Evolution and scaling of atrioventricular conduction time in mammals: part 2

This review in The American Heart Hospital Journal is published in two parts. Part 1 deals with the role of scaling in (patho)physiology and anatomy, or the function and structure of organs and organ systems in relation to body size of mammalian species. An intriguing aspect of scaling is the relation between heart size and the duration of atrioventricular (AV) conduction (Winter 2006 issue-Am Heart Hosp J. 2006;4:53-57.). Part 2 offers a simple mathematic explanation of AV conduction time scaling based on AV hemodynamics in mammalian species of different sizes. 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 function 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.