Elucidating the roles of solubility and ventilation-perfusion mismatch in the second gas effect using a two-step model of gas exchange

Abstract

Gas exchange in the lung can always be represented as the sum of two components: gas exchange at constant volume followed by gas exchange on volume correction. Using this sequence to study the second gas effect, low gas solubility and increased ventilation-perfusion mismatch are shown to act together to enhance second gas uptake. While appearing to contravene classical concepts of gas exchange, a detailed theoretical analysis shows it is fully consistent with these concepts.

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

“…A second contributing factor is the location of gas exchange in the $$$ \dot{V}/\dot{Q} $$$ spectrum for each gas and how the location is affected as the $$$ \dot{V}/\dot{Q} $$$ mismatch is increased. As shown in our previous study, this is significant during induction and maintenance of anesthesia for highly soluble gases whose remainder after Step 1 is not aligned with the contraction in volume which occurs in Step 2 (Korman et al, 2020). In the case of diethyl‐ether, Figure 1 shows excellent overlap in Step 2, between the residual gas (red area) and the volume expansion associated with N 2 O elimination (yellow area).…”

“…According to basic concepts of respiratory physiology, $$$ \dot{V}/\dot{Q} $$$ mismatch should have the opposite effect on gas transfer (Neufeld et al, 1978; West et al, 1974; West & Wagner, 1977). This remained unexplained until it was shown that the process of gas uptake in the lung may be treated theoretically as a two‐step process: gas exchange at constant volume followed by gas exchange on volume contraction (Korman et al, 2020). This is achieved by only allowing the SG to equilibrate with blood in the Step 1.…”

“…Finally, the effect of volume expansion during N 2 O elimination is investigated using a two-step mathematical model of steady-state gas exchange. This follows the sequence of our previous work (Korman et al, 2020), now applied to the elimination phase of anesthesia.…”

“…All programs were written in MATLAB. Further details of our technique are discussed elsewhere (Korman et al, 2018(Korman et al, , 2019(Korman et al, , 2020. The relationships used to generate the equations and figures are discussed further in Appendix 3.…”

“…Exactly how a low SG solubility combines with a high degree of $$$ \dot{V}/\dot{Q} $$$ mismatch to paradoxically increase SG uptake in the SGE remained unexplained until the process of gas uptake in the lung was modeled as a two‐step process: gas exchange for all gases at constant volume followed by gas exchange on volume contraction with FG uptake (see Appendix ) (Korman et al, 2020). The situation during the elimination of N 2 O was not examined and is therefore the subject of the current investigation.…”