A first-degree, or 1 st degree, AV block is indicated on the ECG by a prolonged PR interval, as seen below. Recall that the P wave indicates atrial depolarization, initiated by firing of the SA node.

This ECG has both second degree AV block with 2:1 block and second degree type I AV ... Note the irregular R-R intervals making 3rd degree heart block unlikely. The rhythm on this ECG needs ...