Influence of pulmonary blood flow on gas exchange in piglets.

We previously reported that O2 diffusion was limited in piglets. To test the hypothesis of an inadequacy between diffusion and perfusion in piglets (< 4 wk) vs. older pigs (> 8 wk), we compared in these two age groups the effect of an increase (by opening an arteriovenous fistula) or a decrease (by inflating a balloon in the inferior vena cava) in cardiac output (Q) on gas exchange and on the O2 equilibration coefficient D/Q beta [ratio of the diffusion capacity of O2 (D) to the product of Q and the capacitance coefficient of blood (beta)]. In piglets but not in older pigs, a decrease in Q improved the alveolar-arterial Po2 difference (P < 0.05) and D/Q beta (P < 0.05), whereas an increase in Q had the opposite effect. Changes in the alveolar-arterial O2 difference and D/Q beta were linearly correlated with Q (r = 0.75, P < 0.01 and r = 0.88, P < 0.01, respectively). We suggest that the impaired O2 diffusion in piglets was due to inadequate diffusion-perfusion equilibrium of O2.