Deriving the arterial Po₂ and oxygen deficit from expired gas and pulse oximetry

The efficiency of pulmonary gas exchange is often assessed by the ideal alveolar-arterial partial pressure difference (A-aDO₂). Through a combination of pulse oximetry and rapidly responding gas analyzers to measure the partial pressures of O₂ and CO₂ in expired gas, one can measure the oxygen deficit. Defined as the difference between the measured alveolar Po₂ and the arterial Po₂ calculated from $$, the oxygen deficit is a substitute for the alveolar-arterial Po₂ difference. The oxygen deficit is physiologically reasonable in that it increases with age in healthy subjects and is well correlated with the A-aDO₂. To calculate arterial Po₂ from saturation, the saturation should be below the very flat upper part of the O₂-Hb dissociation curve; good estimates can be made provided the arterial O₂ saturation is below ~95%. Since saturations at or above 95% imply reasonably well-maintained gas exchange efficiency, this limitation is of only minor concern. Calculations show that it is necessary to take into account the change in Po₂ at a saturation of 50% of the O₂-Hb dissociation curve based on the measured alveolar Pco₂. As the measurement is designed to be noninvasive, determination of any base excess is not practical, but calculations show that the effect of assuming a zero base excess is modest, with a similar small effect from an abnormal body temperature. Taken together, these results show that a noninvasive assessment of pulmonary gas exchange efficiency can be obtained from subjects with below-normal arterial O₂ saturations through a combination of expired O₂ and CO₂ measurements and $$ made during quiet breathing.NEW & NOTEWORTHY The details and limitations of a noninvasive measurement of pulmonary gas exchange efficiency, the oxygen deficit, are described. The oxygen deficit, calculated from expired gas measurements made during quiet breathing coupled with pulse oximetry, is a good surrogate measurement of the ideal alveolar-arterial Po₂ difference and does not require arterial blood gas sampling.