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GENERAL ARTICLES

A Demonstration of the Concentration and Second Gas Effects in Humans Anesthetized with Nitrous Oxide and Desflurane

Department of Anesthesia, University of California, San Francisco, California

Address correspondence to Edmond I Eger II, MD, Department of Anesthesia, S-455, University of California, San Francisco, CA 94143-0464. Address e-mail to edmond_eger{at}quickmail.ucsf.edu

In the present study, we explored both the existence of and the basis for the concentration and second gas effects. Groups of six normocapnic patients were given one of three gas mixtures via a nonrebreathing system: 65% nitrous oxide (N₂O) plus 4% desflurane; 5% N₂O plus 4% desflurane; or 65% N₂O plus 0.5% desflurane plus 2% xenon (Xe). End-tidal carbon dioxide (CO₂) was held constant by adjustments in ventilation. Confirming the existence of the concentration effect, the end-tidal (F_A) concentration of N₂O increased toward the inspired (F_I) concentration more rapidly (i.e., F_A/F_I increased more rapidly) when the inspired concentration was 65% than when it was 5%. The F_A/F_I for desflurane also increased more rapidly when desflurane was given with 65% rather than 5% N₂O, confirming the existence of the second gas effect. The small uptake of the second gas (desflurane) did not influence its own F_A/F_I or that of N₂O; that is, the administration of 4%, rather than 0.5%, desflurane did not increase the rate of rise of F_A/F_I of either N₂O or desflurane. One of the bases of the concentration and second gas effects, a concentrating of residual gases, was confirmed: administration of Xe with 65% N₂O produced an F_A/F_I for Xe that exceeded 1.0. Patient sex did not seem to influence the rate of rise of F_A/F_I of either N₂O or desflurane. Finally, we unexpectedly found that, despite an equal solubility in blood, the rise in F_A/F_I for N₂O exceeded that for desflurane, perhaps because of differences in tissue solubilities and intertissue diffusion.

Implications: As predicted by the concentration and second gas effects, increasing the inspired concentration of nitrous oxide accelerated its rate of rise and the rate of rise of concurrently administered desflurane in humans.

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