Increased Excitability of the Heart Induced by Electrical Stimulation in the Absolute Refractory Period

¹ Cardiac Physiology Laboratory, Hadassah University Hospital, Jerusalem, Israel

A series of experiments were designed to study the possible effects of electrical stimuli, delivered during the absolute refractory period, on the threshold of excitability in the relative refractory period. They were carried out in dogs with complete A-V block or with intact conduction system. It was found that such stimuli, which previously were regarded as ineffective, may convert a subthreshold current to a depolarizing one. This effect was shown to be dependent on the time-relation between the "ineffective" and the test stimuli from the driving stimulus and also on the strength of the stimulations. The mechanism of the effectiveness of the ineffective stimulus was demonstrated to be a threshold-reducing, locally induced, anodal, all-or-none phenomenon.