A Holter recording obtained from a patient with atrial fibrillation showed ventricular extrasystoles often in bigeminal rhythm. Most extrasystoles were followed by a long return cycle, and only in a few instances the postextrasystolic interval was short. The latter phenomenon was interpreted as a manifestation of poor retrograde concealed penetration of the ventricular impulse into the atrioventricular (A-V) junction: accordingly, an ensuing relatively early fibrillation impulse reached the ventricular chamber, since it did not find the A-V node refractory. These events are similar to what happens in interpolated ventricular extrasystoles occurring during sinus rhythm, the absent or minimal concealed retrograde penetration of the ectopic impulse into the A-V node being necessary to permit anterograde conduction of the ensuing sinus impulse.
Analysis of the recording also revealed that a very long (>2 second) interval between two consecutive narrow beats only occurred after an “interpolated” extrasystole. This was interpreted with the same mechanism underlying the “postponed compensatory pause” observed at times after interpolated ventricular extrasystoles during sinus rhythm: the minimal or nil penetration of the ventricular ectopic impulse into the A-V junction, followed by conduction of an ensuing early atrial impulse, “shifts to the right” the A-V nodal refractory period, preventing conduction of several further supraventricular impulses and generating a pause.
Both interpolated ventricular extrasystoles and the phenomenon of “postponed compensatory pause” are, thus, conceivable during atrial fibrillation, although no definite demonstration is possible.