An Unusual PR Interval Alternans During 2:1 Atrio-Ventricular Block

In a case of a 2:1 second degree A-V block during sinus rhythm, PR intervals of two different durations occur, either separately or alternating beat-by-beat. The longer intervals are not caused by concealed conduction in the A-V junction of the preceding blocked impulses and the shorter ones are not due to supernormal conduction induced by the blocked impulses. This primary PR interval alternans is alternating from a pothophysiological point of view; it only concerns every other impulse alternately traveling along the faster and slower A-V functional pathway.