Comparison of Cardiovascular Responses to Verapamil during Enflurane, Isoflurane, or Halothane Anesthesia in the Dog

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Abstract
The cardiovascular responses to increasing infusion rates of the slow C channel inhibitor, verapamil [an antiarrhythmic agent], were studied in 3 groups of dogs during either enflurane, isoflurane or halothane anesthesia. Control hemodynamic values and plasma samples were taken after 2 h of anesthesia with the given agent. Increasing infusion rates of verapamil were given to achieve a range of plasma verapamil levels up to .apprx. 500 ng/ml. Each infusion rate was administered for 30 min, at which time repeat measurements and plasma samples for verapamil were taken. Mean arterial blood pressure, cardiac index and left ventricular dP/dt decreased with increasing plasma verapamil levels in the enflurane and isoflurane groups compared with the control values. The values for the enflurane-verapamil combination were significantly lower than those for the other anesthetics at comparable verapamil levels. Compared with enflurane, higher verapamil levels were required with isoflurane to achieve the equivalent degree of hemodynamic depression. A higher incidence of conduction abnormalities also was noted in the enflurane group. In the halothane group, the only significant change observed at these verapamil levels, achieved by continuous infusion, was a prolongation of the PR interval of the ECG. In this animal model, verapamil was least well tolerated by the cardiovascular system during enflurane anesthesia.