Ideal alveolar gas defined by modal gas exchange in ventilation-perfusion distributions

Abstract

Under the three-compartment model of ventilation-perfusion (V_A/Q) scatter, Bohr-Enghoff calculation of alveolar deadspace fraction (V_DA/V_A) uses arterial CO₂ partial pressure measurement as an approximation of "ideal" alveolar CO₂(ideal P_ACO₂). However, this simplistic model suffers from several inconsistencies. Modelling of realistic physiological distributions of V_A and Q instead suggests an alternative concept of "ideal" alveolar gas at the V_A/Q ratio where uptake or elimination rate of a gas is maximal. The alveolar-capillary partial pressure at this "modal" point equals the mean of expired alveolar and arterial partial pressures, regardless of V_A/Q scatter severity or overall V_A/Q. For example, modal ideal P_ACO₂ can be estimated from Estimated modal ideal P_ACO₂ = (P_ACO₂+P_aCO₂)/2 Using a multicompartment computer model of log normal distributions of V_A and Q, agreement of this estimate with the modal ideal P_ACO₂ located at the V_A/Q ratio of maximal compartmental VCO₂ was assessed across a wide range of severity of V_A/Q scatter and overall V_A/Q ratio. Agreement of V_DA/V_A for CO₂ from the Bohr equation using modal idealPCO₂ with that using the estimated value was also assessed. Estimated modal ideal P_ACO₂ agreed closely with modal ideal P_ACO₂, intraclass correlation (ICC) > 99.9%. There was no significant difference between V_DA/V_ACO₂ using either value for ideal P_ACO₂. Modal ideal P_ACO₂ reflects a physiologically realistic concept of ideal alveolar gas where there is maximal gas exchange effectiveness in a physiological distribution of V_A/Q, which is generalizable to any inert gas, and is practical to estimate from arterial and end-expired CO₂ partial pressures.