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-aDO 2 ). Through a combination of pulse oximetry and rapidly responding gas analyzers to measure the partial pressures of O 2 and CO 2 in expired gas, one can measure the oxygen deficit. Defined as the difference between the measured alveolar Po 2 and the arterial Po 2 calculated from [Formula: see text], the oxygen deficit is a substitute for the alveolar-arterial Po 2 difference. The oxygen deficit is physiologically reasonable in that it increases with age in healthy subjects and is well correlated with the A-aDO 2 . To calculate arterial Po 2 from saturation, the saturation should be below the very flat upper part of the O 2 -Hb dissociation curve; good estimates can be made provided the arterial O 2 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 Po 2 at a saturation of 50% of the O 2 -Hb dissociation curve based on the measured alveolar Pco 2 . 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 O 2 saturations through a combination of expired O 2 and CO 2 measurements and [Formula: see text] 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 Po 2 difference and does not require arterial blood gas sampling.