Laboratory investigations have shown that some benzo-diazepines can produce biphasic effects on the actions of neuromuscular blocking agents (155,156), higher doses potentiating the effects (155,157); however, several human investigations have failed to show a significant effect (158-160). It has been suggested that agents that are added to commercial formulations of some benzodia-zepines to render them more water-soluble may mask the benzodiazepine effect (160).
Nevertheless, some interactions of benzodiazepines with muscle relaxants used in anesthesia have been described. Diazepam has been reported to potentiate the effects of tubocurare (161) and gallamine (162) and to reduce the effects of suxamethonium (162). However, in 113 patients undergoing general anesthesia, intravenous diazepam 20 mg, lorazepam 5 mg, and lorme-tazepam 2mg did not potentiate the neuromuscular blocking effects of vecuronium or atracurium (160).
In 113 patients undergoing general anesthesia, intravenous midazolam 15 mg slowed recovery of the twitch height after vecuronium and atracurium compared with diazepam. The recovery index was not altered (160). However, in another study in 20 patients, midazolam 0.3 mg/kg did not affect the duration of blockade, recovery time, intensity of fasciculations, or adequacy of relaxation for tracheal intubation produced by suxa-methonium 1 mg/kg, nor the duration of blockade and adequacy of relaxation for tracheal intubation produced by pancuronium 0.025 mg/kg in incremental doses until 99% depression of muscle-twitch tension was obtained (159). Furthermore, in 60 patients undergoing maintenance anesthesia randomly assigned to one of six regimens (etomidate, fentanyl, midazolam, propofol, thiopental plus nitrous oxide, or isoflurane plus nitrous oxide), midazolam did not alter rocuronium dosage requirements (163).
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