Sinoatrial nodal pause, arrest, and exit block

INTRODUCTION — The sinoatrial (SA) node represents the integrated activity of pacemaker cells, sometimes called P cells, in a compact region at the junction of the high right atrium and the superior vena cava. Perinodal cells, sometimes called transitional or (T) cells, transmit the electrical impulse from the SA node to the right atrium. Each of these cell types has distinct expression profiles of ion channels and gap junctions.

Given the architecture of the SA node, SA nodal dysfunction typically results from either abnormalities in impulse generation by the P cells or abnormalities in conduction across the T cells. SA nodal dysfunction is more commonly an acquired condition, but in some patients it can be inherited, with gene mutations having been described in some forms of inherited SA nodal dysfunction [1]. Patients with SA nodal dysfunction may be asymptomatic or highly symptomatic as in cases of sinus node dysfunction (SND).

Sinoatrial nodal pauses, arrest, and exit block will be discussed here. Additional details regarding the anatomy and electrophysiology of the SA node, as well as a discussion of the SND, are presented separately. (See "Sinus node dysfunction: Epidemiology, etiology, and natural history".)

ETIOLOGY — Sinus pause, arrest, and exit block may arise from hyperkalemia; excessive vagal tone; ischemic, inflammatory, or infiltrative or fibrotic disease of the SA node; sleep apnea; certain drugs (eg, digitalis). The causes of SND are discussed in detail elsewhere. (See "Clinical manifestations of hyperkalemia in adults" and "Sinus node dysfunction: Epidemiology, etiology, and natural history", section on 'Etiology' and "Obstructive sleep apnea and cardiovascular disease in adults", section on 'Other arrhythmias' and "Cardiac arrhythmias due to digoxin toxicity", section on 'Sinus bradycardia, tachycardia, block, and arrest'.)

In patients receiving one or more agents that depress SA node and atrioventricular (AV) node function, a syndrome of bradycardia, renal failure, AV block, shock, and hyperkalemia (BRASH), has been described [2,3]. Patients with BRASH are generally taking therapeutic doses of SA and AV node blocking medications, and the level of hyperkalemia may be mild. However, the severity of bradycardia (caused by sinus arrest and/or AV block) is generally greater than expected for either the dose/level of SA and AV node blocker or the level of hyperkalemia.

TYPES OF SA NODAL DYSFUNCTION

Sinus pause or arrest — A sinus pause or arrest is defined as the transient absence of sinus P waves on the electrocardiogram (ECG) that may last from two seconds to several minutes (waveform 1). This abnormality is an alteration in discharge by the SA pacemaker; as a result, the duration of the pause has no arithmetical relationship to the basic sinus rate (ie, the cycle length of the pause is not a multiple of the basic sinus cycle length as would occur with 2:1 or 3:1 SA nodal block). The pause or arrest often allows escape beats or rhythms to occur, but lower pacemakers may be sluggish or even absent in SND.

A pause that is two seconds and perhaps somewhat longer does not necessarily indicate disease, since it can occur in the normal heart [4,5]. Longer episodes of sinus arrest can produce symptoms of dizziness, presyncope, syncope, and, rarely, death.

SA nodal exit block — SA nodal exit block occurs as a result of interference with the delivery of impulses from the SA node to its neighboring atrial tissue. This results in the absence of a P wave on the surface ECG since the P wave is the only manifestation of SA nodal activity. Following the convention for AV nodal block, SA nodal exit block can be classified as first, second, or third degree. This problem is most easily conceptualized as having three components:

●A relatively constant input – The input is from the SA nodal pacemaker, which is not seen on the surface ECG. The behavior of the SA nodal pacemaker is, as noted above, inferred from the P waves of atrial activation. The rate or cycle length of the input can be presumed from portions on the ECG where normal P-P cycles are observed.

●An area across which block occurs – Exit block is thought to involve the perinodal T cells. The type of exit block in the perinodal tissues must be inferred from the output or response, that is, from the P waves.

●An output – The P wave abnormalities reflect the type of exit block which is present.

First degree SA nodal exit block reflects a slowing of impulse exit but there is still 1:1 conduction. This abnormality cannot be recognized on the ECG recorded from the body surface.

Second degree SA nodal exit block has two types:

●Type I (Wenckebach type) is characterized by progressively decreasing P-P intervals prior to a pause caused by a dropped P wave; the pause has a duration that is less than two P-P cycles (waveform 2). The mechanisms of Wenckebach conduction are considered elsewhere. (See "Second-degree atrioventricular block: Mobitz type I (Wenckebach block)".)

●In type II exit block, the P-P output is an integer multiple of the presumed sinus pacemaker input (eg, 2:1, 3:1, 4:1). Therefore the P-P cycle length surrounding the pause is a multiple of the normal P-P interval (waveform 3).

Third degree SA nodal exit block prevents sinus node pacemaker impulses from reaching the right atrium. This disorder cannot be distinguished from sinus arrest because the P-wave recorded on the surface ECG is not a direct measure of sinus node activity, only of atrial depolarization (which will be absent).

Variants of SA nodal dysfunction

Sinus arrhythmia — Sinus arrhythmia is a term that describes small changes in the sinus cycle length. The formal definition of sinus arrhythmia is a variation in the P-P interval by 0.12 sec (120 msec) or more in the presence of normal P waves and the usual PR interval (waveform 4), or a difference of 10 percent or greater between the longest and shortest P-P intervals. Sinus arrhythmia is more common in the young and also in those exposed to digoxin or morphine, while it is less frequently seen in older patients and those with diabetes. Typically, the P wave morphology remains relatively constant, but small variations in the PR interval may be seen. There are two predominant types of sinus arrhythmia, one occurring as a result of normal respiration and another in the presence of digoxin toxicity. Sinus arrhythmia (except when due to digoxin toxicity) is not considered abnormal and rarely causes symptoms; therefore, no treatment is typically recommended. (See "Normal sinus rhythm and sinus arrhythmia", section on 'Sinus arrhythmia'.)

Wandering atrial pacemaker — Wandering atrial pacemaker refers to a change in the dominant pacemaker focus from the sinus node. A wandering atrial pacemaker is present when there are three or more ectopic foci within the atrial myocardium that serve as the dominant pacemaker (waveform 5). Because the location of the pacemaker changes, the PR interval can vary as well depending on its proximity to the AV node. Wandering pacemaker is not considered pathologic and can often be seen in young, healthy individuals (eg, athletes). Rarely is this symptomatic; therefore, no treatment is typically indicated. (See "ECG tutorial: Atrial and atrioventricular nodal (supraventricular) arrhythmias", section on 'Wandering atrial pacemaker'.)

ELECTROPHYSIOLOGIC AND ELECTROCARDIOGRAPHIC FEATURES

Electrocardiographic features — The mass of the crescent-shaped SA node is too small to create an electrical signal that can be routinely recorded on the surface electrocardiogram (ECG). As a result, we generally infer SA nodal activity from the ECG appearance of the response to that activity (ie, P waves). Although signal-averaged ECGs focusing on the P wave can identify sinus nodal dysfunction, this is not a commonly used clinical tool due in large measure to problems in registration, particularly during a sinus arrhythmia, and because many of the conduction events are transient and are missed by this technique [6]. Hence, common signs of SA nodal dysfunction on the surface ECG include marked sinus bradycardia that is disproportionate to an individual's metabolic or conditioned state and periods of absent P waves (waveform 6 and waveform 7 and waveform 8). In order to more clearly delineate the mechanism underlying periods of SA nodal dysfunction, electrophysiology studies can be attempted, but these are not commonly performed because understanding the precise mechanism for SA nodal dysfunction very infrequently changes patient management decisions.

Patients with BRASH syndrome have ECG findings of sinus arrest, AV block, and may or may not have other signs of hyperkalemia including peaked T waves and QRS prolongation [2,3]. (See 'Etiology' above.)

Electrophysiologic features — SA nodal activity can be recorded using an intracardiac electrode catheter [7-12]. The following observations were noted in a study of 38 patients with severe symptomatic sinus node dysfunction (SND) in whom intracardiac SA nodal electrograms could be recorded [12]:

●Nine had complete SA block, unidirectional in seven and bidirectional in two.

●13 had normal 1:1 SA conduction.

●Nine had second degree SA exit block.

●Seven patients had no recordable sinus nodal electrogram, which could result from technical failure or from SA nodal quiescence.

Electrophysiologic techniques utilizing both pacing at different rates and extra stimuli can be used to assess several characteristics of SA nodal function, including postpacing escape times, SA nodal recovery time, SA nodal conduction time, SA nodal refractoriness, and the assessment of the "intrinsic" heart rate and other parameters following pharmacologic and autonomic intervention. (See "Invasive diagnostic cardiac electrophysiology studies", section on 'Electrocardiographic and electrophysiologic recordings'.)

SND can coexist with AV node conduction disturbances. This has been termed binodal disease. Electrophysiologic studies suggest that individuals with binodal disease also commonly have intra-atrial conduction disturbances [13]. (See "ECG tutorial: Atrioventricular block".)

TREATMENT — Therapy begins by establishing whether the patient is experiencing symptoms as a result of sinus node dysfunction. Asymptomatic patients with SA nodal pauses, arrest, or exit block often do not require treatment. Patients with symptoms are often treated by discontinuing possible offending drugs and/or with a permanent pacemaker. (See "Sinus bradycardia", section on 'Management' and "Sinus node dysfunction: Treatment".)

Patients with BRASH syndrome require prompt therapy to interrupt the vicious cycle of worsening bradycardia causing worsening renal function and worsening hyperkalemia. Management includes treatment of hyperkalemia, management of bradycardia (including withdrawal of drugs inhibiting sinus node and AV node function), and if needed, fluid resuscitation [2,3]. (See "Treatment and prevention of hyperkalemia in adults" and "Beta blocker poisoning" and "Evaluation of and initial approach to the adult patient with undifferentiated hypotension and shock".)

SUMMARY AND RECOMMENDATIONS

●A sinus pause or arrest is the transient absence of sinus P waves on the electrocardiogram (ECG) that may last from two seconds to several minutes. This abnormality is an alteration in discharge by the SA pacemaker; as a result, the duration of the pause has no arithmetical relationship to the basic sinus rate. The pause or arrest often allows escape beats or rhythms to occur. (See 'Sinus pause or arrest' above.)

●SA nodal exit block occurs as a result of interference with the delivery of impulses from the SA node to its neighboring atrial tissue. This results in the absence of a P wave on the surface ECG since the P wave is the only manifestation of SA nodal activity. Following the convention for atrioventricular nodal block, SA nodal exit block can be classified as first, second, or third degree. (See 'SA nodal exit block' above.)

●Other variations of SA nodal dysfunction which are generally non-pathologic include sinus arrhythmia and wandering atrial pacemaker. (See 'Variants of SA nodal dysfunction' above.)

●Asymptomatic patients with SA nodal pauses, arrest, or exit block often do not require treatment. Patients with symptoms are often treated by discontinuing possible offending drugs and/or with a permanent pacemaker. (See 'Treatment' above.)

ACKNOWLEDGMENT — The UpToDate editorial staff acknowledges Alan Cheng, MD, who contributed to an earlier version of this topic review.