Abstract

Arterial oxygen tension (Pa-O2) is known to decrease with age, and this is accompanied by a number of changes in mechanical properties of the lungs, including loss of elastic recoil and increase in closing volume. The changes in respiratory mechanics with age could induce greater ventilation/perfusion ((V) over dot A/(Q) over dot) mismatch and thus explain the decrease in Pa-O2. In 64 normal subjects aged 18 to 71 yr (lifetime nonsmokers with normal spirometry), we measured (V) over dot A/(Q) over dot inequality and arterial respiratory blood gases (Pa-O2 and Pa-CO2) at rest in the seated position. (V) over dot A/(Q) over dot mismatch, represented by the second moments of the blood flow and ventilation distributions (log SDQ and log SDV) increased with age, but only slightly (mean log SDQ was 0.36 at age 20 yr and 0.47 at age 70 yr). Pa-O2 fell by a correspondingly small amount of 6 mm Hg. Previously established upper 95% confidence limits for log SDQ (0.60) and log SDV (0.65) in subjects at age 20 yr were confirmed. At age 70 yr, the upper limits of reference for log SDQ are 0.70 and for log SDV 0.75. The study shows that an increased alveolar-arterial O-2 gradient with age is due to (V) over dot A/(Q) over dot inequality rather than to shunting.