The mismatch between size and function of the heart

The mismatch between size and function of the heart can be exemplified by the relation between atrioventricular transmission time and heart size. While heart size may increase with a factor of 100,000,000 PR interval - on the surface ECG - will only increase with a factor 30. The curve of the PR interval versus the third root of heart weight has an S-shape. This form of relation cannot be explained on the basis of current electrophysiological theories . The contribution of the AV nodal delay to AV transmission time must diminish with increasing heart size. The mechanism of AV nodal delay is therefore not clear. The AV node as an unprotected pacemaker electrotonically modulated by atrial excitation may offer an explanation for the non-linear behaviour of the AV conduction system