[Recognition of first-degree sinoatrial block in the standard electrocardiogram]

This report concerns a patient affected by Sick Sinus Syndrome, whose ECG showed many episodes of second degree sino-atrial (SA) block. The long pause was always less than twice the normal PP interval, but the diagnostic criteria for Wenckebach SA block were not met. The atrial cycle, indeed, did not shorten progressively before the block. Furthermore even Schamroth's diagram, obtained by plotting the length of each PP cycle against the length of the following cycle, was not suggestive of Wenckebach SA block. In type 2 (Mobitz) second degree SA block, the long pause may appear slightly shortened if first degree SA block is also present. In such a situation the SA conduction time is longer than normal, but when a sinus impulse is blocked, i.e. does not reach the atrium, the following sinus impulse may travel the SA junction in a shorter time. The first P wave after the block thus occurs earlier, thereby reducing the duration of the long pause. By the same mechanism, the PP cycle that follows the long pause appears longer than expected. Both these phenomena have been observed in the reported case. First degree SA block appears to be likely when the standard ECG shows the following features: 1) the 2nd degree SA block is not classifiable as type 1 (Wenckebach); 2) the pause that includes the blocked sinus impulse measures always less than twice the sinus cycle; 3) the PP interval that follows the prolonged pause is slightly longer than the one that precedes the pause.