EVALUATION OF CARDIAC DRUGS IN THE PRESENCE OF AN ELECTRICAL PACEMAKER

Quantitative study of the supernormal phase of ventricular excitability in man

The supernormal phase of excitability of the human heart was studied by means of fixed rate endocardial pacing in 11 patients with acute and chronic bradyarrhythmias. Ten of the eleven patients manifested a supernormal phase. The duration of the phase increased with increasing intensity of subthreshold stimuli and ranged from 91 to 148 percent of the Q-T interval. Subthreshold stimuli of a wide range of intensity could elicit a full response. Two types of supernormal phase, early and late in relation to the cardiac cycle, were observed. The latter was attributed to the summation of subthreshold stimuli with either spontaneous phase 4 depolarization of a ventricular ectopic pacemaker or atrial depolarization potentials. Its possible connection with Wedensky facillitation was suggested. The ventricle was less excitable after an ectopic beat than after a normally conducted beat.

Effect of stimulation site on ventricular threshold in dogs with heart block

Ventricular pacing thresholds were measured in barbiturate-anesthetized dogs with complete heart block using an oscilloscope and current probe. All electrode and physiologic variables were carefully controlled in order to compare the effects of stimulus duration and site of ventricular threshold energy requirements. Left ventricular intramyocardial stimulation, at all stimulus durations under 5 msec., required less current and energy, and at all stimulus durations under 2 msec. less voltage, than either right ventricular endocardial or right ventricular intramyocardial stimulation. Chronaxy and polarization losses were lowest and impedance highest with left ventricular intramyocardial stimulation. The two right ventricular sites were almost equivalent electrically. Earlier studies which reported lower thresholds with endocardial stimulation than with intramyocardial stimulation were biased in favor of the endocardial site by lack of control of electrode variables including: material, configuration, cross-sectional area, and epicardial versus intramyocardial location.