Effects of N₂O elimination on the elimination of second gases in a two‐step mathematical model of heterogeneous gas exchange

Abstract

We have investigated the elimination of inert gases in the lung during the elimination of nitrous oxide (N2O) using a two‐step mathematical model that allows the contribution from net gas volume expansion, which occurs in Step 2, to be separated from other factors. When a second inert gas is used in addition to N2O, the effect on that gas appears as an extra volume of the gas eliminated in association with the dilution produced by N2O washout in Step 2. We first considered the effect of elimination in a single gas‐exchanging unit under steady‐state conditions and then extended our analysis to a lung having a log‐normal distribution of ventilation and perfusion. A further increase in inert gas elimination was demonstrated with gases of low solubility in the presence of the increased ventilation‐perfusion mismatch that is known to occur during anesthesia. These effects are transient because N2O elimination depletes the input of that gas from mixed venous blood to the lung, thereby rapidly reducing the magnitude of the diluting action.