Surface Activation of Guinea Pig Ventricle Determined by Intracellular Electrodes

Abstract

Intracellular micro-electrodes have been used to detect the arrival of the activation process in myocardial fibers lying near the epicardial surface of the intact guinea pig ventricle. Arrival of excitation was indicated by the steepest part of the upstroke of the transmembrane action-potential curve obtained. The times of arrival of the activation process at various points on the surface of the ventricles were measured with respect to selected points on the simultaneous ECG, which served as a standard of reference. The scale on the time axis was greatly magnified (paper speed = 75 cm/sec.) in order to determine these times with precision. The ‘relative activation times’ thus obtained indicated that the right ventricle tended to be activated earlier than the left, except on their dorsal aspects, where there was little difference. There were, however, many exceptions to this general pattern, and rather large differences in relative activation times were frequently observed between contiguous regions. The high paper speeds used revealed three different types of upstroke in the monophasic records. An association was observed between the relative activation time of a region and the type of upstroke. An explanation of these findings is offered and a supporting experiment described.