COMPONENTS OF ALVEOLAR-ARTERIAL O2 DIFFERENCE IN NORMAL MAN.

The alveolar-arterial oxygen difference (A-aD) is believed due to true venous shunting, unequal distribution of alveolar ventilation (Va) to pulmonary capillary flow (Q), and diffusion limitations. Measurement of the A-aD was made at varying levels of alveolar oxygen tension in order to separate these three factors. The A-aD averaged 7.3, 15.2, and 37.1 mm Hg during the breathing of 14, 21, and 100% oxygen. The true venous shunt was 3.0 ± 1.9% of the systemic blood flow; unequal distribution of Va to Q resulted in an additional virtual shunt of 3.4 ± 2.6%. Breathing room air the diffusion component of the A-aD was negligible, true shunting accounted for 6.2 ± 4.0 mm Hg of the total A-aD. This created an A-aD of 9 and 0.6 mm for nitrogen and carbon dioxide, respectively. The method used could not distinguish the unequal Va/Q effect from the effect of unequal distribution of diffusion and perfusion. true veno-arterial shunt; alveolar ventilation-pulmonary capillary flow ratio (Va/Q); pulmonary membrane diffusion (D); virtual shunt (unequal Va/Q effect); distributional and diffusion components of A-aD; perfusion of nonventilated alveoli; venous admixture diffusing capacity; venoarterial shunting; alveolar ventilation/perfusion ratios; arterial oxygen tension; platinum electrode Submitted on May 22, 1963