Nonrandom ventricular rhythm in horses with atrial fibrillation and its significance for patients.

RR interval sequences during spontaneous atrial fibrillation in eight horses were analyzed as in previous studies in patients and dogs using histograms and serial auto-correlograms. In patients and dogs with spontaneous atrial fibrillation, ventricular rhythms were always random. In the horses, the histograms were skewed with median RR intervals of approximately 1,000 ms. A striking finding in these animals was the presence of long RR intervals up to 5,000 ms in duration. The shortest RR intervals lasted 400 to 600 ms. In contrast to findings in dogs and patients, the serial autocorrelograms showed periodicity that was reenforced by digitalis (n = 3), but eliminated by quinidine (n = 2) and atropine (n = 2). Quinidine and atropine eliminated the longer RR intervals, whereas digitalis increased the number of long RR intervals. In one horse, it was possible to measure intraarterial pressure, and large fluctuations in pressure were observed as the RR intervals varied from over 3,000 to less than 500 ms. It is postulated that these changes in blood pressure are associated with baroreceptor responses that may alter the electrophysiologic behavior of the atria and atrioventricular node. These changes cause the nonrandom patterns of ventricular rhythm in the horse. Because such very long RR intervals do not occur in human beings or dogs during atrial fibrillation, the random ventricular rhythm in these groups is maintained even during digitalis treatment.