Atrioventricular block with 4:2 conduction pattern: what is the mechanism?

We present the case of a 72-year-old man who was admitted because of acute ST-segment elevation myocardial infarction and presented with second-degree atrioventricular block with 4:2 conduction. We propose 3 alternative explanations for such an unusual conduction pattern: (1) supernormal conduction, (2) 2-level block with 4:1 conduction ratio in the upper level and 3:2 conduction ratio in the lower level, and (3) presence of 2 populations of Purkinje cells in the remaining, critically injured fascicle, one with the phase-3 block and the other with slow diastolic depolarization, leading, during a pause after the first nonconducted P, to phase-4 block.