The magnitude of the electromotive force of canine ventricular myocardium

The isolated and perfused dog heart was placed in a cubic container filled with Tyrode's solution. Ventricular ectopic beats were produced by electrical stimulation of the left ventricular wall, and initial QRS vectors of these beats were determined with orthogonal leads from the surface of the container. At the same instants, the activated area on the epicardial surface was measured by means of a large number of contiguous bipolar leads from the epicardial surface. The QRS vector and the activated epicardial area were found to be nearly porportional. By use of these results and a calibration system with artificial dipoles, the double layer moment of the ventricular activation wave was calculated as 0.13 mA.cm per unit area. This value corresponds to 60% of the maximal possible strength of the tissue electromotive force. Lowering the conductivity of the surrounding solution increased the QRS voltage but not as much as the potential caused by a constant-current dipole within the solution. The relationship between the QRS voltage and the conductivity of the medium was analyzed by a simplified model of the system and was found to correspond approximately to that of a constant-current source within a spherical heart with a resistivity 2 to 3 times that of Tyrode's solution.