Mechanisms of intermittent ventricular parasystole due to type II second degree entrance block

Three patients with intermittent ventricular parasystole are reported in whom the presence of second degree entrance block of type II or a type similar to that was shown. In all the patients, when a sinus beat occurred within a certain (the first) critical period after the preceding ectopic beat, the parasystolic focus was protected from this sinus impulse. When, on the other hand, a sinus beat occurred beyond another (the second) critical period after the ectopic beat, this sinus impulse reached and discharged the focus without an appreciable conduction delay. In one patient the second critical period was equal to the first one, while, in the other two, the second one was longer than the first one. In these two patients, when a sinus beat occurred between the two critical periods after the ectopic beat, this sinus impulse reached and discharged the focus after marked delay, and thereafter became a manifest or concealed re-entrant ventricular extrasystole. On the basis of these observations, an attempt was made to clarify the difference in mechanism between type I and type II second degree entrance block.