Evidence for an alveolar-arterial PO2 gradient threshold during incremental exercise.

Examination of the alveolar-to-arterial O2 tension (A-a PO2 difference) provides a method of examining the efficiency of pulmonary gas exchange during exercise. At present, considerable confusion exists as to the exact pattern of the A-a PO2 difference during incremental exercise. We tested the hypothesis that the A-a PO2 difference during incremental exercise is alinear with respect to metabolic rate. Measurements of the A-a PO2 difference were made on six healthy male subjects during incremental exercise under sea level conditions (PIO2 = 149 torr). An alinear model best described the relationship between the A-a PO2 difference and metabolic rate; only small increases in the A-a PO2 difference occurred at low work rates followed by a rapid increase at higher work rates. The existence of a "A-a PO2 difference threshold" was mathematically confirmed by the use of a computer algorithm to define inflection points. These data provide evidence that the relationship between the A-a PO2 difference and metabolic rate is alinear and that a metabolic threshold exists for a rapid increase in the A-a PO2 difference. We conclude that the efficiency of pulmonary gas exchange during exercise is unaltered from rest during low-to-moderate power outputs, however, high intensity exercise compromises pulmonary gas exchange efficiency as evidenced by a significant widening of the A-a PO2 difference.