Optimum dose of neostigmine at two levels of atracurium-induced neuromuscular block.

There is controversy about the optimum dose of neostigmine for antagonizing neuromuscular block. We have studied 57 patients undergoing gynaecological surgery to establish a dose-response relationship when neostigmine was given to antagonize atracurium-induced block. Anaesthesia was induced with thiopentone and fentanyl and maintained with nitrous oxide and enflurane in oxygen and neuromuscular block was produced with a bolus of atracurium 0.5 mg kg-1. At the time of antagonism of block, three groups received neostigmine 20, 40 or 80 micrograms kg-1 at 5-10% recovery of the compound muscle action potential of the adductor pollicis (profound block) and three groups received one of these doses at 40-50% neuromuscular recovery (light block). At profound block, antagonism was prolonged by reducing the dose of neostigmine from 40 micrograms kg-1 to 20 micrograms kg-1, but not shortened by increasing the dose from 40 micrograms kg-1 to 80 micrograms kg-1. At light block, there was no significant difference between the three groups in the time taken to reach a train-of-four ratio of 0.7. There was little benefit in increasing the dose of neostigmine from 40 micrograms kg-1 to 80 micrograms kg-1 when antagonizing profound neuromuscular block. When light block was antagonized, neostigmine 20 micrograms kg-1 was the optimum dose. We suggest that smaller doses of neostigmine than are given commonly produce adequate antagonism of atracurium-induced neuromuscular block.