The effects of magnesium sulfate on neuromuscular blockade by cisatracurium during induction of anesthesia.

During the induction of anesthesia, patients are at risk of aspiration while awaiting full muscle relaxation. Magnesium has been shown to have synergistic effects with neuromuscular blocking drugs. We tested if magnesium, as an adjunct, increases the speed of onset of muscle relaxation, thereby decreasing the risk of aspiration. Eighty-eight American Society of Anesthesiologists (ASA) physical status 1 or 2 patients were randomly assigned to three groups. Group Mg-0 received 100 mL of normal saline, whereas groups Mg-25 and Mg-50 received magnesium sulfate at doses of 25 and 50 mg/kg, respectively. Anesthesia was induced with thiopental 5 mg/kg and cisatracurium 0.15 mg/kg. A peripheral nerve stimulator and single-twitch test was performed on the ulnar nerve until the twitch responses to stimulation had disappeared, and the times were recorded. Then the patients were intubated and anesthesia was maintained with 100 μg/kg/min of propofol. The intensity of blockade was measured at regular time intervals with the post-tetanic count test. The mean times to muscle relaxation in groups Mg-0, Mg-25, and Mg-50 were 226, 209, and 188 s, respectively (P = 0.047). The intensity of the block increased with the dose of magnesium, and remained highest in group Mg-50 at all times measured (P < 0.05). The speed of onset and the intensity of muscle relaxation increased as higher doses of magnesium were used. The increased speed of onset of muscle relaxation produced by magnesium is not substantial enough to justify its use in combination with cisatracurium in rapid sequence induction.