Anodal Excitation of Cardiac Muscle

The strength-interval curve of dog ventricular myocardium has been measured with anodal and cathodal stimulation. During diastole the anodal threshold is higher than the cathodal. As anodal stimuli are applied progressively earlier the anodal threshold first rises above and then falls to levels below the anodal diastolic threshold. During most of the relative refractory period the anodal threshold is lower than the cathodal threshold. At all times during the late relative refractory period and throughout diastole excitation of double origin (anodal and cathodal) is evoked by sufficiently strong stimuli; this simultaneous origin of excitation at two points does not evoke fibrillation. During the early relative refractory period, however, only the anode is able to excite. Differences between anodal and cathodal thresholds are not attributable to asynchronous repolarization at the two electrode sites. The ‘no-response’ phenomenon occurs only when the anodal threshold is markedly lower than the cathodal.