This ECG is an example of non-conducted atrial premature beats causing group beating that simulates type I 2nd degree SA or AV block. The pause between the groups is 1.52 seconds. This is less than twice the interval between the two QRS complexes of each group (2 x 0.9 = 1.8 seconds) and is suggestive of type I block with 3:2 conduction. The first PR interval of each group is 0.20 seconds and that the second is 0.30 seconds. This lengthening suggests type I AV block. However, there is no sinus P wave during the pause. This eliminates AV bloc, raising the possibility of type I SA block. Note however, the subtle deflection that distorts the descending limb of every other T wave (arrows). These are the P waves of non-conducted atrial premature beats and are easily overlooked. They are coupled to the preceding P wave by an interval of 0.70 seconds and are the cause of the group beating. Had these atrial premature beats not been present, the progressive prolongation of the PR interval might have culminated in a non-conducted sinus beat indicating type I 2nd degree AV block.