INADEQUATE ANTAGONISM OF VECURONIUM-INDUCED NEUROMUSCULAR BLOCK BY NEOSTIGMINE DURING SEVOFLURANE OR ISOFLURANE ANESTHESIA

To examine the effects of discontinuing sevoflurane or isoflurane anes thesia (1 minimum alveolar anesthetic concentration [MAC] of end-tidal concentrations, together with 66% N2O/O-2) on the reversal of vecuron ium-induced neuromuscular blockade (an initial dose = 100 mu g/kg), th e electromyographic response of the abductor digiti minimi was monitor ed at 20-s intervals after train-of-four (TOF) stimulation of the ulna r nerve in 192 ASA grades I and II patients. When the amplitudes of th e first response (T1) had recovered to 10% of the control, neostigmine (0; spontaneous recovery, 10, 20, 30, 40, or 55 mu g/kg, eight patien ts each) was given and the ratio of the fourth TOF to the first respon se (TOFR) was monitored at 1-min intervals for 15 min in the presence of the volatile anesthetics, or after discontinuation of anesthetic ad ministration. Spontaneous TOFR recovery was also assessed in another e ight patients who were anesthetized with 66% N2O/O-2-fentanyl-diazepam . Adequate antagonisms (TOFR>75%) were achieved after discontinuation of both administrations, but not during anesthetic administration at t he neostigmine doses tested. The dose-response curves for neostigmine (10, 20, 30, and 40 mu g/kg) were constructed using the TOFR values at 5-11 min, from which the ED(50) values (a neostigmine dose required f or a TOFR value of 50%) were derived. Sevoflurane impaired neostigmine antagonism more than isoflurane, as demonstrated by the significantly higher ED(50) values at 7-11 min (P < 0.05). Discontinuation of anest hetic administration significantly decreased the ED(50) values at 8-9 min for isoflurane, and at 8-11 min for sevoflurane (P < 0.05, compare d with during anesthesia), without any differences in the ED(50) value s. The more rapid spontaneous recovery from N2O-fentanyl anesthesia th an from isoflurane and sevoflurane anesthesia (P < 0.05, at 14 and 15 min), however, suggested that even if the administration of both anest hetics is stopped at the time of reversal, the impairment is reduced b ut not eliminated.