Anesthesiology
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Comparative Study
Effects of halothane, enflurane, and isoflurane on hypoxic pulmonary vasoconstriction in rat lungs in vitro.
Rat lungs were ventilated and perfused at a constant rate in vitro. The maximal hypoxic pulmonary vasoconstrictor (HPV) response was recorded by measuring the pulmonary artery pressure change when the inspired oxygen concentration was changed from 21% to 3% (with 5.5% carbon dioxide) in the absence of anesthetic vapor. In different experimental groups, the effects of halothane, enflurane, and isoflurane on HPV were examined. ⋯ All three agents depressed HPV in a dose-related manner. The concentrations in MAC units at which 50% depression of HPV (ED50) occurred was 0.47, 0.60, and 0.56 for halothane, isoflurane, and enflurane, respectively, and neither the ED50 values nor the slopes of these dose response curves were significantly different. It was concluded that these halogenated general anesthetics inhibit HPV with essentially the same potency.
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The results of erroneous filling of agent-specific anesthetic vaporizers were studied. The fraction of gas flow through the vaporizer was calculated for three vaporizers set to deliver essentially equipotent final concentrations: halothane, 1% (1.25 MAC); enflurane, 2% (1.19 MAC); and isoflurane, 1.5% (1.30 MAC). These fractional flows, at 22 degrees C, were 0.0188 for 1% halothane, 0.0615 for 2% enflurane, and 0.0295 for 1.5% isoflurane. ⋯ Enflurane and isoflurane could not be separated satisfactorily by gas chromatography. Halothane, when mixed with enflurane or isoflurane, enhanced vaporization of each agent, as well as being somewhat more easily vaporized itself. Halothane, enflurane, and isoflurane do not form ideal solutions when mixed and the resultant vapor concentrations of each of two agents when mixed may be far from those predicted by an assumption of ideality.