Inadequate Antagonism of Vecuronium-Induced Neuromuscular Block by Neostigmine During Sevoflurane or Isoflurane Anesthesia

To examine the effects of discontinuing sevoflurane or isoflurane anesthesia (1 minimum alveolar anesthetic concentration [MAC] of end-tidal concentrations, together with 66% N2 O/O2) on the reversal of vecuronium-induced neuromuscular blockade (an initial dose = 100 micro gram/kg), the electromyographic response of the abductor digiti minimi was monitored at 20-s intervals after train-of-four (TOF) stimulation of the ulnar nerve in 192 ASA grades I and II patients. When the amplitudes of the first response (T1) had recovered to 10% of the control, neostigmine (0; spontaneous recovery, 10,20,30,40, or 55 micro gram/kg, eight patients each) was given and the ratio of the fourth TOF to the first response (TOFR) was monitored at 1-min intervals for 15 min in the presence of the volatile anesthetics, or after discontinuation of anesthetic administration. Spontaneous TOFR recovery was also assessed in another eight patients who were anesthetized with 66% N2 O/O2-fentanyl-diazepam. Adequate antagonisms (TOFR>75%) were achieved after discontinuation of both administrations, but not during anesthetic administration at the neostigmine doses tested. The doseresponse curves for neostigmine (10, 20, 30, and 40 micro gram/kg) were constructed using the TOFR values at 5-11 min, from which the ED50 values (a neostigmine dose required for a TOFR value of 50%) were derived. Sevoflurane impaired neostigmine antagonism more than isoflurane, as demonstrated by the significantly higher ED50 values at 7-11 min (P < 0.05). Discontinuation of anesthetic administration significantly decreased the ED50 values at 8-9 min for isoflurane, and at 8-11 min for sevoflurane (P < 0.05, compared with during anesthesia), without any differences in the ED50 values. The more rapid spontaneous recovery from N2 O-fentanyl anesthesia than from isoflurane and sevoflurane anesthesia (P < 0.05, at 14 and 15 min), however, suggested that even if the administration of both anesthetics is stopped at the time of reversal, the impairment is reduced but not eliminated. (Anesth Analg 1995;80:1175-80)