Single-lead electrocardiogram (ECG) showing lead V1 in type I (Wenckebach type) sinoatrial block

There is a 3:2 SA block, resulting in group beating of pairs of sinus beats. The P-P interval during the pause has a duration (1040 msec) that is less than two P-P cycles (1360 msec); this finding distinguishes this arrhythmia from type II SA block in which the pause duration is the same as that of two P-P cycles.

Normal rhythm strip in lead II. The PR interval is 0.15 sec and the QRS duration is 0.08 sec. Both the P and T waves are upright.