Halothane has a relatively high blood:gas partition coefficient and high fat:blood partition coefficient (Table 13-1). Thus, induction with halothane is relatively slow, and the alveolar halothane concentration remains substantially lower than the inspired halothane concentration for many hours of administration. Since halothane accumulates in tissues during prolonged administration, the speed of recovery from halothane is lengthened as a function of duration of administration.
Approximately 60-80% of halothane taken up by the body is eliminated unchanged via the lungs in the first 24 hours after its administration. A substantial amount of the halothane not eliminated in exhaled gas is biotransformed by hepatic CYPs. Trifluoroacetylchloride, an intermediate in oxidative metabolism of halothane, can trifluoroacetylate several proteins in the liver. An immune reaction to these altered proteins may be responsible for the rare cases of fulminant halothane-induced hepatic necrosis.
CLINICAL USE Halothane is a potent, nonpungent and well-tolerated agent that usually is used for maintenance of anesthesia and is well tolerated for inhalation induction of anesthesia, most commonly in children, in whom preoperative placement of an intravenous catheter can be difficult. Anesthesia is produced by halothane at end-tidal concentrations of 0.7-1%. The use of halothane in the U.S. has diminished substantially since the introduction of newer inhalational agents with better pharmacokinetic and side-effect profiles.
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