Noninvasive measurement of pulmonary gas exchange: comparison with data from arterial blood gases

A new noninvasive method was used to measure the impairment of pulmonary gas exchange in 34 patients with lung disease, and the results were compared with the traditional ideal alveolar-arterial Po₂ difference (AaDO₂) calculated from arterial blood gases. The end-tidal Po₂ was measured from the expired gas during steady-state breathing, the arterial Po₂ was derived from a pulse oximeter if the $$ was 95% or less, which was the case for 23 patients. The difference between the end-tidal and the calculated Po₂ was defined as the oxygen deficit. Oxygen deficit was 42.7 mmHg (SE 4.0) in this group of patients, much higher than the means previously found in 20 young normal subjects measured under hypoxic conditions (2.0 mmHg, SE 0.8) and 11 older normal subjects (7.5 mmHg, SE 1.6) and emphasizes the sensitivity of the new method for detecting the presence of abnormal gas exchange. The oxygen deficit was correlated with AaDO₂ ( R² 0.72). The arterial Po₂ that was calculated from the noninvasive technique was correlated with the results from the arterial blood gases ( R² 0.76) and with a mean bias of +2.7 mmHg. The Pco₂ was correlated with the results from the arterial blood gases (R² 0.67) with a mean bias of -3.6 mmHg. We conclude that the oxygen deficit as obtained from the noninvasive method is a very sensitive indicator of impaired pulmonary gas exchange. It has the advantage that it can be obtained within a few minutes by having the patient simply breathe through a tube.