A demonstration of the concentration and second gas effects in humans anesthetized with nitrous oxide and desflurane

In the present study, we explored both the existence of and the basis for t he concentration and second gas effects. Groups of six normocapnic patients were given one of three gas mixtures via a nonrebreathing system: 65% nitr ous oxide (N2O) plus 4% desflurane; 5% N2O plus 4% desflurane; or 65% N2O p lus 0.5% desflurane plus 2% xenon (Xe). End-tidal carbon dioxide (CO2) was held constant by adjustments in ventilation. Confirming the existence of th e 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 increas ed 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 w as given with 65% rather than 5% N2O, confirming the existence of the secon d gas effect. The small uptake of the second gas (desflurane) did not influ ence its own F-A/F-I or that of N2O; that is, the administration of 4%, rat her than 0.5%, desflurane did not increase the rate of rise of F-A/F-I of e ither N2O or desflurane. One of the bases of the concentration and second g as effects, a concentrating of residual gases, was confirmed: administratio n 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 solubilit y in blood, the rise in F-A/F-I for N2O exceeded that for desflurane, perha ps because of differences in tissue solubilities and intertissue diffusion. Implications: As predicted by the concentration and second gas effects, in creasing the inspired concentration of nitrous oxide accelerated its rate o f rise and the rate of rise of concurrently administered desflurane in huma ns.