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

Abstract

The authors investigated whether the increases in venous admixture and intrapulmonary shunt which occur with increases in cardiac output (Q̇(t)) result from an effect mediated by mixed venous P(O2) (Pv̄(O2)) or an effect mediated by the increase in pulmonary blood flow. Using a veno-venous bypass system they were able to alter Pv̄(O2) independent of variations in Q̇(t) and vice versa. During room air ventilation of dogs with normal lungs at constant Q̇(t), an increase in Pv̄(O2) from 33 ± 7 (mean ± SD) to 54 ± 9 mmHg (P < 0.05) resulted in a decrease in venous admixture from 22 ± 11 to 13 ± 4% (P < 0.05). During room air ventilation of normal dogs at a constant Pv̄(O2), raising Q̇(t) from 2.16 ± .53 to 3.49 ± 0.91 l/min (P < 0.05) increased venous admixture from 10 ± 5 to 16 ± 5% (P < 0.05). During oxygen ventilation in these two groups of dogs, changes in Pv̄(O2) and Q̇(t) had no effect on shunt. During oxygen ventilation of dogs with significant shunts from oleic-acid-induced pulmonary edema, independent increases in either Pv̄(O2) or pulmonary blood flow resulted in increased shunt. At constant Q̇(t), an increase in Pv̄(O2) from 30 ± 8 to 52 ± 3 mmHg (P < 0.05) resulted in an increase in shunt from 39 ± 12 to 43 ± 12% (P < 0.05). When Pv̄(O2) remained constant, increasing Q̇(t) from 1.97 ± 0.42 to 3.61 ± 0.50 l/min (P < 0.05) resulted in an increase in shunt from 47 ± 17 to 53 ± 15% (P < 0.05). The authors conclude that during oxygen ventilation, normal dogs have shunts which are unaffected by changes in blood flow or Pv̄(O2). Increases in pulmonary blood flow increase venous admixture during room air ventilation, while increases in Pv̄(O2) decrease venous admixture during air ventilation. In edematous lungs, increases in either Pv̄(O2) or pulmonary blood flow increase shunt.