Atrioventricular conduction versus heart size from mouse to whale

In normal sinus rhythm, the atrioventricular (AV) conduction system introduces an appropriate delay between atrial and ventricular excitation and contraction. This delay, the AV or PR interval, increases with the size of the animal and, thus,of the heart. This increase, however, is relatively small in comparison with the difference in the sizes of the body and the heart. For instance, the PR interval is only twice as long in mammals such as the elephant or the whale as it is in human beings. Comparison of the AV nodal electrophysiologic properties with the size of the heart in a number of mammalian species raises rather interesting questions regarding the functional properties of the AV conduction system