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ANESTHETIC PHARMACOLOGY

Onset Time for Sevoflurane/Nitrous Oxide Induction in Adults Is Prolonged with Increasing Age

Department of Anesthesiology, University of Miami, Miami, Florida; Division of Anesthesiology, Cleveland Clinic Florida, Weston Florida

Address correspondence and reprint requests to Michael C. Lewis, MD, Department of Anesthesiology, Miller School of Medicine at the University of Miami, P.O. Box 016370 (R-370) Miami FL 33101. Address e-mail to mclewis{at}med.miami.edu.

Inhaled induction of anesthesia is occasionally used in adults. Using a modified vital capacity sevoflurane/nitrous oxide (N₂O) inhaled induction, we evaluated the effect of increasing age on the onset time of anesthesia. Twenty patients, aged 26-65 yr, performed a vital capacity breath followed by regular tidal breathing from an anesthesia circuit primed with sevoflurane 8%/N₂O/O₂. The following values were recorded: time to loss of eyelash reflex (LOER); time to bispectral index 60 (BIS 60); expired fraction of sevoflurane at the time of induction, LOER and BIS 60. The mean times and 95% confidence intervals to LOER and BIS 60 were 54 s (37-70 s) and 175 s (143-207 s), respectively, and were significantly prolonged by aging (r = 0.65; P = 0.002). Times to LOER and BIS 60, predicted from the regression line, were 3.9 and 2 times longer in a 60-yr-old than in a 30-yr-old patient. The expired fraction of sevoflurane measured at time to LOER and BIS 60 decreases with increase in age. We conclude that inhaled induction with sevoflurane/N₂O is dramatically prolonged with increased age.

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