EFFECTS OF RESIDUAL CONCENTRATIONS OF ISOFLURANE ON THE REVERSAL OF VECURONIUM-INDUCED NEUROMUSCULAR BLOCKADE

Thirty-six anesthetized patients (ASA physical status 1 or 2) undergoing elective surgery were monitored (isometric adductor pollicis mechanical activity) to detect the effects of discontinuing isoflurane anesthesia upon the reversal of vecuronium-induced neuromuscular blockade. Neuromuscular blockade was produced by vecuronium 100-mu-g/kg and additional doses of 20-mu-g/kg until completion of surgery. The patients were randomly divided into three groups: in the control group (n = 12), only fentanyl/N2O was given; in the "isostable" group (n = 12), isoflurane at an end-tidal concentration of 1.25% was maintained throughout anesthesia; in the "isostop" group (n = 12), isoflurane 1.25% was discontinued before neostigmine administration. In all groups, paralysis was antagonized with 15-mu-g/kg intravenous (iv) atropine and 40-mu-g/kg iv neostigmine when the twitch height (0.1 Hz) had regained 25% of its control value. The measured parameters were twitch height, train-of-four, and 50-100-Hz tetanic fade. No significant differences were found among the three groups with respect to the final twitch heights and tetanic fades at 50 Hz. In the isostable group, final mean train-of-four was significantly less (75%) than in the other patients (88%) (P < 0.01). Mean tetanic fade at 100 Hz was significantly less in the isostable group (31%) than in the isostop group (57%) (P < 0.01) and control group (84%) (P < 0.01). We conclude that discontinuing isoflurane anesthesia for 15 min improves the reversal of a vecuronium paralysis. In addition, after the antagonism of vecuronium-induced neuromuscular blockade, tetanic fade at 100 Hz appears useful to detect the slight impairment of the neuromuscular transmission that is induced by residual isoflurane concentrations and that is undetected by train-of-four measurements.