach over cardioversion and pharmacologic prevention. Non–isthmus-dependent atypical atrial flutters can be more difficult to ablate. However, with current three-dimensional mapping systems (electroanatomic and noncontact high resolution), even these types of atrial flutter are being ablated with high success rates.

If attempts to ablate atrial flutter fail, the ventricular rate can be controlled by transcatheter ablation of the AV node or His bundle. With a long-standing, there may be a subsequent improvement in left ventricular function.

Stroke Prophylaxis

Atrial flutter is a recognized cause of peripheral embolization and stroke. The current recommendation is to treat patients with atrial flutter just as atrial fibrillation in terms of stroke prophylaxis. Many patients have bouts of atrial fibrillation even after successful atrial flutter ablation. Therefore, rigorous monitoring after atrial flutter ablation is recommended to determine which patients need long-term aspirin or anticoagulant therapy.

Essentials of Diagnosis

• Heart rate: up to 150 bpm.
• P waves: three or more distinct P waves in a single lead.
• Variable P-P, P-R, and R-R intervals.

General Considerations

Multifocal atrial tachycardia (MAT) is an irregular SVT that constitutes less than 1% of all arrhythmias. It is related to pulmonary disease in 60–85% of cases, with chronic obstructive pulmonary disease (COPD) exacerbation being the most common. In addition, MAT is precipitated by respiratory failure, acute decompensated cardiac function, and infection. It has also been reported to be associated with hypokalemia, hypomagnesemia, hyponatremia, pulmonary embolism, cancer, and valvular heart disease, and can also occur in the postoperative setting. It affects children and adults. Distention of the right atrium from elevated pulmonary pressures causes multiple ectopic foci to fire, with ventricular rates not usually exceeding 150 bpm. Whether this rhythm is due to abnormal automaticity or triggered activity is uncertain, but the ability of verapamil to suppress the ectopic atrial activity by virtue of its calcium channel–blocking properties supports the latter assumption.

Three ECG criteria must be met to diagnose MAT (Figure 11–3): (1) the presence of at least three distinct P-wave morphologies recorded in the same lead; (2) the absence of one dominant atrial pacemaker; and (3) varying P-P, P-R, and R-R intervals.

MAT is often misdiagnosed as atrial fibrillation. Although both are irregular, the former has distinct P waves with an intervening isoelectric baseline. At times, MAT may progress to atrial fibrillation.

Treatment

The primary treatment for MAT should be directed at the underlying disease state. Oral and intravenous verapamil and several formulations of intravenous β-blockers have been effective to varying degrees in either slowing the heart rate (without terminating the rhythm) or in converting the arrhythmia to sinus rhythm. Intravenous magnesium and potassium, even in patients with serum levels of these electrolytes within the normal range, convert a significant percentage of these patients to sinus rhythm. Digoxin is not effective in treating this condition. Moreover, treatment with digoxin may precipitate digitalis intoxication. In addition, if the arrhythmia is secondary to delayed after-depolarizations, further aggravation may occur with digitalis because this drug increases delayed after-depolarizations. Medications that cause atrial irritability, such as theophylline and β-agonists, should be withdrawn whenever possible.

Application of radiofrequency energy for both AV node modification and AV node ablation with subsequent implantation of a pacemaker has been reported. The numbers of patients in the studies were very small, and there are no long-term results. Nevertheless, ablation of the AV node has been shown to reduce symptomatic MAT, resulting in improved quality of life, reduced hospital admissions for recurrent symptomatic MAT, and improved left ventricular function.

Prognosis

Because of the severity of the precipitating underlying diseases, MAT portends a poor outcome. Mortality during the hospitalization when the arrhythmia is first diagnosed is between 30% and 60%, with death being attributed to the disease state rather than the tachycardia itself. In one study of patients with pulmonary disease who were admitted for acute respiratory failure, the in-hospital mortality rate for those with MAT was 87%, compared with 24% for those in a different rhythm.

Essentials of Diagnosis

• Onset and termination: sudden.
• Heart rate: 100–180 bpm.
• P wave: distinct P waves that differ from sinus P waves.
• R-P relationship: long.

General Considerations

Atrial tachycardia originates from an ectopic site in the atrium, and therefore, the P waves are usually quite different than the sinus P waves. It has been demonstrated that these arrhythmias arise from well-defined anatomic regions, including the crista terminalis, the tricuspid and mitral annuli, the right and left atrial appendage, and the region within or surrounding the pulmonary veins. In situations where the P wave is identical to the sinus P wave, the SVT is usually designated to be sinus node reentry or sinus tachycardia. The onset of atrial tachycardia is typically sudden; however, there can be some acceleration at the beginning, which is called the “warm-up” phase. Rates can range from 100 to 180 bpm and, rarely, over 200 bpm. The R-P relationship is usually long, unless there is a very long P-R interval, which can sometimes be appreciated on the baseline ECG. The episodes may either be brief and self-terminating or chronic and persistent, eventually leading to a tachycardia-induced cardiomyopathy if left untreated. Short nonsustained bursts of atrial tachycardia can be seen in 2–6% of young healthy adults on Holter evaluations.

In patients with paroxysmal sustained atrial tachycardia, there is a higher likelihood of associated organic heart disease, including coronary artery disease, valvular heart disease, congenital heart disease, and other cardiomyopathies. Frequently, a transient automatic atrial tachycardia will be present, the cause of which can usually be determined from the associated clinical setting. Some of the most frequent causes include acute myocardial infarction (in which case, it is seen in 4–19% of patients), electrolyte disturbances (especially hypokalemia), chronic lung disease or pulmonary infection, acute alcohol ingestion, hypoxia, and use of cardiac stimulants (theophylline, cocaine).

The form that occurs almost exclusively, and not uncommonly, in children is a continuous tachycardia with heart rates of about 175 bpm. Symptoms are severe, and congestive heart failure frequently develops as a result of a tachycardia-induced cardiomyopathy. The arrhythmia may be transient in younger children, but when it persists in older children, it should be considered permanent. Fortunately, if the tachycardia can be terminated, cardiac function returns to normal. When it appears in adults, the continuous variety manifests milder symptoms.

Atrial tachycardias may have an automatic, triggered, or microreentrant mechanism. Although precisely defining the basic mechanism of a particular atrial tachycardia may be difficult, understanding their basic principles may help with choosing therapy.

Treatment

Pharmacologic Therapy

Although there are no large-scale trials in the medical treatment of atrial tachycardias, reported data show that β-blockers and calcium channel blockers are at least partially effective, particularly if the underlying mechanism of the tachycardia is abnormal automaticity or triggered activity. Because of their safety profile, these drugs are usually first-line medical therapy.

Other antiarrhythmic drugs may be effective in treating some patients with atrial tachycardias. However, there are no large-scale trials comparing the drugs or even trials comparing drugs to placebo. Therefore, drug therapy is largely empiric, and drug choice is determined more by side-effect profile and risk of proarrhythmia than by suspected efficacy. The use of class Ic antiarrhythmic drugs may be somewhat successful. Flecainide and propafenone are often well tolerated in patients without structural heart disease and thus can be considered a reasonable first-line antiarrhythmic therapy. Quinidine and procainamide are less well tolerated. Class Ib agents are generally not effective for atrial tachycardias; however, there may be a small subset of lidocaine-sensitive atrial tachycardias in which mexiletine may be effective. Sotalol may also be effective, in part because of its inherent β-blocker (class II) properties. It is generally better tolerated than quinidine and may provide rate control during recurrences. Nevertheless, because sotalol also has class III properties, it will prolong the QT interval and may predispose patients to torsades de pointes, similar to quinidine and procainamide. Amiodarone may be effective, especially in resistant tachycardias. In addition, it is the least proarrhythmic and is generally used as first-line drug therapy in patients with depressed left ventricular function. Newer class III drugs, such as dofetilide, may be effective for atrial tachycardias, but there are few data about their use in atrial arrhythmias, except atrial fibrillation and atrial flutter.

Ablation

Ablation for atrial tachycardias has been proven safe and effective, with reported success rates between 77% and 100%. It also has been shown to improve patient quality of life scores. Therefore, ablation should be indicated for all symptomatic patients who have persistent symptoms despite medical therapy or intolerable side effects from medicines. Furthermore, patients who are not willing to undergo medical therapy should also be considered. Recently, the use of electromagnetic and noncontact mapping systems has significantly improved ablative therapy by decreasing fluoroscopic time, mapping time, and number of radiofrequency applications, thereby increasing efficacy.

Essentials of Diagnosis

• Onset and termination: sudden.
• Heart rate: usually 120–200 bpm but can be faster; neck pulsations correspond to heart rate.
• P waves: retrograde P waves; P waves not visible in 90% of cases.
• R-P relationship: short, if P waves visible.

General Considerations

Atrioventricular nodal reentrant tachycardia (AVNRT) is more common in women than in men. Heart rates usually fall in the range of 120–200 bpm, although rates up to 250 bpm have been recorded. Palpitations are almost universally reported. A feeling of diuresis, noted with other supraventricular arrhythmias, is significantly more common in AVNRT and has been correlated with elevated right atrial pressures and elevated atrial natriuretic peptide. Neck pulsations are common (Brugada phenomenon) and are secondary to simultaneous contraction of the atria and ventricles against closed mitral and tricuspid valves. Dizziness and lightheadedness can occur, but frank syncope is very unusual. Sudden death has been reported but is extremely rare. Although symptoms may occur at any age, the distribution of when the tachycardia and symptoms commonly start appears to be bimodal. The initial episode may begin during the second decade of life, only to disappear and then reappear during the fourth and fifth decades of life.

Pathophysiology

Conduction from the right atrium to the ventricles is normally over a singular AV nodal pathway, with no route of reentry back into the atrium. In persons with dual AV nodal pathways, an atrial impulse may travel antegrade from the atrium over one limb of the AV node and then back to the atrium over another limb in a retrograde fashion. When only one cycle occurs, a single echo beat in the form of a retrograde P wave may be seen on the ECG. If this echo beat can again penetrate the AV node antegrade, the cycle can perpetuate itself, leading to AVNRT. It is estimated that dual AV nodal pathways exist in up to one-fourth of the population, but only a fraction of these people ever manifest a tachycardia.

If each limb of the circuit conducted impulses equally, echo beats and sustained AVNRT would not occur. An atrial impulse traveling down both limbs at the same speed would cause each limb to be refractory when that impulse reached the bottom of the node, preventing the impulse in each limb from going back up the other. Instead, the limbs have varying conduction speeds and refractory periods. The faster conducting limb, called the β or fast pathway, has a longer refractory period, whereas the reverse is true for the second limb, called the α or slow pathway. A premature atrial or ventricular depolarization is often the trigger of AVNRT. The typical AVNRT circuit occurs with a self-perpetuating impulse that travels down the slow pathway and up the fast pathway.

Conduction up the fast pathway is usually so rapid that retrograde atrial depolarization is simultaneous or almost simultaneous with antegrade ventricular activation. This causes the low-amplitude P wave to become obscured in the much higher amplitude QRS complex. Therefore, the P wave is not visible 50–60% of the time. In 20–30% of cases, the P wave distorts the terminal portion of the QRS causing a pseudo-S wave in the inferior leads and a pseudo-R' in lead V1, and in approximately 10% of cases, the P wave distorts the QRS complex (Figure 11–4). Since the P wave usually occurs simultaneous to or just after the QRS, the common variety of AVNRT is a short R-P tachycardia.

AVNRT, once initiated, can perpetuate itself without the participation of either the atria or ventricles. Therefore, on rare occasions, the tachycardia can occur with a 2:1 block in either direction, leading to two atrial depolarizations for every ventricular depolarization or two ventricular depolarizations for every atrial depolarization.

Prevention

Prevention of AVNRT is directed at slowing or blocking conduction in either the fast or slow pathway. Typical AV nodal blocking agents such as β-blockers, calcium channel blockers, and digoxin are most effective on the antegrade slow pathway. The more potent class Ia antiarrhythmic drugs may be necessary to inhibit conduction in the retrograde fast pathway. Class Ic drugs and amiodarone and sotalol (class III) affect both pathways.

Treatment

Vagal Maneuvers

Vagal maneuvers should be considered, barring any contraindications, before embarking on medical therapy. The Valsalva maneuver and carotid sinus massage can immediately terminate the tachycardia by increasing refractory time in the AV node. In patients over 50 years old, bruits should be ruled out before carotid sinus massage to avoid embolic stroke. Medications may render the arrhythmia more susceptible to termination with vagal maneuvers. Therefore, these can be attempted immediately after each round of drug therapy if the tachycardia persists.

Pharmacologic Therapy

Terminating an acute episode of AVNRT in the hospital setting has been made simpler with the availability of intravenous adenosine. Adenosine temporarily stops conduction in the AV node, thereby terminating the reentrant circuit in AVNRT. It is fast acting, typically reaching and acting on the AV node within seconds of administration. With a clinical half-life of 10 seconds, commonly reported sensations such as breathlessness, chest heaviness, and flushing disappear quickly. The routine dosage is 6 mg followed by up to two more boluses of 12 mg. If adenosine is ineffective, intravenous verapamil can be used, but it takes longer to act. Diltiazem, given as intravenous bolus, can also be effective in aborting the tachycardia. Hypotension occurs with about a 10% incidence and is usually rapidly reversed with fluid administration. Although intravenous β-blockers, procainamide, and digoxin are other second-line choices, they can be advantageous in recurrent cases because of their slower clearance from the body.

Catheter Ablation

Radiofrequency lesions delivered via an ablation catheter can be directed at either the fast or slow pathway; the latter is preferred since it is associated with a lower risk of complete heart block.

The inferior origin of the slow pathway is variable within the triangle of Koch, but it is usually anterior and superior to (and sometimes within) the os of the coronary sinus. These anatomic landmarks and gross intracardiac electrogram patterns can be used to position the ablation catheter for successful modification of the AV node. The fast pathway is left unaltered and can still be used for transmission of sinus impulses to the ventricles. Long-term freedom from recurrent AVNRT is about 95%, and the risk of complete heart block as a result of inadvertent fast pathway or nodal damage is 0–5%.

A relatively new form of ablation is cryoablation, using a supercooling catheter, which can reversibly or permanently damage endocardial tissue. This novel approach allows for testing the acceptability of a particular ablation site. Should heart block occur prior to irreversible tissue damage, then rewarming can be performed with no untoward effects.

Essentials of Diagnosis

• Onset and termination: gradual.
• Heart rate: 70–120 bpm.
• P waves: retrograde.
• R-P relationship: short, if P waves visible.

General Considerations

Unlike AVNRT, which presents with recurrent tachycardia episodes of sudden onset, junctional tachycardia, which is sometimes also called nonparoxysmal junctional tachycardia (NPJT), usually starts gradually and sometimes almost imperceptibly. It is easily differentiated from AVNRT by a heart rate between 70 and 120 bpm, gradual onset and termination, and lack of termination with vagal maneuvers. Although heart rates of less than 100 bpm can be seen, it is, nevertheless, a tachycardia because the rates are faster than the 40–60 bpm seen with a junctional escape rhythm.

Clinical Findings

The heart rate at onset is just slightly faster than that of the rhythm preceding it, with gradual acceleration until the final rate is achieved. AV dissociation is common, occurring in 85% of cases caused by digoxin (see following discussion). When the conduction to the atria is intact, retrograde P waves may appear immediately before or after the QRS complex, or they may be obscured within the QRS. Discharge from the AV node is regular, but if antegrade second-degree AV block (almost always the result of digoxin excess), exit block, or atrial capture beats coexist with junctional tachycardia, the rhythm will appear irregular. Enhanced vagal tone or vagolytic agents will either slow down or speed up the arrhythmia, respectively.

Usually seen in the setting of organic heart disease, the cause of this rhythm is almost always identifiable. At one time, digoxin excess accounted for up to 85% of cases of junctional tachycardia. Awareness of the drug's side effects and the availability of other AV nodal blocking agents have diminished the incidence. Nevertheless, in patients being treated with digoxin for atrial fibrillation, clinicians should suspect this arrhythmia when the ECG demonstrates a regularized ventricular response. Acute inferior infarction accounts for 20% of junctional tachycardias, and this rhythm may be present in up to 10% of all infarcts in this location, with onset usually within the first 24 hours and disappearance in several days. Junctional tachycardia may follow open heart surgery (valve replacement more often than bypass surgery), or it can be caused by myocarditis (especially rheumatic) and, rarely, congenital heart disease. In all cases, the tachycardia resolves along with the acute underlying event or with digoxin withdrawal.

Treatment

Treatment is usually directed at the underlying causative factor. Because the rhythm rarely causes deleterious hemodynamic effects, treatment of the rhythm itself is usually not indicated. If digoxin toxicity is the cause, it should be withdrawn. If digoxin is not the cause, digoxin, β-blockers, or calcium channel blockers can be used to slow down the rate if necessary. Catheter ablation can also be performed, but it carries a risk of complete heart block since the origin of the arrhythmia is near the AV node. However, cryoablation has been used in this setting and may be safer.

Essentials of Diagnosis

• Onset and termination: sudden.
• Heart rate: 140–250 bpm.
• P wave: ectopic.
• R-P relationship: short.
• Delta wave on baseline ECG if bypass tract conducts antegrade.

Bypass Tracts & the Wolff-Parkinson-White Syndrome

General Considerations

Congenital bands of tissue that can conduct impulses but lie outside the normal conduction system are called accessory pathways, or bypass tracts. These pathways are responsible for a variety of mechanistically distinct tachycardias by providing preferential conduction between different areas within the heart.

Epidemiology

Accessory pathways are quite prevalent in the general population, with a 2:1 male-to-female predominance. The presence of a bypass tract, however, does not mean that a tachyarrhythmia is a certainty because less than half of persons with documented bypass tracts ever sustain an arrhythmia. The actual number depends on the population studied and varies from 13% in a healthy outpatient population to 80% in the hospital setting.

Approximately 5–10% of patients with documented bypass tracts have concomitant structural heart disease. Ebstein anomaly is the most common, accounting for 25–50% of the anomalies in this group. Of patients with Ebstein anomaly, 8–10% have coexistent bypass tracts, mostly on the right side. The association of right-sided accessory pathways with structural heart disease is strong: 45% of patients with right-sided (and only 5% of those with left-sided) pathways display some type of heart disease.

A familial tendency toward bypass tracts has been seen in some instances, with a fourfold to tenfold increase in incidence among first-degree family members.

Pathophysiology

Anatomy

Anatomically, the atria and ventricles are in apposition, separated by an invagination known as the AV groove. Paroxysmal tachycardias mediated by accessory pathways that cross the groove and electrically link the atria and ventricles, when combined with a short P-R interval (< 0.12 seconds), a wide QRS, and secondary repolarization abnormalities, define the Wolff-Parkinson-White syndrome. When this ECG pattern is seen without the tachycardia, it is called Wolff-Parkinson-White pattern or ventricular preexcitation.

Although the most common site of insertion is between the lateral aspect of the left atrium and left ventricular myocardium, pathways can cross the AV groove anywhere in its course (except the region between the aortic and mitral valves) to connect the left or right atrium to its respective ventricle (Figure 11–5). In noting the distribution of accessory pathways, 46–60% are located in the left free wall, 25% within the posteroseptal space, 13–21% in the right free wall, and 2% in the anteroseptal space. Females have right-sided accessory pathways and Asians have right anterior accessory pathways more frequently than other groups, suggesting that pathogenesis of these pathways has a genetic component. Each location produces a distinct ECG pattern (Figure 11–6), but in the 13% of patients with two or more bypass tracts, the ECG tracing can be confounding and show multiple QRS morphologies.

Cardiac Electrical Conduction

Unlike the AV node, whose function is to delay atrial impulses en route to the ventricles, most bypass tracts conduct rapidly and without delay, which accounts for the short P-R interval often seen in sinus rhythm in these patients.

Impulses that reach the ventricles over a bypass tract spread through cell-to-cell conduction within the myocardium, activating the ventricles in series rather than in parallel. This relatively slow process is manifested as a wide QRS complex.

Sinus impulses are not restricted to using the AV node or the bypass tract only to reach the lower chambers. Instead, both may contribute to ventricular activation. This produces a QRS that is initially wide, reflecting conduction over the bypass tract, with the latter portion of the QRS appearing normal and narrow, indicating that the remainder of the ventricle has been depolarized via the normal conduction system (the AV node and His-Purkinje system). The initial slurred upstroke of the QRS, a delta wave, indicates ventricular preexcitation, which can be defined as ventricular depolarization that begins earlier than would be expected by conduction over the AV node alone. The degree of preexcitation and P-R shortening depends on the proportion of ventricular activation occurring over the AV node and the bypass tract. This, in turn, is related to two factors. The first is the conduction velocity of the bypass tract relative to the AV node. The faster the bypass tract can conduct impulses to the ventricles in relation to the AV node, the earlier the ventricle will preexcite, and vice versa. The second factor is the location of the tract, and more specifically, its proximity to the sinus node and AV node. A sinus impulse will encounter a right-sided free-wall bypass tract earlier than it will the AV node, and this favors a short P-R interval with a high degree of ventricular preexcitation (Figure 11–7A). On the other hand, a sinus beat will encounter the AV node early in its course while traveling to a pathway in the lateral left atrium, allowing ventricular activation to occur primarily by the normal conduction system. A narrow, minimally (if at all) preexcited QRS complex with a normal or near-normal P-R interval may be seen (Figure 11–7B). Changes in autonomic tone, by modifying the conduction velocity and refractoriness over both the pathway and the AV node, can produce varying degrees of preexcitation at different times in the same patient (Figure 11–7C).

If the delta wave axis of a maximally preexcited beat is discordant from the accompanying preexcited QRS axis, or if more than one preexcited QRS morphology is noted, there may be multiple bypass tracts.

Mechanism

Atrioventricular Reciprocating Tachycardia

An inherent property of accessory pathways is their ability to conduct in a retrograde direction more easily than antegrade. The AV node, on the other hand, conducts more efficiently antegrade. For this reason, reentrant rhythms in this setting most commonly use the AV node to go from atrium to ventricle and the bypass tract to return to the atrium. Orthodromic AVRT (antegrade conduction over the AV node) accounts for 70–80% of arrhythmias in patients with AV bypass tracts, with heart rates of 140–250 bpm (Figure 11–8). Antidromic AVRT, in which the atrial impulse is carried to the ventricle over the bypass tract and reenters the atrium via retrograde conduction over the AV node, is rare, occurring in approximately 5–10% of cases. Because conduction to the ventricles during orthodromic AVRT occurs over the normal conduction system, the QRS is narrow, unless bundle branch aberrancy is present. During antidromic AVRT, the QRS is wide and maximally preexcited as a result of the complete lack of AV nodal contribution to ventricular depolarization. When two or more bypass tracts are present, each tract may act as the antegrade or retrograde limb (or both), especially with involvement of the AV node. There is a higher incidence of ventricular fibrillation in patients with multiple accessory pathways. Additionally, multiple pathways are more common in patients with antidromic SVT and in patients with Ebstein anomaly.

Tachycardia is usually initiated by a premature atrial or ventricular beat. In orthodromic tachycardia, a premature atrial beat conducts down the AV node to depolarize the ventricle, and the bypass tract carries the impulse back to the atrium (Figure 11–9). A ventricular premature beat finding the AV node refractory might initiate an identical tachycardia by first conducting up the bypass tract to the atrium. Antidromic tachycardia initiates in an identical fashion but with a reversed direction of conduction.

Atrial fibrillation accounts for only 19–38% of arrhythmias in the population with accessory pathways, but it is potentially more lethal than the reciprocating tachycardias discussed earlier. It is more common in patients with antegrade conducting accessory pathways and in pathways with a short antegrade refractory period. By virtue of their short refractory periods, bypass tracts (unlike the AV node) have the potential to conduct very rapidly to the ventricles at ventricular rates of 250–350 bpm (Figure 11–10) with the possibility of degeneration to ventricular fibrillation. A reputed marker for sudden death in patients with atrial fibrillation is a shortest preexcited R-R interval of ≤ 250 ms (corresponding to a heart rate of ≥ 240 bpm) between two fully preexcited beats. The finding of a short R-R interval actually has a low positive predictive value, however, because sudden death in this syndrome is still rare.

During atrial fibrillation, the ECG reveals an irregularly irregular rhythm with QRS complexes of varying morphologies, representing conduction to the ventricles via the AV node (normally conducted narrow complexes), the bypass tract (wide, preexcited complexes), and both (fusion beats, harboring elements of both the normally conducted and preexcited beats). In this setting, the bypass tract may be called a bystander since it is not integral to the tachycardia. Patients with AV bypass tracts have a higher incidence of atrial fibrillation than does the general population, possibly because of the degeneration of reentrant tachycardia or of microreentry within the atrial portion of the bypass tract. It has been shown that ablation of the bypass tract can frequently also eliminate atrial fibrillation.

Concealed Bypass Tracts

Between 15% and 50% of patients with no evidence of preexcitation during sinus rhythm are found to have bypass tracts that conduct only in the retrograde direction. By definition, concealed bypass tracts (their presence cannot be detected by standard ECG) do not display delta waves on the ECG during sinus rhythm, but they can still support an orthodromic AVRT and account for about 30% of orthodromic tachycardias.

Differentiating orthodromic AVRT from AVNRT on the ECG can be difficult. The incidence of both tachycardias being operative at different times in the same person is reported to be between 1.7% and 7%. Therefore, although the presence of a delta wave on the nontachycardiac tracing makes it statistically unlikely that AVNRT was the documented tachycardia, it does not exclude the possibility completely.

Because of the simultaneous atrial and ventricular activation that occurs during AVNRT, the P waves formed as a result of retrograde conduction to the atrium are usually obscured by or merged with the QRS complex. Likewise, because of the short retrograde conduction time via the bypass tract, orthodromic AVRT usually is a short R-P tachycardia, albeit the R-P interval is somewhat longer than with AVNRT (see Figure 11–8). In AVRT, the P waves are usually located within the ST segment and are not obscured or merged with the QRS.

Treatment

Vagal Maneuvers

The management of orthodromic AVRT or antidromic AVRT is similar to that of AVNRT. Since the AV node is an integral part of the reentry circuit in AVRT, blocking the AV node terminates the tachycardia. Therefore, vagal maneuvers such as the Valsalva maneuver or carotid sinus massage can be tried first.

Pharmacologic Therapy

Intravenous adenosine almost always terminates these tachycardias. Other AV nodal blocking agents such as β-blockers, calcium channel blockers, and digoxin can also be helpful. Just as in AVNRT, vagal maneuvers can be retried after each dose of these longer acting AV nodal blocking agents.

In contrast to orthodromic or antidromic AVRT, patients with atrial fibrillation or atrial flutter and a bystander bypass tract should not be given AV nodal blocking agents. This point is crucial since these types of medicines can precipitate ventricular fibrillation. Blocking the AV node enhances conduction down the bypass tract, making all the complexes wide. Furthermore, the bypass tract can often conduct faster than the AV node, increasing ventricular rates to sometimes over 200 bpm. The aberrant conduction and rapid rate can lead to disorganized ventricular conduction, resulting in ventricular fibrillation. The drug of choice for patients with atrial fibrillation or atrial flutter and a bystander bypass tract is intravenous procainamide or another drug that preferentially blocks the bypass tract. This shunts more conduction through the AV node, which narrows the QRS complexes and often slows down the overall ventricular rate.

Asymptomatic patients showing delta waves on the ECG generally do not require treatment unless they are involved in a high-risk profession such as commercial pilots, police officers, and firefighters. Patients with occasional or rare bouts of minimal or mildly symptomatic palpitations from orthodromic or antidromic AVRT can often be safely treated with such agents as β-blockers or calcium channel blockers to prevent recurrent episodes. However, due to the potential of ventricular fibrillation, these AV node blocking agents should be used with great caution or not at all in patients who have also demonstrated atrial fibrillation or atrial flutter.

Radiofrequency Catheter Ablation Therapy

Patients who experience significant symptoms such as dizziness, presyncope, or syncope should undergo an electrophysiologic study with concomitant radiofrequency ablation. In addition, patients with frequent symptoms who do not respond to or who wish to avoid drug therapy can also undergo ablative therapy. Recent guidelines indicate that ablation can be considered first-line therapy for patients with symptomatic Wolff-Parkinson-White syndrome.

The right internal jugular or femoral vein ablation approach is used for accessory pathways located on the right side of the heart. Left-sided pathways can be approached from the left ventricle with the retrograde technique or transseptally from within the left atrium using the Brockenbrough technique. A steerable catheter is moved around the mitral or tricuspid annulus until the site of shortest impulse transit between the atrium and ventricle is found. This mapping process localizes the bypass tract. Frequently, an impulse can be recorded directly from the bypass tract, further confirming its localization. Once identified, radiofrequency energy delivered to the tract through the mapping catheter permanently destroys the tract and prevents further transmission of electric impulses over it.

Given its curative potential, a high success rate (95% in experienced hands, even with multiple bypass tracts), and a low complication rate, radiofrequency catheter ablation is now a very common treatment for accessory pathways (Table 11–2). As with other supraventricular arrhythmias, new mapping systems have been developed that decrease fluoroscopy and procedure time as well as allowing the operator to return to specific locations if needed.

Surgical Ablation Therapy

In rare instances, patients will have multiple pathways or pathways that are inaccessible to an ablation catheter. These patients may undergo surgical division of their tracts.

Other Bypass Tracts

A variety of bypass tracts other than AV tracts (Kent fibers) also exist. Atriohisian fibers connecting the atrium to the His bundle have been demonstrated. This led to the description of the Lown-Ganong-Levine (LGL) syndrome, which refers to those patients with a short P-R interval, normal QRS, and recurrent SVT. However, current data suggest that LGL syndrome does not truly exist since atriohisian pathways have not been shown to support any type of reentry tachycardia.

Atriofascicular fibers, which are also known as Mahaim fibers, typically run from the lateral right atrium to the right bundle branch. The tract is capable of antegrade conduction only, and therefore, only antidromic AVRT is possible. Because the antegrade reentrant circuit engages the right bundle branch, the tachycardia QRS complex typically has a left bundle branch block pattern. Treatment considerations are the same as for Wolff-Parkinson-White syndrome.

With the exception of the AV node, the refractory period of cardiac tissue for a given beat is directly related to the interval between that beat and the preceding beat; the slower the heart rate, the longer is the recovery period with each beat. Furthermore, because the stability of refractoriness depends on the stability of the heart rate, a beat-to-beat change in refractoriness accompanies variability in the heart rate.

When an early supraventricular beat occurs in the midst of a regular rhythm, the tissues do not have a chance to shorten (or reset) their refractory periods; the result may be aberrant conduction (a transient functional refractoriness, or block) in one of the bundle branches. The shorter the interval between the early beat and the preceding one, the greater is the probability that the early beat will be aberrant (Ashman phenomenon). Ashman phenomenon is also associated with the irregularity and frequent pauses of atrial fibrillation (Figure 11–11). Aberrancy may continue for a variable period before normal conduction resumes.

Aberrant conduction is often seen at the onset of any SVT that uses the AV node for antegrade conduction. (This excludes tachycardias that conduct antegrade over a bypass tract.) The right bundle branch, with its longer refractory period, is more subject to block than is the left, but occasionally the left bundle becomes refractory earlier. Once the tachycardia has been established and refractory periods stabilized, this normal functional aberrancy may give way to normal conduction, with resumption of a narrow QRS.

Deciding whether a wide-complex tachycardia is supraventricular or ventricular in origin is often difficult. Only when AV dissociation or capture or fusion beats are present can a ventricular rhythm be diagnosed with certainty. The low sensitivities of these findings (20% for AV dissociation) do not provide a firm diagnosis in more than a minority of wide-complex tachycardias, however.

Classic criteria that examine V1 when a right bundle branch block pattern is present, the QRS width and axis, the presence of positive or negative concordance across the precordium, and various combinations of QRS patterns in different leads may support a diagnosis, but they have been found lacking in diagnostic power because of their low sensitivity, low specificity, or both. A proposed algorithm separates supraventricular from ventricular tachycardia with 99% sensitivity and 97% specificity. However, this algorithm, which is also known as the “Brugada criteria,” has had lower sensitivity and specificity in recent studies.

Sinus Node Arrhythmia

Essentials of Diagnosis

• Cyclic heart rate variation with respiration.
• P-P interval variability ≥ 160 ms or 10%.
• P-wave morphology identical to normal sinus rhythm P wave.

General Considerations

A cyclic increase and decrease in heart rate normally accompany inspiration and expiration, respectively, and the irregularity in rhythm (mediated by vagal tone) is often imperceptible. More marked degrees of rate excursion can occur, especially at slower heart rates, but these are not considered to be sinus arrhythmia unless the shortest and longest P-P interval varies by 0.16 seconds or more, or by 10% or more. This respiratory form of sinus arrhythmia is common in younger people. It becomes less prevalent with increasing age and in conditions associated with autonomic dysfunction, such as diabetes mellitus. Enhancement of vagal tone with agents such as digoxin and morphine may also cause sinus arrhythmia.

Treatment

Because of its benign nature, no treatment is required for sinus arrhythmia.

Wandering Atrial Pacemaker

Essentials of Diagnosis

• Progressive cyclic alteration in P wave morphology.
• Heart rate: 60–100 bpm.

General Considerations

The presence of more than one pacemaker within the atria (which may or may not include the sinus node) causes variation in the P-P interval, P-wave morphology, and P-R interval. The heart rate remains within the normal range.

There is controversy over the cause of this rhythm. Some authorities believe that wandering atrial pacemaker and MAT are the same rhythm artificially separated by heart rate and that both are attributable to underlying pulmonary disease. Others believe that it is an exaggerated form of a respiratory sinus arrhythmia, with the uncovering of latent atrial and sinus node pacemakers when the primary sinus node pacemaker cycles to a slow rate with expiration.

The significance ascribed to a wandering atrial pacemaker should probably be interpreted in the setting in which it is seen. In those with lung disease, it may simply be a reflection of that process. In the elderly, it may suggest sinus node disease or sick sinus syndrome, and in the young and athletic heart, it may represent heightened vagal tone.

Treatment

The rhythm itself is usually benign and typically requires no intervention. If the rhythm is secondary, treating the underlying etiology may be warranted.