Accuracy of Two-compartment Modeling of Gas Exchange with Ventilation-Perfusion Mismatch in Inhalational Anesthesia

Multicompartment computer models of heterogeneity in alveolar ventilation-perfusion ratios V̇ A /Q̇ scatter) across the lung explain the significant alveolar-arterial partial pressure gradients and associated alveolar dead-space fractions (V da /V a ) seen in anesthetized patients for both carbon dioxide and for anesthetic gases of different blood solubilities. However, the accuracy of a simpler two-compartment model of V̇ A /Q̇ scatter to do this has not been tested or compared to calculations from the traditional Riley model with "ideal," unventilated (shunt) and unperfused (dead-space) compartments.

Measurements of gas partial pressures in inspired and expired gas and arterial and mixed venous blood from 29 patients undergoing inhalational general anesthesia for cardiac surgery were used to compare the accuracy of two simple models of V̇ A /Q̇ scatter and lung gas exchange in predicting measured alveolar and arterial partial pressure differences and the associated alveolar dead-space calculations for the modern anesthetic gases isoflurane, sevoflurane, and desflurane. These models were the Riley model and a two-compartment model with reciprocal proportions of allocation of V̇ A and Q̇, with and without additional true shunt. A multicompartment "log-normal" model was also tested.

Mean (95% CI) of the measured alveolar dead-space fraction for the three anesthetic gases G combined (V da /V aG ) was 0.557 (0.523 to 0.592). Mean V da /V aG from the two-compartment model incorporating an additional true-shunt lung compartment (0.539 [0.498 to 0.580]) was similar to the measured value ( P = 0.347) and was 0.501 (0.457 to 0.546) without a true-shunt compartment. The log-normal model outputs were 0.491 (0.453 to 0.529). The Riley model outputs for V da /V aG severely underestimated this (0.327 [0.294 to 0.361]).

Satisfactory prediction of the alveolar-arterial partial pressure gradients and alveolar dead space for the modern volatile anesthetic gases measured in vivo requires a model with more than one gas-exchanging lung compartment, which the traditional Riley model lacks. A simple "reciprocal" two-compartment model achieves this.