Ventricular Response in Atrial Fibrillation

Abstract

The ventricular response in atrial fibrillation is determined by the long refractory period of the AV junction. Since an atrial impulse is always available for transmission to the ventricles, a regular ventricular rhythm would be expected, the rate of which would reflect the duration of a stable junctional refractory phase. The irregularity of the ventricular action associated with atrial fibrillation, therefore, indicates changes of refractoriness of the AV junctional tissues from cycle to cycle. This can best be attributed to varying degrees of penetration of "blocked" atrial impulses into parts of the AV junction, and to the effect of such concealed conduction on the propagation of subsequent impulses. The following facts are pointed out and illustrated as evidence of concealed AV and VA conduction during atrial fibrillation: (a) Occurrence of a "compensatory pause" following a ventricular premature systole. (b) Failure of an AV nodal escape to appear at the expected time due to concealed discharge of a subsidiary AV nodal pacemaker by a penetrating atrial impulse. (c) Acceleration of the ventricular rate when atrial fibrillation changes to flutter (elimination of concealed conduction with slowing of the atria). (d) A tendency for two or more long cycles to occur in succession (each containing one or more concealed responses). The reasons for the variations of the refractory period at different levels of the AV junction as a result of concealed conduction are analyzed. Their complex interplay may readily account for the over-all irregularity of the ventricular response associated with atrial fibrillation.