Effect of enflurane and fentanyl on the clinical characteristics of long-term succinylcholine infusion.

The characteristics of the neuromuscular block produced by prolonged succinylcholine infusion were compared in 40 patients anaesthetized with either nitrous oxide with enflurane (1-2 per cent inspired) or nitrous oxide and fentanyl. Neuromuscular transmission was monitored using train-of-four stimulation and the infusion rate was adjusted to keep the first twitch at 10-15 per cent of its control value. Initially, all patients, exhibited a depolarizing-type block all twitches of the train-of-four being roughly the same size, and the infusion rates were similar in the enflurane (54 microgram X kg-1/min) and the fentanyl (58 microgram X kg-1/min) groups. Tachyphylaxis developed later in both groups and correlated well with the onset of phase II block (dual block). This occurred sooner and at a lower cumulative dose in the enflurane group. Fourth to first twitch ratios decreased to 50, 25 and 0 per cent in 31, 46 and 59 minutes in the enflurane group, at cumulative succinylcholine doses of 2.2, 3.2 and 4.2 mg X kg-1 respectively. Corresponding figures for the fentanyl group were 52, 73 and 86 minutes, with dose of 3.4, 5.0 and 5.9 mg X kg-1. Infusion rates increased markedly after establishment of dual block, but were similar with enflurane (0.99 mg X kg-1/min) and fentanyl (1.12 mg X kg-1/min). Ten minutes after stopping the infusion fourth to first twitch ratios failed to reach 50 per cent in most patients given enflurane who had received more than 6 mg X kg-1 succinylcholine over more than 90 minutes. Corresponding figures for fentanyl patients were 13 mg x kg-1 and 150 minutes. The block in all 15 patients (9 enflurane, 6 fentanyl) who did not recover spontaneously was successfully antagonized with atropine and neostigmine.