Intermittent ventricular parasystole due to entrance block failure

An interaction between three pacemakers (one artificial) has made possible a number of observations on the nature of intermittent parasystole. Evidence shows that the intermittency is due to temporary failure of entrance block. The fixed coupling of the first beat of an intermittent series is observed to be a consequence of temporary entrance block failure rather than a manifestation of a more complex mechanism. The records also demonstrate that the reason for failure of the entrance block may be due to preferential penetration of the parasystolic focus early in its diastole.