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Abstract:
Anesthetic indices for methoxyflurane, enflurane, and isoflurane in oxygen and halothane in nitrous oxide and oxygen (50:50), were determined in rats using measurements of heart and brain concentrations of the volatile agents at the endpoints of anesthesia, respiratory arrest and cardiac failure. The indices related respiratory arrest to anesthesia (respiratory index-A1r), cardiac failure to anesthesia (cardiac index-AIc) and respiratory arrest to cardiac failure (cardiorespiratory index-AIcr). Isoflurane had a significantly higher AIr (3.1) and AIc (5.7) than enflurane (AIr 1.8, AIc 3.3), methoxyflurane (AIr 2.2, AIc 3.7) and halothane in nitrous oxide and oxygen (AIr 2.4, AIc 3.7). These indices were also higher than those previously determined for halothane (AIr 2.3, AIc 3.0). Isoflurane had a higher AIcr (1.9) than halothane (1.6). Enflurane had a significantly lower AIr (1.8) than any of the other agents studied. These findings suggested a greater margin of safety for isoflurane, especially with respect to the heart, and a greater potential for respiratory depression for enflurane than for the other agents. Nitrous oxide decreased the amount of halothane necessary to produce anesthesia, but also that needed to produce respiratory arrest or cardiac failure. The addition of nitrous oxide, therefore, did not significantly enhance the overall safety of halothane anesthesia with respect to potential respiratory or cardiac depression.