SUMMARY We studied the difference between myocardial vulnerability to arrhythmias caused by cathodal, anodal, and bipolar stimulation in 29 dogs with partial right coronary artery ocdusion. We used 2-msec duration stimuli of up to 8 mA to determine the ventricular vulnerable periods, their relationship to the refractory periods, and the fibrillation or multiple response thresholds for unipolar anodal and cathodal stimulation after two premature ventricular contractions. The vulnerable period for arrhythmias began at the end of the respective refractory periods and terminated at a specific time within the cardiac cycle. Within this period the arrhythmia and excitation thresholds were equal. Because shorter refractory periods were obtained with anodal stimulation than cathodal, the vulnerable periods for anodal stimulation were longer. This indicated that the vulnerable periods for bipolar stimulation also would be longer than for unipolar cathodal stimulation since bipolar and anodal refractory periods are equal when the cathode and anode are of similar surface area. Results from seven of the experiments showed that a dual focus of excitation, which can only occur with bipolar stimulation, did not make the ventricle more vulnerable to arrhythmias than did unifocal stimulation. These results indicate that the difference between the arrhythmia vulnerability to unipolar cathodal, anodal, and bipolar stimulation is dependent on the relationship between their excitability characteristics, i.e., their strength-interval curves.THE EXISTENCE of a brief period in the ventricular cycle during which electrical stimulation can induce arrhythmias such as ventricular fibrillation was demonstrated in animals by Wiggers and Wegria, 1 and often has been verified.
-3The extensive clinical use of cardiac stimulation has made the circumstances under which such electrical stimulation may cause an arrhythmia of importance. Although most investigations indicate that the minimum energy required to precipitate ventricular fibrillation in the normal myocardium exceeds that delivered by a clinical cardiac pacemaker, there have been clinical reports of ventricular fibrillation and tachycardia associated with a pacemaker stimulus falling in the "vulnerable period" of the cardiac cycle. This suggests the presence of pathophysiological and pharmacological factors that lower the threshold to arrhythmias such as myocardial ischemia, sympathetic activity, and premature ventricular beats.
4~6Aside from these factors, the effects of electrode and stimulus characteristics on myocardial vulnerability are not yet well understood.A recent analysis of cases of ventricular fibrillation and tachycardia attributed to pacemaker stimuli found that most such arrhythmias arose in patients undergoing bipolar rather than unipolar cathodal stimulation. 7 In our own review of the literature, 8 we found 26 cases in which the initiation of ventricular fibrillation or tachycardia (defined as at least a run of 3 premature ventricular contractions) was illustrated. In ...