Thenar muscle blood flow and neuromuscular effects of vecuronium in patients receiving balanced or isoflurane anaesthesia.

We have tested the hypothesis that isoflurane potentiates non-depolarizing neuromuscular block via an increase in muscle blood flow. Anaesthesia was induced with thiopentone 4-5 mg kg-1 in 30 adult male patients of ASA physical status I or II and was maintained with 70% nitrous oxide in oxygen supplemented with either a bolus dose of fentanyl 4 micrograms kg-1 followed by an infusion of 1 microgram kg-1 h-1 (balanced anaesthesia group, n = 15) or 1.1% end-tidal isoflurane (isoflurane group, n = 15). Vecuronium 0.1 mg kg-1 was given for neuromuscular block. The force of contraction of the adductor pollicis of the thumb in response to ulnar nerve stimulation was recorded. Thenar muscle blood flow was measured continuously with a laser Doppler flowmeter. Times required for the first twitch in the train-of-four (T1) to recover to 25%, 75% and 90% of its control value were mean 26.3 (SD 5), 35.3 (10), 43.5 (7) min and 39.2 (15), 53 (12.5), 61.2 (10) min in the balanced anaesthesia and isoflurane groups, respectively (P < 0.01). Recovery index (time between T1 25% and 75%) was prolonged significantly in the isoflurane group. Administration of thiopentone significantly increased thenar muscle blood flow from 2.6 (1.9) and 2.2 (1.5) ml min-1/100 g to 19.2 (14) and 21.7 (16) ml min-1/100 g in the balanced anaesthesia and isoflurane groups, respectively (P < 0.001). The addition of fentanyl (balanced) or isoflurane to the anaesthetic mixture produced further increases in thenar muscle blood flow to reach, respectively, 26.2 (16) and 26.8 (13.6) ml min-1/100 g during steady state anaesthesia. Thenar muscle blood flow was comparable in the two groups throughout the study. We conclude that isoflurane prolonged vecuronium-induced neuromuscular block. This prolongation was not related primarily to increase in muscle blood flow.