Abstract
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 O(2) and CO(2) partial pressures. However, the ideal alveolar gas equation most accurately calculates mixed (ideal and alveolar dead space) PAO(2) . A novel "modal" definition has been validated for ideal alveolar CO(2) partial pressure, at the VA/Q ratio in a lung distribution where CO(2) elimination is maximal. A multicompartment computer model of physiological, lognormal distributions of VA and Q was used to identify modal "ideal" PAO(2) , and find a modification of the alveolar gas equation to estimate it across a wide range of severity of VA/Q heterogeneity and FIO(2) . This was then validated in vivo using data from a study of 36 anesthetized, ventilated patients with FIO(2) 0.35-80. Substitution in the alveolar gas equation of respiratory exchange ratio R with modalR = R - (1 - PEtCO2/PaCO2) achieved excellent agreement (r(2) = 0.999) between the calculated ideal PAO(2) and the alveolar-capillary Pc'O(2) at the modal VO(2) point ("modal" Pc'O(2) ), 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 FIO(2) 0.18-1.0, where the modeled PaO(2) was over 50 mm Hg. Modal ideal PAO(2) can be reliably estimated using routine blood gas measurements.