Sinoatrial Exit Block

Abstract

The major electrocardiographic expressions of sinus node dysfunction (SND) are the presence of an abnormal sinus bradycardia and the occurrence of a “sinus pause.” A “sinus pause” is the occurrence on ECG of a sudden pause in the underlying sinus rhythm that can be due to the failure of the sinoatrial (SA) node to initiate (sinus arrest) or to conduct the impulse (sinus exit block). Sinoatrial exit block can be divided into three types: first, second, and third degrees. The second-degree exit block is further classified into type I (SA block with Wenckebach conduction) and type II (SA Mobitz II). SA block is included in a broader clinical scenario called “sick sinus syndrome” (SSS) that may consist in inappropriate sinus bradycardia, SA exit block or sinus arrest, prolonged sinus arrest with failing ectopic pacemaker, persistent atrial or atrioventricular escape rhythm, episodes of alternating supraventricular tachyarrhythmias with bradyarrhythmias, long pause following cardioversion of atrial tachyarrhythmia, or chronotropic incompetence. Symptoms are related to low cardiac output that occurs with brady- and/or tachyarrhythmias. The causes of SSS i nclude intrinsic and extrinsic factors. The cornerstone for the diagnosis of SSS is the correlation of end-organ perfusion symptoms with the occurrence of bradyarrhythmias. When an SSS is suspected, prolonged cardiac monitoring should be considered. Exercise testing, intracardiac electrophysiologic tests, and external or internal long-term devices are helpful in SSS diagnosis. There are no medications that reliably increase the heart rate in patients with bradyarrhythmias. Cardiac pacing therapy through an artificial pacemaker is the only choice, and an early identification of a potential reversible cause is always the first step of treatment.