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Different Effects of Halothane and Enflurane on Diaphragmatic Contractility In Vivo

Department of Anesthesiology, Chiba University School of Medicine and the Department of Anesthesiology, National Cancer Center Hospital, Chiba, Japan.

Abstract

We examined the effects of halothane and enflurane on diaphragmatic contractility in 12 anesthetized, mechanically ventilated dogs. The diaphragmatic force was assessed from transdiaphragmatic pressure (P_di) developed at functional residual capacity against an occluded airway during cervical phrenic nerve stimulation. Animals were randomly assigned to two groups, a halothane group (n = 6) and an enflurane group (n = 6). The P_di stimulus-frequency relationship was compared at anesthetic levels of 1, 1.5, and 2 MAC (minimum alveolar concentration) in each group. The sequence of changing anesthetic concentration was randomized. In addition, the P_di- frequency relationship was also compared between 1 MAC of halothane and enflurane in 8 of 12 dogs. In animals anesthetized with enflurane, P_di significantly decreased with 50– and 100-Hz stimulation in the presence of increasing MAC values, whereas P_di at10-Hz stimulation was not affected by the depth of anesthesia. P_di with 20-Hz stimulation during 2 MAC enflurane also decreased significantly below P_di levels seen at 1 and 1.5 MAC. By contrast, with halothane there was no difference in P_di at any of the stimulation frequencies during any of the three levels of anesthesia. There was no statistical difference, however, between P_di-frequency relationships during 1 MAC of halothane and enflurane in eight animals. From these results, we conclude that halothane does not impair diaphragmatic contractility any more than enflurane does, but enflurane decreases force generation of the diaphragm at high stimulation frequencies in a dose-related fashion. This depressant effect of enflurane occurs mainly through the impairment of neuromuscular transmission and/or membrane excitability. Part of its effect is probably related, however, to the impairment of excitation-contraction coupling, as suggested by the depression of P_di at 2 MAC in response to 20-Hz stimulation.

Key Words: ANESTHETICS, VOLATILE—halothane, enflurane. • MUSCLE, SKELETAL—diaphragm.

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