A modal definition of ideal alveolar oxygen

In the three-compartment model of lung ventilation-perfusion heterogeneity (VA/Q scatter), both Bohr dead space and shunt equations require values for central "ideal" compartment O₂ and CO₂ partial pressures. However, the ideal alveolar gas equation most accurately calculates mixed (ideal and alveolar dead space) PAO₂ . A novel "modal" definition has been validated for ideal alveolar CO₂ partial pressure, at the VA/Q ratio in a lung distribution where CO₂ elimination is maximal. A multicompartment computer model of physiological, lognormal distributions of VA and Q was used to identify modal "ideal" PAO₂ , and find a modification of the alveolar gas equation to estimate it across a wide range of severity of VA/Q heterogeneity and FIO₂ . This was then validated in vivo using data from a study of 36 anesthetized, ventilated patients with FIO₂ 0.35-80. Substitution in the alveolar gas equation of respiratory exchange ratio R with $$ achieved excellent agreement (r²  = 0.999) between the calculated ideal PAO₂ and the alveolar-capillary Pc'O₂ at the modal VO₂ point ("modal" Pc'O₂ ), across a range of log standard deviation of VA 0.25-1.75, true shunt 0%-20%, overall VA/Q 0.4-1.6, and FIO₂ 0.18-1.0, where the modeled PaO₂ was over 50 mm Hg. Modal ideal PAO₂ can be reliably estimated using routine blood gas measurements.