Evolution and scaling of atrioventricular conduction time in mammals: part 1.

Scaling can be defined as the adjustment of a structure, a function, or an organ to the size of the mammalian body. An example is the size of the heart in relation to the size of the body. The duration of the PR interval on the electrocardiogram (atrioventricular delay) in relation to the size of the heart is a perplexing example of scaling. During evolution, mammalian species changed their shape, size, and function while adapting to the habitat in which they had to live and survive. This review deals with the problem of the apparent mismatch in scaling of the atrioventricular delay (PR interval) in relation to the size of the mammalian heart from mouse to whale.