Prolonged suxamethonium infusion during nitrous oxide anaesthesia supplemented with halothane or fentanyl

The neuromuscular blockade produced by a prolonged infusion of suxamethonium was studied using train-of-four stimulation in 40 patients receiving either halothane-nitrous oxide or fentanyl-nitrous oxide anaesthesia. Initially, a depolarizing (phase I) block was observed in all patients followed by phase II block which was associated with tachyphylaxis to suxamethonium; the latter changes occurring more rapidly in the halothane group. Infusions were continued for more than 150 min in 17 patients and there was a late decrease in suxamethonium requirement in those who received halothane, but not fentanyl. Ten minutes after the suxamethonium infusion was stopped, most patients received neostigmine which was followed by rapid recovery of neuromuscular transmission.