Effect of inspired nitrous oxide concentration on the rate of fall of alveolar concentration: reversed concentration effect.

The effect of inspired nitrous oxide concentration on the rate of fall of alveolar nitrous oxide concentration was studied in six patients ranging in age from 25 to 46 years. After equilibrium was reached with nitrous oxide, nitrous oxide administration was stopped and replaced by the same concentration of nitrogen. The decrease in alveolar concentration of nitrous oxide (FE/FE0) was measured continuously, where FE = measured end-tidal concentration of nitrous oxide, and FE0 = the end-tidal concentration immediately preceding the cessation of nitrous oxide administration. The rate of decrease in FE/FE0 was more rapid when 70% nitrous oxide was inspired (Part I) than when 30% nitrous oxide was inspired (Part II). One minute after the cessation of nitrous oxide administration, FE/FE0 was 0.30 +/- 0.03 (mean +/- s.d.) in Part I and 0.36 +/- 0.02 in Part II (P less than 0.05). Mean expired tidal volume increased from 515 +/- 43 ml to 552 +/- 48 ml in Part I and from 510 +/- 48 ml to 526 +/- 39 ml in Part II (P less than 0.05). The authers conclude that the more rapid decrease in alveolar concentration of nitrous oxide (FE/FE0) at higher concentrations of nitrous oxide results from augmentation of expired ventilation. We have called this phenomenon the "reversed concentration effect".