When an anesthetic such as halothane (second gas) is administered with nitrous oxide (first gas) the rate of rise of alveolar halothane is more rapid than when it is given alone. This "second gas effect" has been attributed to the increased volume of inspired gas which results from the large volume of nitrous oxide taken up. We find that a second explanation, a concentrating effect, is also necessary.
Dogs were equilibrated with ethylene, cyclopropane or halothane as the second gas. Administration of 70 per cent nitrous oxide plus the equilibration concentration of the second gas caused the second gas concentration to rise above the inspired concentration, thus precluding the possibility that the second gas effect was due to an increased volume of inspired gas. Only a concentration of the second gas in a smaller volume could explain the increase in second gas concentration. The rise was greatest with the least soluble gas, ethylene and greatest when ventilation was least. At a ventilation which produced a PACO? of 80 mm Hg, the alveolar concentration of ethylene reached a peak of 15.6 per cent, that of cyclopropane 10 per cent, and that of halothane S.3 per cent, above the equilibrium values.
(C) 1969 American Society of Anesthesiologists, Inc.