Preanesthetic train-of-four fade predicts the atracurium requirement of myasthenia gravis patients

Background: The most sensitive diagnostic criterion of myasthenia gravis is a decrement in the muscular response to repetitive stimulation. The author s hypothesized that myasthenia gravis patients who show a train-of-four rat io (T4/T1) < 0.9 in the preanesthetic period will have increased sensitivit y to nondepolarizing neuromuscular blocking agents compared with myasthenia gravis patients with preanesthetic T4/T1 greater than or equal to 0.9. Methods: After institutional review board approval was obtained, 20 electro physiologically documented myasthenia gravis patients were studied, Current pyridostigmine therapy was continued until the morning of surgery. Before induction of anesthesia, neuromuscular transmission was recorded from the h ypothenar muscles using electromyography with train-of-four stimulation of the ulnar nerve. According to the T4/T1 ratio, patients were assigned to th e "normal'' group (T4/T1 greater than or equal to 0.9) or the "decrement" g roup (T4/T1 < 0.9). After induction of intravenous anesthesia, the effectiv e dose to achieve a 95% neuromuscular blockade (ED95) for atracurium was as sessed with a cumulative bolus technique. Postoperatively, pyridostigmine w as titrated to obtain a T4/T1 > 0.75 and to treat residual myasthenic sympt oms. Results: In 14 patients, preanesthetic T4/T1 was greater than or equal to 0 .9 (normal), whereas 6 patients presented with T4/T1 < 0.9 (decrement). Dec rement patients had a lower ED95 of 0.07 +/- 0.03 mg/kg atracurium(mean +/- SD) compared with normal patients with an ED95 of 0.24 +/- 0.11 mg/kg atra curium (P = 0.002). All patients were extubated within 30 min after surgery . Postoperative pyridostigmine infusion did not differ significantly betwee n groups. Conclusions: The requirement for atracurium is significantly reduced in mya sthenia gravis patients with a T4/T1 ratio < 0.9 before anesthesia. This st udy indicates that routine neuromuscular monitoring in myasthenia gravis pa tients should be extended into the preinduction period to identify patients who require less atracurium.