Atrial fibrillation is one ofthe most common\ud cardiac arrhythmias in humans. It a1so occurs\ud quite frequent1y in dogs and horses. Comparative study of this arrhythmia may contribute to better understanding of the pathophysiologica1 mechanisms involved.\ud In this study, we present a quantitative analysis of atrial fibrillation in humans, dogs, horses, and in a kangaroo, making use of histograms and serial autocorrelograms of the ventricular rhythm with and without digitalis medication.\ud Increase in the size of the animal and thus in the size of the heart is accompanied by a decrease in ventricular rate. The ventricular rhythm was random in the dog, kangaroo, and man, but periodicity\ud was present in the horse. Digitalis decreased the ventricular rate in all species studied and enforced the periodicity in the horse.\ud The differences in the atrial excitation process,atrioventricular (AV) conduction, and ventricular behavior between the four species studied are small when compared with the differences in their heart size.\ud We conclude that in evolution, as far as the heart is concerned, cell size and morphology probably prevail over cell-function
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