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