Deriving the arterial P<scp>o</scp>2 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 [Formula: see text], 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 [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 Po2 difference and does not require arterial blood gas sampling.