Pulmonary gas exchange during graded exercise in normal, sedentary, non-smoking subjects

There is some discordance in the literature on the evolution of the alveolo-arterial oxygen tension difference (A-a)DO2 from rest (R) to graded exercise (E) in healthy subjects. For some authors and not for others (A-a)DO2 increased during exercise. In order to solve this problem we have examined 9 healthy, non-smoking sedentary students (5 males). In every subject PaO2 increased from rest to the highest level of exercise. (A-a)DO2 significantly increased from R and E1 to E2 and from E3 to E4. (A-a)DO2 at rest was 7.5 +/- 4.1 torr; at E1: 7.0 +/- 4.3 torr; at E2: 11.5 +/- 4.2 torr; at E3: 12.5 +/- 3.9 torr; and at E4: 16.5 +/- 4.3 torr. The oxyhemoglobin dissociation curve (ODC) was significantly shifted to the right from 53 to 92% saturation. This shift favours the liberation of O2 from hemoglobin to the tissues but cannot explain the evolution of (A-a)DO2. We believe that the increase in (A-a)DO2 is due to an increased maldistribution of the ventilation-perfusion ratio.