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 Poi difference (AaDO_2) calculated from arterial blood gases. The end-tidal Pen was measured from the expired gas during steady-state breathing, the arterial Po_2 was derived from a pulse oximeter if the Spo_2 was 95 or less, which was the case for 23 patients. The difference between the end-tidal and the calculated Po_2 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.
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