Abstract
Under the three-compartment model of ventilation-perfusion ([Formula: see text]) scatter, Bohr-Enghoff calculation of alveolar deadspace fraction (VDA/VA) uses arterial CO2 partial pressure measurement as an approximation of "ideal" alveolar CO2 (ideal [Formula: see text]). However, this simplistic model suffers from several inconsistencies. Modeling of realistic physiological distributions of [Formula: see text] and [Formula: see text] instead suggests an alternative concept of "ideal" alveolar gas at the [Formula: see text] ratio where uptake or elimination rate of a gas is maximal. For alveolar-capillary partial pressure, this "modal" point equals the mean of expired alveolar and arterial partial pressures, regardless of [Formula: see text] scatter severity or overall [Formula: see text]. For example, modal ideal [Formula: see text] can be estimated from the following equation: [Formula: see text]Using a multicompartment computer model of log-normal distributions of [Formula: see text] and [Formula: see text], agreement of this estimate with the modal ideal [Formula: see text] located at the [Formula: see text] ratio of maximal compartmental [Formula: see text] was assessed across a wide range of severity of [Formula: see text] scatter and overall [Formula: see text] ratio. Agreement of VDA/VA for CO2 from the Bohr equation using modal ideal PCO2, with that using the estimated value was also assessed. Estimated modal ideal [Formula: see text] agreed closely with modal ideal [Formula: see text], intraclass correlation (ICC) > 99.9%. There was no significant difference between VDA/VA using either value for ideal [Formula: see text]. Modal ideal [Formula: see text] reflects a physiologically realistic concept of ideal alveolar gas where there is maximal gas exchange effectiveness in a physiological distribution of [Formula: see text], which is generalizable to any inert gas and is practical to estimate from arterial and end-expired CO2 partial pressures.NEW & NOTEWORTHY The three-compartment model of lung ventilation-perfusion mismatch postulates definitive alveolar deadspace and "ideal gas" lung compartments, but these are, in fact, widely different for gases of different blood solubility. Physiologically realistic distributions of ventilation, perfusion, and gas exchange instead suggest an individual "ideal" ventilation-perfusion ratio for every gas, where its alveolar-capillary uptake or elimination rate is maximal (modal). For carbon dioxide, this "ideal" partial pressure is the mean of the arterial and end-tidal partial pressures.
Highlights
The three-compartment or Riley model of lung ventilation-perfusion inhomogeneity (V_ A=Q_ scatter) uses simple and familiar mixing equations to quantify the effect of V_ A=Q_ scatter on inefficiency of gas exchange [1,2,3], which becomes significant in a variety of lung conditions and in normal lungs under general anesthesia [4,5,6,7,8,9,10]
The principle of a modal point of gas exchange and ideal alveolar gas is illustrated in Fig. 1, which shows theoretical distributions of V_A and Q_and calculated gas exchange relative to V_A=Q_ratio across all lung compartments n in a lung with log SD of V_A of 1.5, overall V_A=Q_ratio of 0.8 and V_O2 1⁄4 200 mL=min with respiratory quotient (RQ) = 0.8
The mixing equations employed require that an “ideal” reference point is nominated against which measurable variables can be compared
Summary
The three-compartment or Riley model of lung ventilation-perfusion inhomogeneity (V_ A=Q_ scatter) uses simple and familiar mixing equations to quantify the effect of V_ A=Q_ scatter on inefficiency of gas exchange [1,2,3], which becomes significant in a variety of lung conditions and in normal lungs under general anesthesia [4,5,6,7,8,9,10]. High V_ A=Q_ ratio lung regions contribute to alveolar deadspace, whereas low V_ A=Q_ ratios contribute to venous admixture or shunt. In both these theoretical compartments, no gas exchange interface is present and all gas exchange is conceived as occurring in an “ideal” lung compartment of uniform V_ A=Q_ ratio, where for any gas G, “ideal alveolar” (ideal PAG) and “pulmonary endcapillary” (Pc0G) partial pressures are identical. An estimate of the gas content of the ideal compartment is required by the mixing equations used to calculate both deadspace fraction (the Bohr equation) and venous admixture (shunt equation of Berggren) [1,2,3]. Alveolar deadspace fraction (VDA/VA) for CO2 is calculated from the same mixing principle by measuring the Downloaded from journals.physioloPguyb.loisrhge/djobuyrnthael/Ajampeprlic(a0n5P2h.0y2si3o.l2o0gi8c.a1l8S5o)cioenty.December 29, 2021
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