This tracing is an example of non-conducted atrial premature beats causing group beating that mimics type I 2nd degree AV block. In this tracing, the P waves have quite similar waveforms and there is progressive prolongation of the PR interval (0.26, 0.32 and 0.34 seconds) prior to the non-conducted P wave (arrow). In addition, the pause between the groups (1.24 seconds) is less than twice the preceding RR interval (2 x 0.74 = 1.48 seconds). These features all suggest type I 2nd degree AV block. However, note that the PP interval preceding the blocked P wave is 0.40 seconds whereas the PP intervals between the 1st and 2nd, and between the 2nd and 3rd P waves in the group, i.e. the sinus PP intervals, are 0.72 seconds. This indicates that the blocked P waves are actually non-conducted atrial premature beats. The similarity of their waveform to that of the sinus P waves suggests that they may be arising from a focus that is located in close proximity to the sinus node. As before, had the atrial premature beat not occurred, the progressive prolongation of the PR interval might have culminated in a non-conducted sinus beat.