Determination of Myocardial Depolarization and Repolarization Times Using the Unipolar Ventricular Evoked Potential: Contrasting Effects of Stimulus Interval and Isoprenaline in the Isolated Perfused Rahhit Heart

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Abstract
The effects of changes in stimulus interval and the infusion of isoprenaline upon myocardial depolarization and repolarization times have been determined using the unipolar ventricular evoked potential. An isolated perfused rabbit heart was the experimental vehicle and the stimulus-to-R wave (St-R) and R wave-to-end (R-E) of complex time intervals were used as measures of depolarization and repolarization times, respectively. Variation in the stimulus interval was shown to have highly statistically significant effects upon both of the parameters of the unipolar paced evoked potential that were investigated. Myocardial repolarization time is increased at longer intervals, while activation time is reduced. Isoprenaline was found to reduce the duration of the R-E interval of the unipolar ventricular evoked potential when infused at a constant rate at a fixed stimulus interval. At the maximum concentration of the drug, when stimulus interval was just shorter than the intrinsic RR interval, it was estimated that one third of the total change in the R-E interval was due purely to the effects of decreased stimulus interval. It proved impossible to identify any effect of isoprenaline infusion upon the St-R interval. The absence of any effect of isoprenaline on the St-R interval of the evoked potential suggests that it may serve as a monitor of purely stimulus interval dependent variations of the signal.