Time dependence of unipolar cathodal and anodal strength-interval curves.

Following pacemaker electrode attachment, greater ventricular vulnerability to arrhythmias exists in an acutely ischemic ventricle with a unipolar anodal than cathodal stimulus falling on the T wave, due to the shorter refractory period exhibited by an anodal stimulus. In order to determine the relationship between anodal and cathodal refractory period at various times following endocardial electrode insertion, ventricular strength-interval curves were determined in nine dogs. With stable electrode position, serial measurements were made for a mean duration of 44 days. It was observed that during the first five days, the anodal refractory period was significantly shorter than cathodal, but within 15-20 days this relationship was reversed in all dogs. The longer anodal refractory period was observed up to the end of each study. These results suggest that due to shorter anodal refractory period, greater vulnerability to arrhythmias may occur with an anodal than cathodal stimulus in the initial 15-20 days following electrode insertion.