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Wilkins Christopher J. MB ChB; Cramp, Paul G. W. MB, chB; Staples, James MD; Stevens, Wendell C. MD
Anesthesia & Analgesia: November 1992

We tested the hypothesis that the laryngeal mask airway (LMA) is tolerated at lighter levels of anesthesia than an endotracheal tube (ET). We studied 20 unpremedicated, nonsmoking ASA physical status I or II patients aged 18–40 yr whose surgery lasted >1 h. Subjects were randomly assigned to receive either an ET or LMA. Anesthesia was induced with intravenous propofol and the LMA or ET was inserted. The ET-group patients received 1.5 mg/kg of succinylcholine, preceded by vecuronium (0.015 mg/kg IV). Maintenance of anesthesia was with only isoflurane and approximately 66% N2O in O2 by spontaneous ventilation. All gas concentrations were measured by a Raman spectrometer sampling from the breathing circuit end of the LMA or ET. Toward the end of the procedure, the end-tidal N2O and isoflurane concentrations were allowed to decrease to <3 vol% and 0.8 ± 0.05 vol%, respectively. The end-tidal isoflurane concentration was then decreased in 0.1% ± 0.05% decrements, each stable value being held for 5 min. The patient was observed for signs of reaction to the presence of the LMA or ET. The mean (range) end-tidal isoflurane concentrations for reaction to ET and LMA were 0.55% (0.4–0.7) and 0.35% (0.2–0.51), respectively (P < 0.001). These data confirm the original hypothesis of the study.

Address correspondence to Mr. Wilkins, Department of Anesthesiology, UHS-2, Oregon Health Sciences University, 3181 S.W. Sam Jackson Park Road, Portland, OR 97201–3098. Reprints will not be available.

© 1992 International Anesthesia Research Society