Sinus Exit Block does looks very much the same as Sinus Arrest with one important distinction. With SA Block the R-R interval measurement is within plus or minus 2 small boxes.
Second degree type I sinoatrial exit block (Wenckebach) demonstrates progressive shortening of the R-R or P-P intervals until a P wave is blocked in the SA node (which would not appear on the ECG). A "sinus pause" ensues afterward which would be shorter than two of the preceding R-R intervals.
Second degree type II sinoatrial exit block occurs when there are consistent R-R and P-P intervals, then a P wave is blocked in the SA node (not seen on the ECG). The subsequent "sinus pause" is an exact interval of the preceding R-R intervals (usually two times).
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 and the rhythm may be confused with a junctional rhythm. If there is no junctional escape rhythm that is present, a long pause can occur resulting in asystole and cardiac arrest.
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