Relationship between the muscle relaxation effect and body muscle mass measured using bioelectrical impedance analysis: A nonrandomized controlled trial

OBJECTIVE: The dose of neuromuscular blocking drugs is commonly based on body weight, but using muscle mass might be more effective. This study investigated the relationship between the effect of neuromuscular blocking drugs and muscle mass measured using bioelectrical impedance analysis. METHODS: Patients who were scheduled for elective surgery using a muscle relaxant were screened for inclusion in this study. Under intravenous anaesthesia, 12 mg or 9 mg of rocuronium was administered to males and females, respectively; and the maximal relaxation effect of T1 was measured using a TOF-Watch-SX® acceleromyograph. RESULTS: This study enrolled 40 patients; 20 males and 20 females. For both sexes, the maximal relaxation effect of T1 did not correlate with the body weight-based dose of neuromuscular blocking drugs (males, r(2) = 0.12; females, r(2) = 0.26). Instead, it correlated with the dose based on bioelectrical impedance analysis-measured muscle mass when injected with the same dose of rocuronium (males, r(2) = 0.78, female, r(2) = 0.82). CONCLUSIONS: This study showed that the muscle relaxation effect of rocuronium was correlated with muscle mass and did not correlate with body weight when using the same dose. Therefore, a muscle mass-based dose of neuromuscular blocking drugs is recommended.