Role of the compensatory pause in the production of concealed bigeminy

Abstract

In experiments conducted on anesthetized dogs, a single prolonged cardiac cycle caused an increase in the duration of the refractory period of the ventricular myocardium. The refractory period remained prolonged for one or two additional cardiac cycles after the basic cycle length was resumed. In concealed bigeminy, a 2:1 block in a reentry path is postulated. The compensatory pause that follows a manifest extrasystole results in a prolongation of the refractory period. If the refractory period exceeds the propagation time of the next ectopic impulse conducted through the reentry path, that ectopic impulse will be concealed. However, if the refractory period does not exceed the propagation time, another extrasystole will occur, and a bigeminal sequence will ensue. Consecutive compensatory pauses appear to exert a cumulative effect on refractory period duration, so that for critical propagation times, the bigeminal pattern will be terminated briefly when an ectopic impulse becomes concealed. A quadrigeminal pattern occurs when the propagation time in the reentry loop lies between the refractory period durations of the beat terminating a compensatory pause and the beat that occurs two sinus cycles later. In concealed bigeminy where an odd number of conducted beats greater than five lies between extrasystoles, the propagation time in the reentry path is just less than the refractory period of the ventricular cells during normal sinus rhythm at the prevailing heart rate. Random increases in propagation time or decreases in refractory period result in manifest extrasystoles.