Effect of ventricular aberrancy on fibrillation threshold

VFT's were determined in 12 open-chest dogs at epicardial sites in the right and left ventricles (RV and LV) following normally or aberrantly conducted beats. The normal beats were produced by right atrial pacing, and the aberrant beats by surgical RBBB or ventricular pacing at an RV or LV site. The mean VFT following normal beats was 21.2 plus or minus 1.8 ma. in RV and 23.0 plus or minus 2.7 ma. in LV. The mean VFT following aberrant beats of RBBB was 21.3 plus or minus 2.6 ma. in RV and 25.0 plus or minus 2.8 ma. in LV. The difference between the mean VFT of normal beats and that of aberrant beats was not statistically significant. The mean values of VFT's determined in RV or LV following aberrant beats produced by pacing of the contralateral ventricle were not significantly different from those of the normal beats. The mean VFT was 22.9 plus or minus 3.1 ma. in RV and 20.1 plus or minus 2.1 ma. in LV. These results indicate that the aberrancy of ventricular beats per se is not associated with decreased VFT or increased ventricular vulnerability to fibrillation. The most predictable observation was that the mean VFT's were significantly lower in both ventricles when they were determined at the site of application of pacing stimuli. The mean values were 12.4 plus or minus 1.2 ma. in RV and 13.6 plus or minus 1.6 ma. in LV. This decrease in VFT may be due to slow conduction and increased asynchrony of recovery of excitability at or near the site of application of pacing stimuli.

Effects of unipolar cathodal and bipolar stimulation on vulnerability of ischemic ventricles to fibrillation

The effects of unipolar and bipolar stimulation on ventricular fibrillation threshold were studied during acute occlusion of the left anterior descending coronary artery in 13 anesthetized dogs. Values for ventricular fibrillation threshold were determined by delivering trains of rapid bipolar or unipolar pulses (100/sec) during the vulnerable period. The mean threshold value was found to be 13.0 ma for bipolar, 13.9 ma for unipolar anodal and 21.0 ma for unipolar cathodal stimulation. Ventricular fibrillation threshold was significantly lower (P less than 0.01) with both unipolar anodal and bipolar stimulation than with unipolar cathodal stimulation. In these animals, the first premature beats induced by the rapid stimuli occurred significantly earlier with unipolar anodal and bipolar stimulation than with unipolar cathodal stimulation. The effect of competition of unipolar or bipolar pacing stimuli with normally conducted ventricular beats was also studied in a group of 16 dogs. Repeated trials of competitive pacing during coronary occlusion showed that the incidence of ventricular fibrillation was significantly greater (P less 0.05) with bipolar pacing (36 percent) than with unipolar cathodal pacing (15 percent). These results indicate that bipolar pacing is potentially more dangerous than unipolar cathodal pacing and suggest that the incidence of pacemaker-induced ventricular fibrillation might be further reduced by the use of unipolar cathodal stimulation during acute myocardial infarction.