Cardiac Extracellular Potentials

Extracellular recordings of normal action potentials, action potentials at reduced temperature, and action potentials initiated when the membrane was partially depolarized due to elevated extracellular potassium were made at varying distances from the surface of Purkinje strands of the dog left ventricle. As the electrode was withdrawn from the surface, the time between the positive and negative peaks increased; the amplitude of the simple biphasic wave form decreased markedly at first, and, thereafter, it decreased more gradually. The extracellular wave forms could be described by a single equation for all distances right up to the membrane surface even in the presence of interventions which drastically changed the intracellular potential and the conduction velocity. Wave forms of extracellular potentials at varying distances from the strand were computed from intracellular potentials and showed good agreement with the measured wave forms. Most major bundles in the dog conduction system have polyphasic extracellular wave forms rather than the relatively simple kind characteristic of the single strand. By using the equation that describes how the extracellular potential about a single strand varies as a function of distance, it was shown that the recorded polyphasic wave forms of the bundle studied resulted from the superposition of potentials from a number of asynchronously excited strands situated at varying distances from the recording electrode instead of from an underlying complex intracellular action potential. Extracellular recordings were chosen over intracellular recordings for this study, because they can indicate the presence of more than one excitation wave in a single record, thus permitting the resolution of the sequence of activation of the conduction system to its most elementary level.