Arrhythmias

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[edit] Arrhythmias

David T. Martin

The term arrhythmia is used to describe any disorder of cardiac impulse formation or conduction, whether it be for a single beat or sustained for minutes, days, or decades. A healthy adult heart beats approximately 100,000 times a day, and the proportion of irregular or premature beats increases gradually with increasing age. This chapter will describe the general and specific mechanisms of arrhythmias, their clinical presentation and evaluation, and the range of therapies currently available. In addition, this chapter will provide guidelines for specialist referral.

[edit] PATHOPHYSIOLOGY

Cardiac myocytes are excitable cells and have intrinsic pacemaker activity. The rate of depolarization is dependent on the location and autonomic, pharmacologic, and pathologic state of these cells within the heart; in the normal heart the sinus node drives cardiac activation because these cells depolarize more rapidly than those cells in subsidiary pacemakers such as the atrioventricular (AV) node or intraventricular conduction (His-Purkinje) system. Enhanced automaticity of subsidiary pacemaker tissue causes premature beats and other arrhythmias (Fig. 61-1); this mechanism is probably responsible for atrial and ventricular premature beats and therefore is the most common arrhythmia mechanism in clinical practice, being responsible both for single premature beats and for the initiation of many sustained tachycardias. Reentry is an arrhythmia mechanism of both healthy and diseased cardiac cells; it requires unidirectional conduction block (usually caused by a premature beat) and an anatomic barrier that permits the formation of a circuit for the wavefront of cardiac activation to become self-perpetuating. Almost all sustained tachycardias observed in clinical practice are mediated by a reentrant mechanism; there may be a single (macro-reentrant) wavefront of activation as in ventricular tachycardia, atrial flutter, or those tachycardias associated with accessory pathways, or there may be multiple wavefronts as in atrial or ventricular fibrillation. Finally, disorders of action potential repolarization leading to “triggered” automaticity are responsible for arrhythmias associated with the congenital or acquired long QT syndrome, quinidine use, and digoxin toxicity.^[1]

Figure 61-1 Arrhythmia mechanisms. Panel I, Enhanced automaticity. This shows a normal rate of depolarization in phase 4 of the action potential in the upper portion. In contrast, the lower portion shows inadequate repolarization of the action potential during phase 3, which, given that the rate of depolarization in phase 4 is unchanged, leads to more rapid heart rates due to more rapid achievement of threshold voltage for cellular depolarization. Panel II, Triggered activity. This shows abnormal membrane depolarization during phase 3 of the action potential, which causes rapid and irregular heart rates when these brief oscillations in voltage reach cellular threshold. Panel III, Reentry. This is the most common clinical mechanism of tachycardia, and both an anatomic (or functional) barrier and unidirectional block are required to initiate and sustain such arrhythmias. (a) Cardiac impulse conducts via pathway β since the α pathway has a longer conduction time. (b) A premature beat finds the β pathway blocked (due to refractoriness) and therefore conducts down the slower α pathway, which has a shorter refractory period. Retrograde penetration of this impulse into the β pathway renders it refractory for antegrade conduction and (c) allows for continuous reentry of the impulse down the α pathway and up the β pathway.

[edit] CLINICAL PRESENTATION AND PATIENT EVALUATION

The various manifestations of arrhythmias are protean and often highly challenging for the physician (Box 61-1). Patients with the potential for life-threatening arrhythmias may have no symptoms; conversely, patients with benign arrhythmias caused by atrial premature beats may be disabled by palpitations. In general the degree of concern about the patient's symptoms or arrhythmia is more dependent on the context in which the symptoms occur than on the phenomenology of the symptoms themselves.^[2] The presence or absence of heart disease is often the key determinant of the pace and location of the evaluation of the arrhythmia patient; prompt hospitalization is essential for the patient with a history of heart failure who presents with near-syncope, but would be inappropriate for a patient without clinical evidence of heart disease presenting with a hemodynamically stable episode of atrial fibrillation.

[edit] History

The specific history of palpitations is often revealing; the patient should be asked to indicate the rate and regularity of the heart during symptoms, as well as any subjectively apparent provoking or relieving characteristics. Are episodes abrupt in onset? Are they more likely to occur during exercise or rest? Are they affected by vagal maneuvers? Associated symptoms during episodes of palpitations may also be helpful in determining the mechanism of tachycardia: the sensation of throbbing in the neck during regular tachycardia has been associated with AV nodal reentry presumably because of the simultaneous atrial and ventricular activation during this arrhythmia leading to atrial contraction against a closed tricuspid valve. A careful medical history for evidence of cardiac disease (particularly coronary artery disease with left ventricular dysfunction, which predisposes to ventricular tachycardia), pulmonary disease (which predisposes to atrial fibrillation, atrial flutter, and atrial tachycardia), and thyroid disease (which predisposes to atrial fibrillation) is essential. A family history of sudden cardiac death should raise concern about hypertrophic cardiomyopathy or long QT syndrome, but is also relevant in the patient with coronary artery disease.

Use of medications and nonprescription remedies that may be arrhythmogenic is also an important component of the history (Table 61-1); similarly, alcohol and other recreational substance consumption, particularly cocaine, may predispose to both cardiac structural disease and arrhythmia. Alcohol is a well-recognized cause of atrial fibrillation, even in the absence of an associated cardiomyopathy, and long-term abstinence may be sufficient to eliminate recurrences of this arrhythmia.

Table 61-1 Potentially Arrhythmogenic Drugs

✢Torsades de pointes is a polymorphic ventricular tachycardia associated with prolonged QT interval that usually occurs in the context of an underlying bradycardia.

[edit] Physical Examination

Although such opportunities are rare, the physical examination may be most helpful during the arrhythmia: a tachycardia is present when the heart rate exceeds 100 beats per minute. Marked irregularity suggests atrial fibrillation. Inspection of the venous pulse may reveal intermittent cannon waves indicative of AV dissociation (most commonly caused by ventricular tachycardia). The neck veins may exhibit regular rapid flutter waves, which are suggestive of atrial flutter, particularly if the heart rate is approximately 150 beats per minute.

[edit] DIAGNOSIS

It is a helpful exercise for the evaluating physician to classify each patient after obtaining the history, physical examination, and surface electrocardiogram (ECG) into one of three clinical categories:

• Definite arrhythmia: The symptoms and ECG are recorded together and the diagnosis is not in doubt; all that remains is for appropriate therapy to be selected.
  
• Potential arrhythmia: Specific symptoms of arrhythmia are not present, but significant structural heart disease or other arrhythmia substrates such as Wolff-Parkinson-White pattern on the ECG raise the question of arrhythmia potential. Such patients may be at high risk despite the apparent absence of symptoms, and ambulatory (Holter) monitoring or electrophysiologic evaluation may be advisable.
  
• Possible arrhythmia: The patient may have symptoms suggestive of arrhythmia but will require further investigation to correlate the symptom(s) with the cardiac rhythm. The evaluation of such patients is often expedited by ambulatory patient-activated event monitoring.
  

[edit] Diagnostic Procedures

For the last hundred years the gold standard in arrhythmia diagnosis has been the 12-lead ECG, and despite the advent of intracardiac electrophysiologic recording techniques, this simple noninvasive tool remains central to the investigation of arrhythmias. The key features of the ECG that are relevant in arrhythmia interpretation are given in Box 61-2. In many patients it may not be possible to obtain an ECG recording of all 12 leads during an arrhythmia; this difficulty in capturing brief episodes of arrhythmia led to the development of monitoring techniques, including Holter monitoring in which the patient wears the recording unit attached to skin electrodes for at least a 24-hour period and keeps a diary of symptoms during that time. The recording is analyzed later and provides a quantitative evaluation of the cardiac rhythm during the recording period. This quantitative report is useful in defining the frequency and grade of atrial (particularly atrial fibrillation) and ventricular arrhythmia, which may be asymptomatic. However, because this technique does not easily allow for correlation of symptoms with cardiac rhythm, and is likely to yield little in the patient with infrequent symptoms, this test has in many cases been superseded by event monitoring in which the patient wears the monitoring equipment for 30 days (changing skin electrodes daily) and activates the device only when symptoms are present; the monitor records continuously for a programmable period prior to patient activation (continuous loop), as well as for some minutes after activation. The captured rhythm is then transmitted transtelephonically by the patient to a central monitoring station where the technician is able to provide immediate advice to the patient as well as notify the physician if necessary.^[3]

Other noninvasive techniques have recently been developed for evaluation of arrhythmia risk, particularly regarding ventricular tachycardias and sudden death. The earliest of these to gain widespread use in clinical practice was the signal-averaged electrocardiogram; this technique records, averages, and amplifies a few hundred beats of a resting ECG, with the focus on the terminal portion of the QRS complex where “late potentials” may be found. This subtle widening of the QRS complex represents the slowed conduction of the cardiac impulse that occurs around the border zone of the scar that is present after myocardial infarction. The presence of a late potential indicates that the substrate for ventricular tachycardia exists. However, it does not necessarily suggest that ventricular tachycardia will actually occur clinically, and studies of this technique indicate that the sensitivity for ventricular tachycardia is quite low. The specificity, however, is high, suggesting that a patient with a negative test after myocardial infarction would not be expected to have inducible ventricular tachycardia at electrophysiologic study.^[4] It should be noted that signal averaging of the ECG cannot be interpreted in the presence of an intraventricular conduction defect or bundle branch block, significantly limiting its clinical application.

Prediction of arrhythmic events in patients with heart disease (usually after myocardial infarction) has been studied using other noninvasive techniques including heart rate variability analysis and T wave alternans testing.^[5][6] These tests provide information about autonomic function in the heart and electrical stability regarding the risk of ventricular fibrillation, respectively, but neither has gained wide currency in the United States. All of the noninvasive tests used for risk stratification regarding sudden death have the significant limitation of relatively low sensitivity; integrating such test data with the clinical context and most importantly with the ejection fraction provides more clinically useful information to aid decision making in patients who may be candidates for prophylactic defibrillator implantation.

[edit] MANAGEMENT

[edit] General Principles

In the last 10 years clinical cardiac electrophysiology has evolved from a predominantly academic enterprise of little practical benefit to patients, to a recognized subspecialty of cardiology in which effective therapies can be offered for management of both atrial and ventricular arrhythmias. Referral for electrophysiologic evaluation and possible definitive therapy should be considered for many patients (Table 61-2).

Table 61-2 Rationale for Electrophysiologic Referral: Common Clinical Scenarios

Management strategies for tachycardias include suppression or rate control using drugs or implantable devices, repeated termination using implantable antitachycardia devices, and ablation (destruction) of the tachycardia substrate using catheter or surgical techniques (Table 61-3).

Table 61-3 Established and Evolving Indications for Radiofrequency Catheter Ablation

The treatment of tachycardias may be considered in two phases: acute management and long-term therapies. The acute termination of any tachycardia in which the patient is hemodynamically unstable should be performed using cardioversion or defibrillation. This procedure requires intimate familiarity with the functioning of the defibrillator and the need to synchronize energy discharge to the QRS complex. Intravenous sedation or anesthesia is often required, and it should never be necessary to deliver a transthoracic shock to a patient who is alert. This procedure is effective for termination of all reentrant arrhythmias, but often needs to be supplemented with agents designed to prevent recurrence.

In the diagnosis and termination of tachycardias in the emergency setting, the use of the endogenous nucleoside adenosine has recently gained popularity because of its favorable safety profile when compared with verapamil; both agents safely terminate narrow QRS complex regular tachycardias about 90% of the time.^[7] Adenosine has a half-life of only a few seconds, and although it may produce complete AV block, such effect is transient and permits safe arrhythmia diagnosis even when a wide complex tachycardia is present; the use of verapamil under such circumstances is absolutely contraindicated because of its hypotensive effects and the risk that a relatively stable ventricular tachycardia will degenerate to ventricular fibrillation.

Intravenous lidocaine is indicated for the acute treatment of the stable patient with ongoing ventricular tachycardia, and intravenous procainamide may be used as a second-line agent for this indication also, although intravenous amiodarone is probably more efficacious. Procainamide has also been used for the acute termination of atrial fibrillation, and in particular for treatment of preexcited atrial fibrillation complicating the Wolff-Parkinson-White syndrome. However, the recently marketed agent, ibutilide, may have more utility for the acute termination of both atrial fibrillation and flutter in the emergency setting; this short half-life agent prolongs action potential repolarization and appears to have about 30% efficacy in restoring sinus rhythm, with a risk of proarrhythmia (polymorphic ventricular tachycardia) of about 2%.^[8]

Catheter ablation using radiofrequency energy has revolutionized the treatment of supraventricular tachycardias and other less common arrhythmias.^[9] In the treatment of ventricular arrhythmias, automatic defibrillator implantation has become the standard of care for patients who have suffered a life-threatening arrhythmic event or who are at high risk for such an event.^[10][11]

[edit] Atrial and Ventricular Premature Beats

Premature beats are ubiquitous; more than 60% of healthy adults undergoing ambulatory (Holter) monitoring for a single 24-hour period exhibit asymptomatic atrial or ventricular premature beats, and because of the unpredictability and inherent variation of such events, it is thought that they are more widely present in the general population. The frequency of both atrial and ventricular premature beats is increased in the presence of heart disease; over 80% of postinfarction patients exhibit ventricular premature beats, and when more than 10 are present per hour the mortality risk is elevated. However, the Cardiac Arrhythmia Suppression Trial (CAST) clearly demonstrated that adequate suppression of such arrhythmia with the class Ic drugs flecainide and encainide does not reduce the risk of death but actually increases it.^[12][13] Therefore specific treatment of premature beats with antiarrhythmic agents is not indicated; the risk that such beats confer in the postinfarct patient is related more to the presence of associated ventricular dysfunction and ongoing ischemia than to the arrhythmia itself.

Atrial premature beats are identified on the ECG by the occurrence of an early P wave, which often has a configuration different from the sinus node P wave. Atrial premature beats may originate in the left or right atrium, and may be multifocal in origin; they usually conduct to the ventricle normally, but very early premature beats may conduct aberrantly (with a widened QRS complex) or not at all. Such blocked atrial premature beats are often misinterpreted as second-degree heart block, and it is important to emphasize that the normal physiology of the AV node in these circumstances is to delay or block conduction in a fashion that is directly related to the degree of prematurity of the early P wave. The presence of AV nodal blocking drugs increases the likelihood that atrial premature beats will be nonconducted.

The management of atrial and ventricular premature beats in asymptomatic patients consists of reassurance alone that no specific treatment is required. In a significant minority of patients premature beats cause disabling symptoms, usually palpitations described as “thumping,” “fluttering,“ or “flip flop“ in the chest. Occasionally a patient complains of a sensation that the heart is stopping. Again, once the symptoms have been correlated with premature beats (since up to 40% of patients with such complaints have no arrhythmia whatsoever during their symptoms), it is important to reassure the patient that these symptoms reflect a benign ubiquitous condition and that elimination of alcohol and adrenergic stimulants may be sufficient to abolish the palpitations. However, if such simple alterations in lifestyle are ineffective, then the use of β-blockers is recommended; these agents do not abolish premature beats but reduce cardiac contractility and thereby eliminate the vigorous postextrasystolic beat that causes the symptom of palpitations.

[edit] Tachycardias

Tachycardia is the occurrence of three or more consecutive complexes faster than 100 beats per minute. If the patient is hemodynamically stable, an attempt should be made to determine the arrhythmia mechanism because this will usually lead to appropriate therapy. Narrow QRS complex tachycardias are supraventricular in origin, and wide QRS complex tachycardias may be either supraventricular or ventricular, although in the absence of data to suggest otherwise, it is best to assume that the arrhythmia is ventricular tachycardia.

The availability of a sinus rhythm ECG for comparison with that recorded during the tachycardia often aids prompt arrhythmia diagnosis. Bedside techniques that facilitate identification of the arrhythmia mechanism include carotid massage and other vagal maneuvers and the administration of intravenous adenosine. Neither intervention should be performed without the ready availability of continuous ECG monitoring and emergency resuscitation equipment, including a defibrillator. Carotid massage should not be performed in patients with neck bruits.

[edit] Sinus Tachycardia.

In the adult, sinus tachycardia rarely exceeds 200 beats per minute and is not considered a primary arrhythmia; it is characterized by gradual increases and decreases in rate, with typical sinus P waves preceding each QRS complex. The PR interval is usually not prolonged, reflecting the enhanced adrenergic tone, and carotid massage has little or no effect on the rate or PR interval. The presence of sinus tachycardia should prompt a search for the underlying cause, on which diagnosis the management should be based. Occasionally an apparent sinus tachycardia is observed to begin and terminate abruptly; this arrhythmia is seen in patients with sinus node disease and is referred to as sinus node reentrant tachycardia. It is exquisitely sensitive to vagal maneuvers, which abruptly terminate it, and it has been successfully treated with verapamil.^[14] Patients with sinus node reentry may also have significant bradycardias due to sinus node disease and may require permanent pacing.

[edit] Paroxysmal Supraventricular Tachycardia.

There are two reentrant mechanisms that account for over 80% of the arrhythmias with this common presentation (Fig. 61-2).^[15] AV nodal reentrant tachycardia (AVNRT) is the most common and is more likely to occur in older patients with paroxysmal supraventricular tachycardia (Fig. 61-3). The ECG during this arrhythmia usually shows a narrow complex tachycardia that may be very rapid (up to 260 beats per minute) and in which P waves are not visible because there is simultaneous atrial and ventricular activation and P waves are buried within the QRS complex. Orthodromic AV reentrant tachycardia (AVRT) is mediated by an accessory pathway and is one of the arrhythmias that occurs in the Wolff-Parkinson-White syndrome. Most patients with this form of supraventricular tachycardia, however, do not evidence the Wolff-Parkinson-White pattern on the ECG during sinus rhythm, suggesting that the pathway conducts only retrogradely. During tachycardia the P waves may or may not be visible, depending on the rate of the arrhythmia and the location of the accessory pathway; however, since most pathways are located laterally on the mitral annulus and the ventriculoatrial conduction time is therefore relatively long, inverted P waves are often seen in the inferior and lateral ECG leads between QRS complexes (Fig. 61-4). Since the AV node participates in the tachycardia circuit in both these mechanisms, the response to vagal maneuvers and adenosine is usually abrupt termination of the arrhythmia.

Figure 61-2 Common mechanisms of paroxysmal supraventricular tachycardia. The left panel shows a reentrant circuit within the region of the atrioventricular (AV) node. Because the circuit is short and conduction is rapid within it, impulses exit to activate atria and ventricles virtually simultaneously, thus producing the typical ECG appearance where the P wave is buried within the QRS complex during such tachycardias. The right panel shows a reentrant circuit involving activation of the ventricles via the normal AV nodal pathway, but retrograde activation of the atria via an accessory pathway, which is often some distance from the AV node. Therefore the reentrant circuit is relatively long, and there is a significant difference in the timing of activation of the atria and ventricles, causing the typical ECG appearance where the P wave is visible between QRS complexes.

Figure 61-3 ECG example of atrioventricular nodal reentrant tachycardia. This ECG shows a regular narrow QRS complex tachycardia at 200 bpm. There is a 1:1 relationship between P waves and QRS complexes; P waves are visible in the terminal portion of the S wave in the inferior leads, particularly lead II.

Figure 61-4 ECG example of atrioventricular reentrant tachycardia. This ECG shows a regular narrow QRS complex tachycardia at 145 bpm. The P waves deform the T waves in the inferior leads and leads I and V1. In addition, there is QRS alternans where the amplitude of the R and S waves varies beat-by-beat in most leads.

Typically, the patient is aware of sudden onset of rapid palpitations with or without concomitant symptoms of chest discomfort, dizziness, or syncope. Episodes may be brief and self-terminating or may last for hours until terminated in an emergency department. The degree to which these symptoms cause anxiety and interfere with lifestyle varies widely and reflects the wider variation in arrhythmia frequency. Where episodes are infrequent and symptoms are well tolerated, treatment may consist merely of advice about how to perform Valsalva's maneuver at home; alternatively drug or ablation therapy is appropriate for many patients with frequent or bothersome symptoms. Commonly used drugs include β-blockers, digoxin, and calcium channel blockers such as verapamil or diltiazem. Drug selection should be empiric, but it is important to note that neither digoxin nor calcium channel blockers should be used if there is any possibility of Wolff-Parkinson-White syndrome, since these agents may enhance anterograde conduction in the accessory pathway and predispose the patient to ventricular fibrillation should atrial fibrillation occur. In general, patients with recurrent tachycardia after a single drug trial should be offered catheter ablation early in the management of this condition, since it has a salutary effect on quality of life and has been shown to be more cost-effective than lifelong drug therapy.^[16][17]

Catheter ablation of these tachycardias is a safe and effective procedure and is advised by many authorities as first-line therapy.^[18] The target site for ablation in AVNRT is the slow pathway, which is in the floor of the right atrium between the orifice of the coronary sinus and the annulus of the tricuspid valve; this area is relatively remote from the compact AV node, and therefore the risk of heart block complicating the procedure is low (1% to 2% at most) compared with the previously used technique targeting the fast pathway following which the risk of heart block was reported to be up to 8%. In AVRT the target for ablation is the accessory pathway itself, either at its atrial or ventricular insertion point around the mitral or tricuspid annulus. The risk of heart block is present in these cases only where the pathway itself is close to the AV node, a rare occurrence. For both arrhythmias ablation is successful in approximately 98% of cases.^[19][20]

[edit] Atrial Fibrillation.

Atrial fibrillation is the most common sustained cardiac arrhythmia (Fig. 61-5).^[21][22] It is a major public health problem in the United States, afflicting approximately 1.7% of the population aged 60 to 64 and up to 12% of those over the age of 75; as the general population ages, and appears to be surviving other cardiac conditions due possibly to improved therapies for coronary artery disease, the frequency of atrial fibrillation is increasing markedly. Although atrial fibrillation is regarded by some physicians as an acceptable alternative to sinus rhythm, it must be remembered that this arrhythmia is a major cause of thromboembolism and is responsible for both impaired left ventricular function (due probably to uncontrolled ventricular rates) as well as poor quality of life. Many of the symptoms are subtle and nonspecific; the effects of sustained atrial fibrillation may be insidious, suggesting this condition be compared more with hypertension than with acute disorders such as ventricular tachycardia or unstable angina. Most patients complain of fatigue and exertional shortness of breath, palpitations being surprisingly uncommon; in one study the initial quality of life of patients with atrial fibrillation undergoing AV junction ablation was less than that of patients recovering from myocardial infarction or with severe rheumatoid arthritis.

Figure 61-5 ECG example of atrial fibrillation. The ECG shows complete irregularity in timing of the QRS complexes in association with a baseline reflective of disorganized atrial activity.

The importance of atrial fibrillation as a cause of stroke is well known. The annual risk of stroke ranges from about 1.5% in the 40 to 50 age group to >20% in patients older than 70; the Framingham data suggest that atrial fibrillation is an independent risk factor for stroke^[22] (risk ratio 5.6 compared with matched controls in sinus rhythm) and that this risk is increased significantly in the presence of hypertension (risk ratio 12), heart failure (risk ratio 12), or mitral stenosis (risk ratio 17). It has been estimated that atrial fibrillation accounts for 15% of all symptomatic cerebral emboli, and studies of computed tomographic (CT) scanning in asymptomatic patients with nonrheumatic atrial fibrillation have shown silent cerebral infarctions in up to 26%.^[23][24] Prospective trials have shown repeatedly that the risk of stroke can be essentially eliminated by warfarin anticoagulation targeted to an international normalized ratio (INR) of 2 to 3; as the INR falls below 2 the risk of stroke progressively and steeply rises^[25]; above an INR of 4 the risks of bleeding increase significantly.^[26] Unfortunately, precise anticoagulant control within the 2 to 3 range is not always possible, and warfarin is considered contraindicated in many patients; moreover, anticoagulation is a lifelong undertaking that is associated with substantial costs and restriction of activities. Therefore, although warfarin anticoagulation should generally be employed in patients with atrial fibrillation, it may be forgone in some patients considered at low risk for stroke; such patients include younger individuals (<65) with lone atrial fibrillation (i.e., no known heart disease, hypertension, or alcoholism) or infrequent paroxysmal atrial fibrillation.

It has recently been recognized that atrial fibrillation may be responsible for a reversible dilated cardiomyopathy, and that left ventricular function can be normalized by either cardioversion or rate control achieved by AV junction ablation^[27][28]; atrial fibrillation frequently coexists with heart failure since it is the quintessential arrhythmia of patients with significant left ventricular dysfunction. Often there may be a “chicken and egg” situation in which rapidly conducting atrial fibrillation leads to worsening heart failure but it may not be clear which is primarily responsible for the patient's deterioration. It is often unclear whether rapid ventricular rates (and consequent impaired cardiac filling), loss of atrial transport (which in heart failure may be responsible for up to 30% of cardiac output), or the irregular ventricular response is responsible alone or in combination for decompensated heart failure. It is certainly a common paradox that those patients who most need to have sinus rhythm (and atrial transport) restored are those in whom the use of antiarrhythmic drugs presents most risk. Generally, however, vigorous attempts at cardioversion (usually using adjunctive amiodarone) or heart rate control (often by AV junction ablation) are rewarded by marked symptomatic improvement as well as by objectively demonstrable increases in ventricular function.

The goals of treatment in atrial fibrillation are to control ventricular rate, to prevent thromboembolism, and to both convert to and maintain sinus rhythm. The order and urgency with which these goals are achieved vary widely according to clinical circumstances. It is likely that the tacit acceptance of atrial fibrillation by many physicians is contributed to by the lack of effective therapies. Although external direct current (DC) cardioversion is about 90% effective, atrial fibrillation recurs in a majority of patients within a year. Even in those treated with antiarrhythmic drugs such as quinidine or sotalol the recurrence rate is approximately 50% at 1 year, and with these agents the risk of proarrhythmia and sudden death may be up to 15% per year in patients with structural heart disease.^[29][30] Amiodarone may be associated with a greater success rate, but this agent has other significant toxicities that limit its long-term use and its applicability to younger patients.

Cardioversion may be achieved by pharmacologic or electrical means. Oral quinidine loading has been used for over 40 years for cardioversion but has only a 20% efficacy rate at best. Ibutilide has the advantage of a shorter half-life and probably greater efficacy than these other agents; in addition, there is evidence from a randomized trial of ibutilide pretreatment prior to transthoracic electrical cardioversion that this drug may also reduce the energy requirement and thereby facilitate the restoration of sinus rhythm.^[31] However, there remains a need to further define the optimal management strategy for conversion to sinus rhythm since the relative cost-effectiveness of ibutilide versus (or in combination with) DC cardioversion is unknown.

Internal cardioversion has a higher success rate than external cardioversion, and has led to the concept of the automatic implantable atrial defibrillator, a device currently in clinical development; this notion has been supported by recent data from a goat model suggesting that the ease of cardioversion is directly related to the duration of the arrhythmia and that “atrial fibrillation begets atrial fibrillation.”^[32] Therefore longer term maintenance of sinus rhythm may be possible by repeated and early cardioversion; however, this theory remains to be validated by clinical data.

Of the drugs available for ventricular rate control, β-blockers have become first-line agents since it has been shown that digoxin is neither effective in restoring nor maintaining sinus rhythm; rate control with digoxin is difficult to achieve under circumstances of heightened sympathetic tone such as with exercise or after surgery.

In patients who receive pacemakers for the treatment of sinus node disease, the incidence of atrial fibrillation is lower in those who receive dual chamber or atrial-based systems compared with those who receive ventricular pacemakers.^[33] In addition, there is retrospective data suggesting that the frequency of preexisting paroxysmal atrial fibrillation in patients receiving dual chamber or atrial-based pacemakers is reduced after implantation. In some patients it is clearly important to prevent sinus bradycardia in order to reduce the frequency of atrial fibrillation, and there is an additional subgroup of patients with “vagally mediated” atrial fibrillation who seem to benefit from atrial-based pacing.^[34] In conditions associated with sinus bradycardia, pacing is thought to have a salutary effect by overdrive suppression of atrial ectopy and by decreasing dispersion of atrial refractoriness.

Radiofrequency ablation of the AV junction combined with permanent pacing has now become an accepted approach. There is strong evidence of a marked improvement in both quality of life and functional status of patients who have undergone this procedure.^[35] Although there remains a need for anticoagulation, and patients are often highly dependent on pacing for survival, this procedure offers the opportunity to avoid treatment with drugs that are usually associated with significant adverse effects. With the advent of pacemakers that can automatically alter the mode of pacing when atrial arrhythmias appear, the use of dual chamber pacing in patients with paroxysmal atrial fibrillation now offers the benefits of AV synchrony between arrhythmia episodes.

Finally, the use of catheter ablation has been explored for the definitive cure of atrial fibrillation. The surgical experience with a range of procedures designed to abolish atrial fibrillation has led to the use of specially designed catheters to create linear atrial lesions that reduce the mass of left and right atrial myocardium in continuity and prevent the propagation of reentrant wavelets.^[36] This catheter-based maze procedure is still experimental and is likely to remain restricted in its application until better catheters are available. Such approaches are predicated on the conventional notion that atrial fibrillation is a generalized phenomenon of both atria in which there are multiple circulating wavelets of reentry. However, in a selected group of generally younger patients with lone atrial fibrillation it may be the case that a single focus is responsible for paroxysmal atrial fibrillation; recent reports suggest that such foci of atrial fibrillation in or around the orifices of the pulmonary veins may be identified and successfully ablated using radiofrequency current.

[edit] Atrial Flutter.

Atrial flutter is common and usually occurs in patients with structural heart disease or pulmonary disease, or after cardiothoracic surgery. Atrial flutter is characterized by rapid atrial rates of approximately 280 to 340 beats per minute; the ventricular response is often rapid, since it is constrained to a fixed ratio of the atrial rate. The ECG typically shows prominent atrial activity with a “saw-tooth” pattern in the inferior leads and no evidence of an isoelectric period during the cardiac cycle (Fig. 61-6).^[37] The most common clinical presentation is with a regular tachycardia of 150 beats per minute, and any tachycardia that is fixed at this rate should raise the clinical suspicion of atrial flutter. Although the ratio of atrial to ventricular events during flutter may vary, the ventricular response is often rapid and difficult to control medically. Prolonged periods of sustained rapidly conducting atrial flutter may cause a tachycardia-induced cardiomyopathy, and elimination of the arrhythmia has been associated with normalization of ventricular function in some patients. The use of AV nodal blocking drugs is commonly ineffective and poorly tolerated; antiarrhythmic agents such as quinidine or flecainide have been used with some efficacy, but these drugs are associated with significant slowing of the atrial rate that may lead to paradoxical increase in ventricular rate with the development of 1:1 AV conduction.^[38]

Figure 61-6 ECG example of atrial flutter. This ECG shows a regular narrow QRS complex tachycardia at approximately 150 bpm. There is no isoelectric period in the baseline between QRS complexes, and there is a clear saw-tooth pattern in the inferior leads suggestive of common or type I atrial flutter.

In the intensive care or postoperative setting cardioversion is the preferred management approach for termination of atrial flutter. Although very low electrical energies (10 J or less) can often be used successfully, it is well recognized that such low energy discharges may convert atrial flutter to atrial fibrillation because the discharge (synchronized to ventricular activation) may be delivered during the vulnerable period of atrial repolarization. Therefore an initial energy of 50 J or higher is generally recommended.

The reentrant circuit in atrial flutter is now well characterized^[39]; typically the activation wavefront circles the right atrium, activating the left atrium passively. Activation proceeds up the interatrial septum and down the lateral atrial wall anterior to the crista terminalis before entering the narrow isthmus in the floor of the right atrium between the tricuspid annulus and the orifice of the inferior vena cava. It is in this compact area of right atrial tissue that the zone of slow conduction is localized; catheter ablation designed to create a line of bidirectional conduction block in this subeustachian isthmus has been successfully employed for the long-term elimination of atrial flutter.^[40] This procedure is effective in approximately 80% of cases, and offers safe control of an arrhythmia that is not easily treated medically. Because atrial flutter may be the primary arrhythmia in some patients with atrial fibrillation, elimination of this tachycardia may also be effective in preventing recurrences of atrial fibrillation.

Some patients manifest primarily atrial fibrillation, although atrial flutter may also occur from time to time; in these patients antiarrhythmic drugs may effectively eliminate fibrillation and organize reentry within the atria such that atrial flutter is the dominant tachycardia. In such circumstances catheter ablation of the flutter circuit combined with long-term antiarrhythmic drug treatment of atrial fibrillation (so-called hybrid therapy) may be salutary.^[41]

Following cardiac surgery in which an atrial incision is made there is a risk of reentrant arrhythmias occurring around the fibrous scar; these tachycardias often appear decades after surgery. Such atrial flutters are also successfully treated with catheter ablation, as are many other apparently atypical atrial flutters in which the usual ECG appearances discussed above are absent.^[42]

[edit] Atrial Tachycardias.

There are a variety of tachycardias of atrial origin that have varying mechanisms and are of importance in clinical practice. These tachycardias are associated with rapid atrial rates, usually with consistent P wave morphology and an isoelectric period between P waves. The ventricular response may be rapid or there may be varying degrees of AV block (Fig. 61-7). Atrial tachycardias are often associated with advanced atrial disease such as that found in severe heart failure; such patients may also be digoxin toxic (occasionally with normal blood levels of digoxin) with or without hypokalemia and the arrhythmia is truly an ectopic atrial tachycardia. In approximately 70% of such cases the arrhythmia can be treated by merely normalizing the metabolic abnormalities. In other patients, particularly those with pulmonary disease, the tachycardia may be reentrant in mechanism (intraatrial reentry) or ectopic with multiple foci of activation (multifocal atrial tachycardia [MAT]). In MAT the P wave morphology is variable with at least three different P waves and PR intervals identifiable on the ECG; this arrhythmia is closely associated with severe chronic lung disease and theophylline therapy but may occur in other chronically ill patients; verapamil has been recommended as the treatment of choice. It is important to recognize that atrial tachycardias in general exist on a spectrum of atrial arrhythmias with atrial flutter and fibrillation; many patients with advanced atrial disease may exhibit all of these arrhythmias in an evanescent fashion that precludes specific ablation therapy. However, when these tachycardias are difficult to control and are hemodynamically significant, catheter ablation of the AV junction may be appropriate if a specific target is not identifiable at electrophysiologic study.^[43]

Figure 61-7 ECG example of atrial tachycardia. This ECG shows an irregular rhythm where the R-R intervals are not random as in atrial fibrillation, but are somewhat consistent during the recording. The atrial activity is rapid and regular, with discrete P waves particularly prominent in lead V1.

[edit] Preexcitation Syndromes.

The Wolff-Parkinson-White syndrome is the most common example of the rare preexcitation syndromes; during sinus rhythm part of the ventricle is prematurely activated by an accessory pathway that bypasses the normal conduction route via the AV node. The ECG during sinus rhythm therefore shows a short PR interval and a widened QRS complex with a slurred upstroke known as the delta wave (Fig. 61-8). The frequency of such congenital anomalies is thought to be about 1:10,000 live births but it is important to note that most patients probably never experience tachycardia even though the ECG may demonstrate that the substrate exists.

Figure 61-8 ECG example of Wolff-Parkinson-White pattern. This ECG shows sinus rhythm with a short PR interval and a wide QRS complex due to delay in the upstroke of the R wave. This slurred upstroke is referred to as a delta wave. Note that in the inferior leads the delta wave polarity is negative and this often leads to inappropriate diagnosis of inferior myocardial infarction. This pseudo-inferior infarct pattern is typical of a manifest posteroseptal accessory pathway.

The most common form of tachycardia in Wolff-Parkinson-White syndrome is orthodromic AVRT (described earlier). This is typically a regular narrow complex QRS tachycardia, although the QRS complex may be wide if bundle branch block occurs during the arrhythmia. It is important to recognize the Wolff-Parkinson-White pattern on the ECG in a patient with symptoms of arrhythmia because of the potential risk of sudden death if atrial fibrillation occurs (Fig. 61-9). In a minority of patients the accessory pathway may be capable of such rapid conduction during atrial fibrillation that ventricular fibrillation may supervene. Although rare, this is a well-described mechanism of sudden death in otherwise healthy patients, and can be prevented by catheter ablation.

Figure 61-9 ECG example of atrial fibrillation in a patient with Wolff-Parkinson-White syndrome. This ECG shows a completely irregular wide complex tachycardia that is very rapid and associated with a bizarre appearance and leftward axis of the QRS complex. This arrhythmia may degenerate to ventricular fibrillation and therefore requires prompt termination followed by catheter ablation of the accessory pathway.

[edit] Ventricular Tachycardia and Sudden Cardiac Death.

Sudden death strikes approximately 300,000 individuals in the United States every year; the resuscitation rate varies greatly according to the geographic setting and sophistication of local rescue services, but the overall survival is no better than 20%. Most deaths (>80%) are related to coronary artery disease and the final event is thought to be ventricular fibrillation in approximately 90%.^[44] In many of these cases there is preexisting left ventricular dysfunction due to previous myocardial infarction and the inciting event is ventricular tachycardia, which then degenerates to ventricular fibrillation. Acute ischemic events also play a significant role in the etiology of sudden cardiac death and are important to identify and treat aggressively in resuscitated patients as for other patients with acute myocardial infarction. The clinical setting in which cardiac arrest occurs is relevant in that patients with primary ventricular fibrillation associated with acute myocardial infarction have a good long-term prognosis, whereas those patients who experience ventricular fibrillation unassociated with an acute ischemic event have a recurrence rate of up to 30% in the year following the event; these patients therefore require ongoing protective therapy, usually an implantable cardioverter-defibrillator (ICD).

Sustained ventricular tachycardia may be monomorphic or polymorphic in ECG characteristics; in the former, the QRS complexes are uniform beat-by-beat with a fixed appearance often from one episode to another (Fig. 61-10). This reflects the abnormal cardiac activation in this reentrant arrhythmia originating in the border zone at the margin of a (usually chronic) infarct. The ECG appearances of polymorphic ventricular tachycardia show a constantly shifting QRS axis and amplitude in each ECG lead, and the ECG may not be distinguishable from ventricular fibrillation, both of which arrhythmias are thought to be caused by multiple circulating reentrant wavelets in ventricular myocardium.

Figure 61-10 ECG example of monomorphic ventricular tachycardia. This ECG shows a regular wide complex tachycardia at 190 bpm in which P waves are intermittently visible in the inferior leads and V1. This lack of a relationship between P waves and QRS complexes is diagnostic of ventricular tachycardia.

Ventricular tachycardia occurring in the context of structural heart disease with left or right ventricular dysfunction is a life-threatening arrhythmia and must be treated aggressively. Historically, treatment of such tachycardias relied on the use of antiarrhythmic drugs, but recent data from such large trials as AVID (Antiarrhythmics Versus Implantable Defibrillators) demonstrate that the ICD offers a clear survival advantage in such patients.^[11]

Data from the CAST study and others suggested that certain antiarrhythmic drugs may adversely affect survival, and the results of two large trials of the most efficacious agent, amiodarone, demonstrate that although this drug may reduce arrhythmic events, the overall survival of patients treated after myocardial infarction is not improved.^[45][46] In current U.S. practice, the majority of patients with ventricular tachycardia or fibrillation that is not caused by a reversible factor receive an ICD; antiarrhythmic agents are used to supplement such therapy in order to reduce the frequency of device discharges as necessary.

Occasionally ventricular tachycardia is amenable to definitive therapy and it is important to recognize such arrhythmias.^[47] Such tachycardias include bundle branch reentrant tachycardia in which the reentrant circuit involves the left and right bundle branches of the His-Purkinje system; this circuit can be readily abolished by ablation of the right bundle branch, but such patients usually have extensively diseased and dilated hearts and therefore often require ICD therapy for other ventricular tachycardias. Patients without evidence of structural heart disease using conventional clinical and imaging techniques who present with recurrent ventricular tachycardia represent a small but important group of patients, since these arrhythmias are usually curable by ablation. Repetitive monomorphic idiopathic ventricular tachycardia is usually suggested by one of two ECG appearances; the more common exhibits a left bundle branch block configuration with markedly positive QRS complex in the inferior leads (II, III, aVF) and originates in the outflow tract of the right ventricle. A less common idiopathic ventricular tachycardia exhibits a QRS morphology of right bundle branch block with left axis deviation and originates in the posterior fascicle of the left bundle branch; it may also be sensitive to verapamil, which may be an effective long-term treatment alternative to ablation.

[edit] Bradycardias

Traditionally, a bradycardia has been defined as a heart rate lower than 60 beats per minute; however, with the widespread use of β-blockers and other negatively chronotropic agents this definition has now been revised to include only heart rates that are below 50 beats per minute. Irrespective of the mechanism of bradycardia, obvious pauses in heart rhythm not due to a reversible factor that lead to syncope or dizziness require pacing, as do most pauses of greater than 3 seconds that are apparently asymptomatic. It is, however, important to consider the relationship between symptoms and heart rate, since a heart rate of 60 beats per minute in the face of fever or heart failure may reveal underlying pathology that justifies long-term pacing in a patient who has no overt symptoms of bradycardia. Often such symptoms of bradycardia are recognized only in retrospect after pacemaker implantation.

Sinus node disease or sick sinus syndrome is a heterogeneous entity in which various bradycardias and tachycardias may coexist at different times; common to all is that the origin of the rhythm disturbance is in or surrounding the sinus node. Sinus bradycardia is common but probably underrecognized and must be evaluated in the context of the patient's physiologic state and drug history; it is often associated with chronotropic incompetence, an inability of the sinus node to respond to the usual stimuli that increase heart rate during exercise or other stressful physiologic states. Such patients complain of fatigue and an inability to exercise, which may be difficult to assess clinically; exercise testing may reveal blunted heart rate response, with typical peak exercise heart rates of less than 100 beats per minute. Rate-modulated pacing in which the implanted pacemaker has a biosensor that permits physiologic heart rates with daily activity usually has a beneficial impact on the quality of life of these patients.^[48] Sinus pauses, exit block, and sinus arrest are common indications for permanent pacing; identification of these often intermittent problems may be difficult, but the use of long-term event monitoring has enhanced diagnostic yield. Bradycardias related to sinus node disease may be secondary to tachycardias originating in the sinus node region. These tachycardias suppress sinus node automaticity such that when the tachycardia terminates there is a long offset pause that provokes syncope, which may be the only overt symptom. Adequate treatment of the tachycardia with β-blocker or other negatively chronotropic agents will require concomitant pacemaker therapy in most of these patients.

AV block is common and represents the most common indication for cardiac pacing worldwide. The classification of AV block is given in Box 61-3; it is important to emphasize that the goal of the clinical and ECG evaluation of AV block is to identify those patients at risk for progression to high-grade, third-degree AV block in which the escape mechanism is likely to be unstable. Localizing the site of block to the AV node or the His-Purkinje system is key to this process of defining risk, since AV nodal block is associated with a stable junctional escape of 40 to 50 beats per minute, whereas infranodal block is often associated with a slow and erratic ventricular escape of only 20 to 30 beats per minute. Localizing the site of block cannot be done reliably using noninvasive tools, but the presence of a wide QRS complex and/or Mobitz type II second-degree AV block both suggest a tenuous conduction system; conversely, a long PR interval and Wenckebach's periodicity with a narrow QRS complex all suggest AV nodal disease with a more stable escape mechanism.

Current pacemaker technology permits the implantation of sophisticated small devices with an expected longevity of 6 to 8 years on average.^[49] Both single and dual chamber pacemakers feature flexible programming of multiple parameters and additional memory storage for internal diagnostic data and arrhythmic events. Rate-responsive pacemakers incorporate a sensor, based usually on body activity or minute ventilation, that accelerates the paced rate according to a predetermined algorithm when triggered. Although data on the mortality benefit (if any) are lacking, it is clear that cardiac pacing improves quality of life and functional status of patients who have sinus and AV node disease. Pacing therapy is cost-effective at any age, an important consideration given that most patients requiring pacemakers are of advanced age.

[edit] REFERENCES