Effects of pulmonary blood flow and mixed venous O2 tension on gas exchange in dogs.

The authors investigated whether the increases in venous admixture and intrapulmonary shunt which occur with increases in cardiac output (Qt) results from an effect mediated by mixed venous PO2 (PVO2) or an effect mediated by the increase in pulmonary blood flow. Using a veno-venous bypass system thay were able to alter PVO2 independent of variations in Qt and vice versa. During room air ventilation of dogs with normal lungs at constant Qt, an increase in PVO2 from 33 +/- 7 (mean +/- SD) to 54 +/- 9 mmHg (P less than 0.05) resulted in a decrease in venous admixture from 22 +/- 11 to 13 +/- 4% (P less than 0.05). During room air ventilation of normal dogs at a constant PVO2, raising Qt from 2.16 +/- .53 to 3.49 +/- 0.91 l/min (P less than 0.05) increased venous admixture from 10 +/- 5 to 16 +/- 5% (P less than 0.05). During oxygen ventilation in these two groups of dogs, changes in PVO2 and Qt had no effect on shunt. During oxygen ventilation of dogs with significant shunts from oleic-acid-induced pulmonary edema, independent increases in either PVO2 or pulmonary blood flow resulted in increased shunt. At constant Ot, an increase in PVO2 from 30 +/- 8 to 52 +/- 3 mmHg (P less than 0.05) resulted in an increase in shunt from 39 +/- 12 to 43 +/- 12% (P less than 0.05). When PVO2 remained constant, increasing Qt from 1.97 pm 0.42 to 3.61 +/- 0.50 l/min (P less than 0.05) resulted in an increase in shunt from 47 +/- 17 to 53 +/- 15% (P less than 0.05). The authors conclude that during oxygen ventilation, normal dogs have shunts which are unaffected by changes in blood flow or PVO2. Increases in pulmonary blood flow increase venous admixture during room air ventilation, while increases in PVO2 decrease venous admixture during air ventilation. In edematous lungs, increases in either PVO2 or pulmonary blood flow increase shunt.