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ANESTHETIC PHARMACOLOGY

The Effect of Nitrous Oxide on the Dose-Response Relationship of Rocuronium

Aaron F. Kopman, MD^*, Wanda A. Chin, MD, Jimmy Moe, MD, and Rawshan Malik, MD

Departments of Anesthesiology, *New York Medical College, Valhalla; and Saint Vincent’s Hospital Manhattan, New York City, New York

Address correspondence to Aaron F. Kopman, MD, Department of Anesthesiology, Room N.R. 408, St. Vincent’s Hospital Manhattan, 170 West 12th St., New York City, NY 10011. Address e-mail to akopman{at}nyc.rr.com.

It has been generally assumed that nitrous oxide (N₂O) enhances the effects of nondepolarizing muscle relaxants only weakly if at all. More recent evidence suggests that drug potency may be more intense under N₂O anesthesia compared with total IV anesthesia (TIVA). However, the magnitude of this effect has not been well defined. We measured the 50% effective dose of rocuronium in 35 patients receiving N₂O-propofol-opioid anesthesia and a comparable group receiving TIVA. A single dose of rocuronium was given to each patient and drug potency was calculated for each individual from the Hill equation assuming a log-dose/logit slope of 4.5. In both groups, the relaxant was administered 15 min after induction of anesthesia. Neuromuscular function was measured using electromyography with single stimuli at 0.10 Hz. We measured a 50% effective dose of 0.209 ± 0.051 mg/kg during TIVA and of 0.166 ± 0.041 mg/kg during N₂O anesthesia, a decrease of 20% (P < 0.001). The clinical importance of this effect must be considered modest; however, estimates of potency that are usually obtained during N₂O anesthesia may underestimate drug requirements at the time of induction of anesthesia.