Intrinsic Deflections, Local Excitation and Transmembrane Action Potentials

The steepest rise of the action potential obtained by an ultramicroelectrode from the most superficial subepicardial muscle fiber in situ of the tortoise was compared with the intrinsic deflection of the unipolar direct electrocardiogram and the surface electrocardiogram obtained from approximately the same region. No specific point on the intrinsic deflection, such as the highest or lowest point, could be regarded as indicating the arrival of excitation. Surface electrocardiograms obtained by the capillary ultramicroelectrode could not be regarded as simple second derivatives of the action potential, they resembled more unipolar direct electrocardiograms.