Direct Current Make and Break Thresholds for Pacemaker Electrodes on the Canine Ventricle

Thresholds to anodal make, anodal break, cathodal make and cathodal break have been studied throughout the cardiac cycle in 13 dogs under pentobarbital anesthesia. Unipolar direct current pulses were applied through epicardial pacemaker electrodes to the left ventricle. Make and break responses were separated by letting the break and make occur in the refractory periods of the following and preceding cycle, respectively. Four distinct threshold interval curves were obtained from each of 17 electrodes. The anodal break and cathodal break curves showed an early diastolic dip. Dips were deepest in the anodal break curves. These also had the shortest effective refractory period. Thresholds for cathodal make and anodal make first dipped steeply, then gradually sloped down towards end-diastolic levels. The magnitude of end-diastolic mean threshold values increased in the following order: cathodal make, anodal make, cathodal break, anodal break. Comparison of threshold interval curves for make and break responses with those for shorter rectangular anodal and cathodal curves throughout the cardiac cycle supports the hypothesis that the threshold behavior to a short rectangular pulse is determined by whichever of two thresholdsis lowest: make or break at that particular cycle interval.