Sinoatrial exit block occurs when the action potential initiated by the sinoatrial node is inhibited or completely blocked before it is able to leave the SA node and reach the atrium, and thus no P wave will appear on the ECG.
First-degree sinoatrial exit block occurs when there is a delay from the time the SA node creates an action potential and the time the atrium is actually depolarized. This is not recognizable on the 12-lead ECG because the time that SA node fires is not able to be determined. The ECG may simply show sinus rhythm or sinus bradycardia. This is different than a first-degree atrioventricular, or AV, block, which shows a prolonged PR interval.
Second-degree sinoatrial exit block is categorized into type I and type II.
Second-degree SA exit block type I (Wenkebach) demonstrates progressive shortening of the RR or PP intervals until a P wave is blocked in the SA node, which would not appear on the ECG. A “sinus pause” ensues afterward and would be shorter than two of the preceding RR intervals. Here is an example:
Second Degree Type I Sinoatrial Exit Block (Wenckebach)
Second-degree SA exit block type II occurs when there are consistent RR and PP intervals, then a P wave is blocked in the SA node, also not seen on the ECG. The subsequent sinus pause here is an exact interval of the preceding RR intervals, usually two times.
Second Degree Type II Sinoatrial Exit Block
Third-degree sinoatrial exit block occurs when no SA nodal action potentials are able to leave the SA node. No P waves are seen on the ECG. When a junctional escape ensues, the rhythm may be confused with a junctional rhythm. If no junctional escape rhythm is present, a long pause resulting in asystole and cardiac arrest can occur.
1. Chou’s Electrocardiography in Clinical Practice: Adult and Pediatric, 6e
2. Surawicz B, et al. AHA/ACCF/HRS Recommendations for the Standardization and Interpretation of the Electrocardiogram. Circulation. 2009;doi:10.1161/CIRCULATIONAHA.108.191095.