Magnesium potentiates neuromuscular blockade with cisatracurium during cardiac surgery.

Purpose: Magnesium potentiates the effect of nondepolarizing neuromuscular blocking agents. It is used in cardiac anesthesia to prevent hypertension and arrhythmias. This study was performed to measure the interaction between magnesium and cisatracurium in cardiac surgery.

Methods: Twenty patients scheduled for elective cardiac surgery were randomly assigned to receive magnesium sulfate (70 mg x kg(-1) at induction followed by 30 mg x kg(-1) x hr(-1)) or placebo. The ulnar nerve was stimulated and the electromyographic response of the adductor pollicis was measured. Cisatracurium 0.1 mg x kg(-1) was given at induction, followed by 0.05 mg x kg(-1) when the first twitch in the train-of-four reached 25%.

Results: Ionized magnesium was 1.32 +/- 0.24 mmol x L(-1) in the treatment group vs 0.47 +/- 0.4 mmol x L(-1) in the control group. Duration of action of the intubating dose was longer in the magnesium group (74 +/- 20 min) than in the placebo group (42 +/- 6 min, P = 0.0001). Duration of the first maintenance dose was 69 +/- 16 min in the magnesium group vs 35 +/- 7 min in the placebo group (P = 0.0001). Total dose of cisatracurium administered throughout surgery was 0.19 +/- 0.07 mg x kg(-1) in the magnesium group compared with 0.29 +/- 0.01 mg x kg(-1) in the placebo group (P = 0.017). Hemodynamic variables and temperature were similar in both groups.

Conclusion: In patients undergoing cardiac surgery, administration of magnesium sulfate, resulting in ionized levels of 1.3 mmol x L(-1), results in a 30-35 min prolongation of the neuromuscular blockade induced with intubating and maintenance doses of cisatracurium and does not alter hemodynamic stability.