Comparative effects of halothane, enflurane, and isoflurane at equipotent anesthetic concentrations on isolated ventricular myocardium of the ferret. II. Relaxation.

The effects of halothane, enflurane, and isoflurane on myocardial relaxation were compared in papillary muscles of the right ventricle of adult male ferrets at 30 degrees C. The sensitivity of cardiac relaxation to the loading conditions was determined by examining the time course of relaxation before, during, and after exposure to incremental concentrations of halothane (n = 9 muscles), enflurane (n = 9 muscles), and isoflurane (n = 9 muscles) in steps of 0.25 MAC up to 1.5 MAC of halothane and of enflurane and up to 2.0 MAC of isoflurane. Load sensitivity of relaxation was quantified by comparing force and time coordinates at the onset of the isometric relaxation phase in several afterloaded isotonic twitch contractions with relaxation of the isometric twitch. Load sensitivity of relaxation, which is of particular benefit during early rapid filling of the heart, was decreased in a dose-dependent reversible fashion by halothane, enflurane, and, to a lesser extent, by isoflurane. These anesthetics abbreviated isometric relaxation, yet prolonged the time course of muscle lengthening which is suggestive of an impairment of calcium uptake by the sarcoplasmic reticulum and of a decrease in calcium sensitivity of the contractile proteins.