Evolution and scaling of atrioventricular conduction time in mammals. [Pt. 2]

Abstract

Changes of the PR interval (atrioventricular delay) in relation to changes of heart size in mammalian species (scaling) confront us with a perplexing lack of understanding of an essential funetion of the heart. The PR interval controls the duration of late diastolic blood flow from the atria to the ventricles. There is good evidence that blood flow velocity is fairly constant in all mammalian species, meaning it does not scale. Also, in all mammalian species, the mitral orifice does not offer any resistance to atrioventricular blood flow. It follows that blood flow duration is directly dependent on the distance between the atria and the ventricles. Although the heart is not a cube, this distance is defined as the third root of heart mass. The third root of any value changes little in relation to the value itself. This simple mathematic fact is an easy explanation for PR interval behavior in relation to heart and/or body size. However, the atrioventricular intranodal electrophysiology of this behavior is not known.