• Philip J Peyton.
• Department of Critical Care, Melbourne Medicine School, Faculty of Medicine, Dentistry and Health Sciences, University of Melbourne, Victoria, Australia.
• Anesthesiology. 2025 Jan 30.

BackgroundMulti-compartment computer models of heterogeneity in alveolar ventilation-perfusion ratios (VA/Q scatter) across the lung explain the significant alveolar-arterial (A-a) partial pressure gradients and associated alveolar dead-space fractions (VDA/VA) 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 VA/Q scatter to do this has not been tested or compared to calculations from the traditional Riley model with "ideal", unventilated (shunt) and unperfused (deadspace) compartments.MethodsMeasurements 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 was used to compare the accuracy of two simple models of VA/Q scatter and lung gas exchange in predicting measured alveolar and arterial partial pressure differences, and 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 VA and Q, with and without additional true-shunt. A multi-compartment "log-normal" model was also tested.ResultsMean (95% confidence interval) of the measured alveolar dead-space fraction for the three anesthetic gases G combined (VDA/VAG) was 0.557 (0.523, 0.592). Mean VDA/VAG from the two-compartment model incorporating an additional true-shunt lung compartment (0.539 (0.498, 0.580) was similar to the measured value (p = 0.347), and was 0.501 (0.457, 0.546) without a true-shunt compartment. The log-normal model outputs were 0.491 (0.453, 0.529)). The Riley model outputs for VDA/VAG severely underestimated this (0.327 (0.294, 0.361)).ConclusionSatisfactory prediction of the A-a 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 compartments, which the traditional Riley model lacks. A simple "reciprocal" two-compartment model achieves this.Copyright © 2025 American Society of Anesthesiologists. All Rights Reserved.

13 keywords...

hide…