Second degree entrance block with supernormal conduction in intermittent ventricular parasystole

A patient with intermittent ventricular parasystole is reported in whom the presence of second degree entrance block with supernormal conduction was suggested for the first time. In this patient, ventricular extrasystoles with variable coupling frequently occurred. The QRS configuration of the extrasystoles was different from that of the parasystolic beats. When extrasystoles did not occur, the parasystolic beat was never seen because the conducted sinus impulse always reset the parasystolic rhythm. When an extrasystole occurred 0.52 sec or more after the preceding sinus beat, this extrasystolic impulse also reset the parasystolic rhythm. On the other hand, when an extrasystole occurred between 0.47 and 0.51 sec after the sinus beat, the parasystolic focus was protected from this extrasystolic impulse. When, however, an extrasystole occurred in a short terminal portion of the T wave of the preceding sinus beat, this extrasystolic impulse reset the parasystolic rhythm again, suggesting entrance block failure during the supernormal phase.