Observations on the Ventricular Dysrhythmia Associated with Atrial Fibrillation in the Dog Heart.

• The mechanism of irregular ventricular responses to atrial fibrillation has not been established. If atrioventricular transmission occurred via a single path with uniform properties, ventricular cycle lengths would be governed by the refractory period of the path and the temporal pattern of the input supplied by the fibrillating atria. Since the frequency of stimuli available to the A-V conduction system during atrial fibrillation is undoubtedly high, activation of the system would be expected at, or shortly after, each moment at which the system became excitable. Only slight variation of ventricular cycle lengths would be expected under these circumstances. The gross irregularity of ventricular rhythm usually associated with atrial fibrillation excludes such simple behavior of the A-V transmission system. It is known that the various ventricular cycle lengths which occur during atrial fibrillation are not uniformly distributed.' Frequency distribution curves of ventricular cycle lengths often show multiple peaks indicating the more frequent occurrence of certain cycle lengths. This finding is further evidence of the structural and/or functional complexity of the A-V transmission system. The phenomenon of incomplete penetration of the transmission system which has been called "concealed conduction" has been postulated to explain the ventricular irregularity during atrial fibrillation.' It is evident that if occasional or frequent atrial impulses enter