Intermittent atrioventricular block: what is the mechanism?

Discussion The rhythm strip shows sinus rhythm with right bundle branch block morphology. There are premature ventricular contractions (PVCs); the coupling intervals of the PVCs are variable, indicating the possibility of a parasystolic focus. However, the diagnosis of parasystole cannot be made with confidence without a long rhythm strip to measure the interectopic intervals. Figure 2A shows sinus rhythm at a cycle length of 1,000 ms. A PVC occurring at a coupling interval of 440 ms is not resetting the sinus node, as evident by a full compensatory pause of 2,000 seconds. Potential mechanisms are that the PVC