Effects of Pulmonary Blood Flow and Mixed Venous O2 Tension on Gas Exchange in Dogs

Abstract

Whether the increases in venous admixture and intrapulmonary shunt which occur with increases in cardiac output (.ovrhdot.Qt) result from an effect mediated by mixed venous O2 partial pressure (P.hivin.VO2) or an effect mediated by the increase in pulmonary blood flow were studied. Using a veno-venous bypass system they were able to alter P.hivin.VO2 independent of variations in .ovrhdot.Qt and vice versa. During room air ventilation of dogs with normal lungs at constant .ovrhdot.Qt, an increase in P.hivin.VO2 from 33 .+-. 7 (mean .+-. SD) to 54 .+-. 9 mm Hg (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 P.hivin.VO2, raising .ovrhdot.Qt from 2.16 .+-. 0.53 to 3.49 .+-. 0.91 1/min (P < 0.05) increased venous admixture from 10 .+-. 5 to 16 .+-. 5% (P < 0.05). During O2 ventilation in these 2 groups of dogs, changes in P.hivin.VO2 and .ovrhdot.Qt had no effect on shunt. During O2 ventilation of dogs with significant shunts from oleic-acid-induced pulmonary edema, independent increases in either P.hivin.VO2 or pulmonary blood flow resulted in increased shunt. At constant .ovrhdot.Qt, and increase in P.hivin.VO2 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 P.hivin.VO2 remained constant, increasing .ovrhdot.Qt from 1.97 pm 0.42-0.50 l/min (P < 0.05) resulted in an increase in shunt from 47 .+-. 17 to 53 .+-. 15% (P < 0.05). During O2 ventilation, normal dogs apparently have shunts which are unaffected by changes in blood flow or P.hivin.VO2. Increases in pulmonary blood flow increase venous admixture during room air ventilation, while increases in P.hivin.VO2 decrease venous admixture during air ventilation. In edematous lungs, increases in either P.hivin.VO2 or pulmonary blood flow increase shunt.