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 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 - PEtC O 2 / P aCO 2 $$ \kern0.5em \mathrm{modalR}=\mathrm{R}\hbox{--} \left(1\hbox{--} \mathrm{PEtC}{\mathrm{O}}_2/\mathrm{P}{\mathrm{aCO}}_2\right) $$ 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.