Sinus bradycardia (sinus rate <60 beats per minute) is considered inappropriate when it is persistent and does not increase appropriately with exercise. This arrhythmia should be distinguished from asymptomatic resting sinus bradycardia in young athletes and in normal adults during sleep (8,9). Chronotropic
incompetence is not present in these individuals, as it is in patients with sinus nodal dysfunction.

The terms sinus arrest and sinus pause are used interchangeably and refer to the condition in which the sinus node’s principal pacemaker cells fail to fire. The pause is not an exact multiple of the preceding PP interval. Pauses greater than 3 seconds are rare in normal individuals and may or may not be associated with symptoms but are usually caused by sinus nodal dysfunction (10,11). In contrast, asymptomatic pauses greater than 2 seconds (but <3 seconds) are seen in 11% of normal patients during 24-hour Holter monitoring and are especially common in trained athletes (12).

The presence of chronic atrial fibrillation in a patient with a slow ventricular response not secondary to drug therapy is a sign of sinus nodal dysfunction. In some cases, cardioversion results in a long sinus pause before the appearance of sinus rhythm, or junctional escape rhythm. Although a combination of sinus nodal and AV nodal conduction disease may be present in many instances, examples of rapid ventricular responses during atrial tachyarrhythmias can frequently be found.

Tachycardia-bradycardia syndrome refers to the presence of intermittent sinus or junctional bradycardia alternating with atrial tachycardia (usually paroxysmal atrial fibrillation) in the same patient. This condition, frequently referred to as sick sinus syndrome, is a common manifestation of sinus nodal dysfunction. The highest incidence of syncope associated with sinus nodal dysfunction probably occurs in this group. Syncope typically occurs secondary to a long sinus pause after the spontaneous termination of atrial fibrillation (Fig. 62.1).

Idiopathic degenerative disease is probably the most common cause of intrinsic sinus nodal dysfunction. Fibrous tissue replacement with age explains some but not all cases (13,14). Coronary artery disease may be responsible for one third of cases of sinus nodal dysfunction (15). Transient slowing of the sinus rate, or sinus arrest, can complicate an acute myocardial infarction. This is usually seen with an acute inferior wall myocardial infarction, is caused by neural influences, and rarely persists (16). Cardiomyopathy, long-standing hypertension, infiltrative disorders, collagen vascular diseases, inflammatory processes, orthotopic cardiac transplantation, myotonic dystrophy or Friedreich ataxia, and surgical trauma can also result in sinus nodal dysfunction (17,18).

In the pediatric population, most cases of sinus nodal dysfunction can be attributed to surgical trauma from repairs of various congenital heart disease syndromes (19,20,21).

In the absence of structural abnormalities, the predominant causes of sinus nodal dysfunction are drug effects and autonomic influences. Drugs known to depress sinus nodal function include β-blockers, calcium channel blockers, digoxin (22), and sympatholytic antihypertensives (e.g., clonidine, methyldopa, and reserpine) (23). Types IA, IC, and III antiarrhythmic drugs can depress sinus nodal function (22) and occasionally may produce proarrhythmias in patients with sick sinus syndrome. Paroxysmal primary atrial tachycardias and pause-dependent ventricular tachycardia have been described (24,25). Other drugs reported to affect sinus nodal function include lithium, cimetidine, amitriptyline, and phenytoin (22).

Sinus nodal dysfunction may sometimes result from excessive vagal tone in individuals without intrinsic sinus nodal disease. Autonomically induced asystole has been reported to occur rarely and may result in sudden death (26). Hypervagatonia can be seen in carotid sinus syndrome and vasovagal syncope (Fig. 62.2). Well-trained athletes with increased vagal tone may require some deconditioning to help prevent symptomatic bradyarrhythmias (27).

The estimated incidence of sinus nodal dysfunction in patients older than 50 years of age is 3 in 5,000 (28). Syncope and presyncope are the most frequent symptoms associated with significant bradycardia. Fatigue, angina, and shortness of
breath can also be seen. Elderly patients may have more subtle symptoms of gastrointestinal distress or change in mental status. Patients with tachycardia-bradycardia syndrome may only experience palpitations associated with tachycardia or embolic events. Syncope in an individual with paroxysmal atrial fibrillation is classically associated with a long sinus pause at the spontaneous termination of the tachycardia. The intermittent nature of these symptoms makes documentation of the associated rhythm disturbance difficult at times. In other cases, marked sinus bradycardia and sinus pauses may be asymptomatic (29).

Both noninvasive and invasive means of diagnosing sinus nodal dysfunction are available. Generally, the noninvasive methods of electrocardiographic (ECG) monitoring, exercise testing, and autonomic testing are used first. However, if symptoms are infrequent, invasive electrophysiologic testing or an implantable monitor may be required.

A 12-lead ECG correlating bradycardia during syncope or near-syncopal episodes can be diagnostic. However, the diagnosis of sinus nodal dysfunction as the etiology of these symptoms is rarely made from the simple ECG. If symptoms are frequent, 24- or 48-hour ambulatory Holter monitoring can be useful. Documentation of symptoms in a diary by the patient while wearing the Holter monitor is essential for correlation of symptoms with the heart rhythm at the time. Often the sinus pauses recorded are not associated with symptoms. Several Holter monitor studies (11,30) demonstrated the futility of treating asymptomatic pauses, even if they were 3 seconds or longer. Most pauses, ranging from 2 to 15 seconds, were asymptomatic, and pacing did not benefit those without associated symptoms. The length of the pause correlated poorly with symptoms and prognosis. In patients whose symptoms occur infrequently, a loop recorder or a home recording device such as Cardionet is useful. Exercise testing is of limited value in diagnosing sinus nodal dysfunction. However, in some cases, it is useful in differentiating those patients with chronotropic incompetence from patients with resting bradycardia who demonstrate a normal heart rate increase with exercise.

Autonomic testing of the sinus node includes various pharmacologic interventions and maneuvers to test reflex responses. An abnormal response to carotid sinus massage (pause >3 seconds) may indicate sinus nodal dysfunction, but this response may also occur in asymptomatic elderly individuals (31). The most commonly used pharmacologic intervention is used to determine the intrinsic heart rate. Complete autonomic blockade is accomplished by administering atropine, 0.04 mg/kg, and propranolol, 0.20 mg/kg. The resulting intrinsic heart rate represents the sinus nodal rate without autonomic influences. The normal intrinsic heart rate is age dependent and can be calculated using the following equation: intrinsic heart rate (in beats per minute) = 118.1 – (0.57 × age [in years]) (32). A low intrinsic heart rate is consistent with abnormal intrinsic sinus nodal function. A normal intrinsic heart rate in a patient with known sinus nodal dysfunction suggests abnormal autonomic regulation.

Sinus nodal function can also be evaluated invasively with an electrophysiologic study. This is usually reserved for symptomatic patients in whom sinus nodal dysfunction is suspected but cannot be documented in association with symptoms by noninvasive means. The pacing tests most commonly used are sinus nodal recovery time and sinoatrial conduction time (33). However, even when they are performed appropriately, the ability of these tests to provide a definitive diagnosis is limited (34). The reader is referred to Chapter 62 for further information on sinus nodal function tests.

On the surface ECG, the PR interval is normally between 120 and 200 msec in duration. This interval reflects the conduction time from the high right atrium to the point of ventricular activation. To measure the different components of the conduction system that the PR interval includes, intracardiac tracings from the high right atrium and bundle of His region are needed. The PA interval, measured from the high right atrial electrogram to the low right atrial deflection in the bundle of His electrogram, gives an indirect approximation of the right atrial conduction time. The AH interval, measured from the low atrial to the bundle of His deflection, reflects conduction time through the AV node. The HV interval, measured from the bundle of His deflection to the earliest ventricular depolarization on the surface electrogram, represents the conduction time from the proximal bundle of His to the ventricular myocardium (44) (Table 62.1).

First-degree AV block on the surface ECG is seen as a PR interval greater than 0.20 second. Each P wave is followed by a QRS complex with a constant, prolonged interval. Although the conduction delay can be anywhere along the system, the PR prolongation is usually caused by delay within the AV node (87% when the QRS complex is narrow). On the bundle of His electrogram, this would be seen as an AH interval greater than 130 msec with a normal HV interval. In cases in which first-degree AV block is seen in the presence of a bundle branch block, a bundle of His electrogram is necessary to localize the site of block (45,46,47). Infranodal conduction delay is present in 45% of these cases. A combination of delay within the AV node and in the His-Purkinje system must also be considered (48). In certain cases of congenital structural heart disease, such as Ebstein anomaly of the tricuspid valve or endocardial cushion defects, intraatrial conduction delay can cause first-degree AV block. In addition, intra-Hisian conduction delay can cause first-degree AV block. On the bundle of His electrogram, a split His potential can be seen, resulting in a prolonged His potential, HV, and PR intervals (52). Dual-AV-nodal physiology can produce transient, abrupt, or alternating first-degree block caused by block in the fast AV nodal pathway (which is normally used), with conduction down the slow pathway instead.

Third-degree, or complete, AV block is seen on the surface electrogram as completely dissociated P waves and QRS complexes, each firing at its own pacemaker rate. The atrial impulse is never conducted to the ventricles, but different levels of block are possible. The level of block determines the QRS morphology along with the site and rate of the escape rhythm. Congenital complete heart block is characterized by a narrow QRS complex with an escape rate between 40 and 60 beats per minute, which tends to increase with exercise or atropine. This is consistent with block within the AV node. Acquired complete heart block is usually associated with block in the His-Purkinje system, resulting in a wide QRS complex with an escape rate between 20 and 40 beats per minute. The intracardiac electrogram shows bundle of His deflections consistently following the atrial electrograms, but the ventricular
depolarization is completely dissociated from these. Block below the bundle of His is thus demonstrated. In contrast, complete heart block at the AV nodal level is seen on the intracardiac tracings as bundle of His potentials consistently preceding each ventricular depolarization. The atrial electrograms are dissociated from the HV complexes (Fig. 62.4). The sinus rate is faster than the ventricular rate in patients with complete heart block. Data collected from patients with congenital complete heart block have shown the atrial rate to usually be age appropriate (52). It is important to note that complete antegrade AV block does not always predict retrograde (VA) conduction. Retrograde conduction may be intact in an individual with complete antegrade AV block (Fig. 62.5).