Effect of Light and Moderate Exercise on Alveolar‐Arterial O₂Tension Difference in Man

The effect of light and moderate exercise (400, 625, and 700 kpm/min) on the alveolar‐arterial O 2 tension difference (A—aD) in the sitting position was studied in healthy, young adults by comparing the mean alveolar and arterial gas tensions determined over a 3 min rest period and over the ensuing 5th and 6th min of exercise. Arterial gas tensions were obtained from continuous measurements of arterial pH, oxygen saturation, and temperature. Alveolar O 2 tension was calculated by the alveolar gas equation, substituting arterial for alveolar CO 2 . At all three levels of exercise the arterial P O2 increased significantly by about 5mm Hg. In the experiments at 625 kpm/min, in which the respiratory exchange ratio (R) was also determined, A–aD decreased from a mean value of 14.7 mm Hg at rest, to 11.0 mm Hg during exercise. In the experiments at 400 and 700 kpm/min, where no ventilatory measurements were made in order to avoid interference with free respiration, it was found that, for the range of possible R values, the A–aD remained unchanged or decreased during exercise. The arterial O 2 deficit, i. e. the difference in O 2 content between pulmonary end‐capillary blood and systemic arterial blood, was calculated to decrease from a mean value of 0.27 vol‐% O 2 at rest, to 0.18–0.22 vol‐% O 2 during exercise. Likewise, the calculated total venous admixture to the systemic blood flow decreased from a mean value of 4.7 per cent of cardiac output at rest, to 1.7–1.8 per cent of cardiac output during exercise. All the above changes are ascribed mainly to a reduction of the virtual shunt created by uneven distribution of alveolar ventilation to pulmonary capillary blood flow.