Decrease in dose requirement of d-tubocurarine by volatile anesthetics

Volatile anesthetics are known to decrease the requirements for neuromuscular blocking agents. To obtain a quantitative measure of the extent of this drug interaction, studies were performed on isolated guinea pig nerve--lumbrical muscle preparations exposed to methoxyflurane, halothane, isoflurane, diethyl ether, fluroxene, and enflurane in concentrations equal to MAC. From the relationship between indirect twitch height and d-tubocurarine concentration, the concentration depressing the twitch height by 50 per cent was determined. In the presence of MAC levels of anesthetic, the ED50 was decreased by the following fractional amounts: methoxyflurane, 0.311; halothane, 0.334; isoflurane, 0.335; diethyl ether, 0.462; fluroxene, 0.580; enflurane, 0.697. Comparison of the fractional decrease of d-tubocurarine dose requirement by an anesthetic at MAC and previously obtained values for the fractional depression of end-plate depolarization by an anesthetic at MAC showed that the more the anesthetic depresses depolarization, the smaller the d-tubocurarine dose requirement. Thus, clinically observed decreases in dose requirements may be explained by the effects of the anesthetics on chemosensitivity of the end-plate region.