Introduction
Abnormalities of conduction can occur between the sinus node and atrium, within the atrioventricular node, and in the intraventricular conduction pathways.

Sick sinus syndrome

This imprecise diagnosis is applied to patients with sinus arrest, sinoatrial exit block (recognized by a pause equal to a multiple of the underlying PP interval or progressive shortening of the PP interval prior to a pause), or persistent sinus bradycardia. These rhythms are often caused or exacerbated by drug therapy (digitalis, calcium channel blockers, ß-blockers, sympatholytic agents, antiarrhythmics), and agents that may be responsible should be withdrawn prior to making the diagnosis. Another presentation is of recurrent supraventricular tachycardias (paroxysmal reentry tachycardias, atrial flutter, and atrial fibrillation), associated with bradyarrhythmias (“tachy-brady syndrome”). The long pauses that often follow the termination of tachycardia cause the associated symptoms. Sick sinus syndrome occurs most commonly in elderly patients.

The pathologic changes are usually nonspecific, characterized by patchy fibrosis of the sinus node and cardiac conduction system. Sick sinus syndrome may be caused by other conditions, including sarcoidosis, amyloidosis, Chagas’ disease, and various cardiomyopathies. Coronary disease is an uncommon cause.

Most patients with electrocardiographic evidence of sick sinus syndrome are asymptomatic, but rare individuals may experience syncope, dizziness, confusion, palpitations, heart failure, or angina. Because these symptoms are either nonspecific or are due to other causes, it is essential that they be demonstrated to coincide temporally with arrhythmias. This may require prolonged ambulatory monitoring or the use of an event recorder. Pharmacologic therapy for sick sinus syndrome has been difficult, but recent studies have indicated that oral theophylline may be effective, especially when sinus bradycardia is the major manifestation. Most symptomatic patients will require permanent pacing. Dual-chamber pacing is preferred because ventricular pacing is associated with a higher incidence of subsequent atrial fibrillation, and subsequent atrioventricular block occurs at a rate of 2% per year. Treatment of associated tachyarrhythmias is often difficult without first instituting pacing, since digoxin and other antiarrhythmic agents may exacerbate the bradycardia. Unfortunately, symptomatic relief following pacing has not been consistent, largely because of inadequate documentation of the etiologic role of bradyarrhythmias in producing the symptom. Furthermore, many of these patients may have associated ventricular arrhythmias that may require treatment; however, carefully selected patients may become asymptomatic with permanent pacing alone.

Adan V et al : Diagnosis and treatment of sick sinus syndrome. Am Fam Physician 2003;67:1725.
Brignole M: Sick sinus syndrome. Clin Geriatr Med 2002;18:211.
Lamas GA et al: Ventricular pacing or dual-chamber pacing for sinus-node dysfunction. N Engl J Med 2002;346:1854.

Atrioventricular Block

Atrioventricular block is categorized as first-degree (PR interval > 0.21 second with all atrial impulses conducted), second-degree (intermittent blocked beats), or third-degree (complete heart block, in which no supraventricular impulses are conducted to the ventricles).

Second-degree block is subclassified. In Mobitz type I (Wenckebach) atrioventricular block, the atrioventricular conduction time (PR interval) progressively lengthens, with the RR interval shortening, before the blocked beat; this phenomenon is almost always due to abnormal conduction within the atrioventricular node. In Mobitz type II atrioventricular block there are intermittently nonconducted atrial beats not preceded by lengthening atrioventricular conduction. It is usually due to block within the His bundle system. The classification as Mobitz type I or Mobitz type II is only partially reliable, because patients may appear to have both types on the surface ECG, and the site of origin of the 2:1 atrioventricular block cannot be predicted from the ECG. The width of the QRS complexes assists in determining whether the block is nodal or infranodal. When they are narrow, the block is usually nodal; when they are wide, the block is usually infranodal. Electrophysiologic studies may be necessary for accurate localization. Management of atrioventricular block in acute myocardial infarction has already been discussed. This section deals with patients in the nonischemic setting.

First-degree and Mobitz type I block may occur in normal individuals with heightened vagal tone. They may also occur as a drug effect (especially digitalis, calcium channel blockers, ß-blockers, or other sympatholytic agents), often superimposed on organic disease. These disturbances also occur transiently or chronically due to ischemia, infarction, inflammatory processes, fibrosis, calcification, or infiltration. The prognosis is usually good, since reliable alternative pacemakers arise from the atrioventricular junction below the level of block if higher degrees of block occur.

Mobitz type II block is almost always due to organic disease involving the infranodal conduction system. In the event of progression to complete heart block, alternative pacemakers are not reliable. Thus, prophylactic ventricular pacing is required.

Complete (third-degree) heart block is a more advanced form of block often due to a lesion distal to the His bundle and associated with bilateral bundle branch block. The QRS is wide and the ventricular rate is slower, usually less than 50 beats/min. Transmission of atrial impulses through the atrioventricular node is completely blocked, and a ventricular pacemaker maintains a slow, regular ventricular rate, usually less than 45 beats/min. Exercise does not increase the rate. The first heart sound varies in intensity; wide pulse pressure, a changing systolic blood pressure level, and cannon venous pulsations in the neck are also present. Patients may be asymptomatic or may complain of weakness or dyspnea if the rate is less than 35 beats/min; symptoms may occur at higher rates if the left ventricle cannot increase its stroke output. During periods of transition from partial to complete heart block, some patients have ventricular asystole that lasts several seconds to minutes. Syncope occurs abruptly.

Patients with episodic or chronic infranodal complete heart block require permanent pacing, and temporary pacing is indicated if implantation of a permanent pacemaker is delayed.

Barold SS: Atrioventricular block revisited. Compr Ther 2002; 28:74.
Bourke JP: Atrioventricular block and problems with atrioventricular conduction. Clin Geriatr Med 2002;18:229. 

Atrioventricular Dissociation

When a ventricular pacemaker is firing at a rate faster than or close to the sinus rate (accelerated idioventricular rhythm, ventricular premature beats, or ventricular tachycardia), atrial impulses arriving at the atrioventricular node when it is refractory may not be conducted. This phenomenon is atrioventricular dissociation but does not necessarily indicate atrioventricular block. No treatment is required aside from management of the causative arrhythmia.

Intraventricular Conduction defects

Intraventricular conduction defects, including bundle branch block, are common in individuals with otherwise normal hearts and in many disease processes, including ischemic heart disease, inflammatory disease, infiltrative disease, cardiomyopathy, and postcardiotomy. Below the atrioventricular node and bundle of His, the conduction system trifurcates into a right bundle and anterior and posterior fascicles of the left bundle. Conduction block in each of these fascicles can be recognized on the surface ECG. Although such conduction abnormalities are often seen in normal hearts, they are more commonly due to organic heart disease — either an isolated process of fibrosis and calcification or more generalized myocardial disease. Bifascicular block is present when two of these — right bundle, left anterior and posterior hemibundle — are involved. Trifascicular block is defined as right bundle branch block with alternating left hemiblock, alternating right and left bundle branch block, or bifascicular block with documented prolonged infranodal conduction (long His-ventricular interval).

The prognosis of intraventricular block is generally that of the underlying myocardial process. Patients with no apparent heart disease have an overall survival rate similar to that of matched controls. However, left bundle branch block — but not right — is associated with a higher risk of development of overt cardiac disease and cardiac mortality. Even in bifascicular block, the incidence of occult complete heart block or progression to it is low, and pacing is not usually warranted. In patients with symptoms (eg, syncope) consistent with heart block and intraventricular block, pacing should be reserved for those with documented concomitant complete heart block on monitoring or those with a very prolonged HV interval (> 90 ms) with no other cause for symptoms. Even in the latter group, prophylactic pacing has not improved the prognosis significantly, probably because of the high incidence of ventricular arrhythmias in the same population.

Permanent Pacing

The indications for permanent pacing have been discussed: symptomatic bradyarrhythmias, asymptomatic Mobitz II atrioventricular block, or complete heart block. The versatility of pacemaker generator units has increased markedly, and dual-chamber multiple programmable units are being implanted with increasing frequency. A standardized nomenclature for pacemaker generators is employed, usually consisting of four letters. The first letter refers to the chamber that is stimulated (A = atrium, V = ventricle, D = dual, for both). The second letter refers to the chamber in which sensing occurs (also A, V, or D). The third position refers to the sensory mode (I = inhibition by a sensed impulse, T = triggering by a sensed impulse, D = dual modes of response). The fourth letter refers to the programmability or rate modulation capacity (usually P for programming for two functions, M for programming more than two, and R for rate modulation).

A pacemaker that senses and paces in both chambers is the most physiologic approach to pacing patients who remain in sinus rhythm. Atrioventricular synchrony is particularly important in patients in whom atrial contraction produces a substantial increment in stroke volume and in those in whom sensing the atrial rate to provide rate-responsive ventricular pacing is useful. Dual-chamber pacing is most useful for individuals with left ventricular systolic or — perhaps more importantly — diastolic dysfunction and for physically active individuals. In patients with single-chamber pacemakers, the lack of an atrial kick may lead to the so-called pacemaker syndrome, in which the patient experiences signs of low cardiac output while upright. Uncontrolled data suggest that chronic dual-chamber pacing is associated with a lower incidence of chronic atrial fibrillation than single-chamber ventricular pacing. However, patients with intermittent or potential bradyarrhythmias or conduction disturbances in whom pacing is primarily prophylactic should undergo ventricular pacing.

Pulse generators are also available that can increase their rate in response to motion or respiratory rate when the atrial rate is not an indication of the optimal heart rate. These are most useful in active individuals. Follow-up after pacemaker implantation, usually by telephonic monitoring, is essential. All pulse generators and lead systems have an early failure rate that is now below 5% and an expected battery life varying from 4 years to 10 years.

ACC/AHA/NASPE 2002 guideline update for implantation of cardiac pacemakers and antiarrhythmia devices: summary article. J Am Coll Cardiol 2002;40:1703.
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Faddis MN et al: Pacing interventions for falls and syncope in the elderly. Clin Geriatr Med 2002;18:279.
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Wong GC et al: Single chamber ventricular compared with dual chamber pacing: a review. Can J Cardiol 2002;18:301.

Evaluation of syncope

Syncope, defined as a transient loss of consciousness and postural tone due to inadequate cerebral blood flow with prompt recovery without resuscitative measures, is a common clinical problem, especially in the elderly. Thirty percent of the adult population will experience at least one episode, and syncope accounts for approximately 3% of emergency room visits. Causes include cardiac abnormalities (either disturbances of rhythm or hemodynamics), vascular disorders, or neurologic processes. A specific cause is identified in about 50% of cases during the initial evaluation. The prognosis is relatively benign except when accompanying cardiac disease is present. Syncope is more likely to occur in patients with known heart disease, older men, and young women (who are prone to vasovagal episodes). Syncope is characteristically abrupt in onset, often resulting in injury, transient (lasting for seconds to a few minutes), and followed by prompt recovery of full consciousness.

Vasomotor syncope may be due to excessive vagal tone or impaired reflex control of the peripheral circulation. The most frequent type of vasodepressor syncope is vasovagal hypotension or the “common faint,” which is often initiated by a stressful, painful, or claustrophobic experience, especially in young women. Premonitory symptoms, such as nausea, diaphoresis, tachycardia, and pallor, are usual. Episodes can be aborted by lying down or removing the inciting stimulus. Enhanced vagal tone with resulting hypotension is the cause of syncope in carotid sinus hypersensitivity and postmicturition syncope; vagal-induced sinus bradycardia, sinus arrest, and atrioventricular block are common accompaniments and may themselves be the cause of syncope. Carotid sinus massage under carefully monitored conditions or tilt-table testing may be diagnostic (see above under Autonomic Testing). Treatment consists largely of counseling patients to avoid predisposing situations. Paradoxically, ß-blockers may be helpful in patients with altered autonomic function uncovered by head-up tilt testing. Permanent pacing may benefit patients with documented bradycardiac responses.

Orthostatic (postural) hypotension is another common cause of vasomotor syncope, especially in the elderly, in diabetics or other patients with autonomic neuropathy, in patients with blood loss or hypovolemia, and in patients taking vasodilators, diuretics, and adrenergic blocking drugs. In addition, a syndrome of chronic idiopathic orthostatic hypotension exists primarily in older men. In most of these conditions, the normal vasoconstrictive response to assuming upright posture, which compensates for the abrupt decrease in venous return, is impaired. A greater than normal decline (20 mm Hg) in blood pressure immediately upon arising from the supine to the standing position is observed, with or without tachycardia depending on the status of autonomic (baroreceptor) function. Studying patients with a tilt table can establish the diagnosis with more certainty. Autonomic function can be assessed by observing blood pressure and heart rate responses to Valsalva’s maneuver and by tilt testing. In older patients, vasoconstrictor abnormalities and autonomic insufficiency are perhaps the most common causes of syncope. Thus, tilt testing should be employed before proceeding to invasive studies unless clinical and ambulatory electrocardiographic evaluation suggests a cardiac abnormality.

Cardiogenic syncope can occur on a mechanical or arrhythmic basis. Mechanical problems that can cause syncope include aortic stenosis (where syncope may occur from autonomic reflex abnormalities or ventricular tachycardia), pulmonary stenosis, hypertrophic obstructive cardiomyopathy, congenital lesions associated with pulmonary hypertension or right-to-left shunting, and left atrial myxoma obstructing the mitral valve. Episodes are commonly exertional or postexertional. More commonly, cardiac syncope is due to disorders of automaticity (sick sinus syndrome), conduction disorders (atrioventricular block), or tachyarrhythmias (especially ventricular tachycardia and supraventricular tachycardia with rapid ventricular rate).

The evaluation for syncope depends on findings from the history and physical examination (especially orthostatic blood pressure evaluation, examination of carotid and other arteries, cardiac examination, and, if appropriate, carotid sinus massage). The resting ECG may reveal arrhythmias, evidence of accessory pathways, prolonged QT interval, and other signs of heart disease (such as infarction or hypertrophy). If the history is consistent with syncope, ambulatory electrocardiographic monitoring is essential. This may need to be repeated several times, since yields increase with longer periods of monitoring, at least up to 3 days. Event recorder and transtelephone electrocardiographic monitoring may be helpful in patients with intermittent presyncopal episodes. Electrophysiologic studies to assess sinus node function and atrioventricular conduction and to induce supraventricular or ventricular tachycardia are indicated in patients with recurrent episodes and nondiagnostic ambulatory ECGs. They reveal an arrhythmic cause in 20-50% of patients, depending on the study criteria, and are most often diagnostic when the patient has had multiple episodes and has identifiable cardiac abnormalities.

Faddis MN et al: Pacing interventions for falls and syncope in the elderly. Clin Geriatr Med 2002;18:279.
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Weimer LH et al: Syncope and orthostatic intolerance. Med Clin North Am 2003;87:835.

Recommendations for resumption of driving

An important management problem in patients who have experienced syncope, symptomatic ventricular tachycardia, or aborted sudden death is to provide recommendations concerning automobile driving. According to a survey published in 1991, only eight states had specific laws dealing with this issue, whereas 42 had laws restricting driving in patients with seizure disorders. There are no adequate data to support driving restrictions in patients with asymptomatic arrhythmias, though patients with frequent nonsustained ventricular tachycardia, associated heart disease, and significant left ventricular dysfunction are at high enough risk to warrant cautioning. Patients with syncope or aborted sudden death thought to have been due to temporary factors (acute myocardial infarction, bradyarrhythmias subsequently treated with permanent pacing, drug effect, electrolyte imbalance) should be strongly advised after recovery not to drive for at least 1 month. Other patients with symptomatic ventricular tachycardia or aborted sudden death, whether treated pharmacologically, with antitachycardia devices, or with ablation therapy, should not drive for at least 6 months. Longer restrictions are warranted in these patients if spontaneous arrhythmias persist. The physician should comply with local regulations and consult local authorities concerning individual cases.

Akiyama T et al: Resumption of driving after life-threatening ventricular tachyarrhythmia. N Engl J Med 2001;345:391.