Alveolar-Arterial Oxygen Pressure Gradient: I. Comparison between an Assumed and Actual Respiratory Quotient in Stable Chronic Pulmonary Disease; II. Relationship to Aging and Closing Volume in Normal Subjects

An actual value for respiratory exchange ratio (R) was compared with a value assumed to be 0.8 in the calculation of the alveolar-arterial oxygen pressure gradient (P(A-a)O 2 ), both by theoretical analysis and analysis of results in patients with stable chronic pulmonary disease. These comparisons demonstrated that except for a few well-defined exceptional circumstances, the assumed value is sufficiently accurate for clinical purposes. Using an assumed R of 0.8, the P(A-a)O 2 was measured, as was the closing volume (CV) of the lung in 66 normal nonsmoking subjects between 20 and 82 years of age. A significant correlation between P(A-a)O 2 and CV was demonstrated and it is concluded that the increasing P(A-a)O 2 with advancing age results from a concomitant progressive occurrence of airway closure in the tidal volume range.