Atrial rhythm during ventricular fibrillation in the dog

Depolarization of the atrioventricular junctional tissues and of the atrial septum was examined in the perfused dog heart before and during ventricular fibrillation by (1) recording the potentials from the junctional tissues in the regions of the interatrial and interventricular septum and examining the relationship of activity at these sites to atrial depolarization, (2) computing histograms and autocorrelograms of atrial firing intervals to study atrial rhythmicity, and (3) plotting the sequence of atrial septal depolarization. The junctional tissue was generally randomly depolarized by the fibrillating ventricles. The histograms and autocorrelograms indicate that during ventricular fibrillation the atrial depolarization intervals do not remain constant, but vary widely. This seems to be due to the retrograde excitation from the junctional tissues. Plots of the depolarization sequences of the interatrial septum also indicate that retrograde depolarization takes place. The junctional tissues decrease the number of impulses that can pass from ventricle to atrium, and they similarly decrease the number of impulses that pass in an antegrade direction during atrial fibrillation.