Influence of Mixed Venous Oxygen Tension (PVO2) on Blood Flow to Atelectatic Lung

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Abstract
The influence of mixed venous O2 tension (P.hivin.VO2) on blood flow to the atelectatic left lung was studied at normal and reduced cardiac outputs (CO) using extracorporeal veno-venous bypass in 6 pentobarbital anesthetized, mechanically ventilated dogs. Aortic and left pulmonary artery flows; airway, left atrial, central venous, pulmonary and systemic arterial pressures; Hb, arterial, and mixed venous blood gases were measured. The blood flow reduction observed in atelectasis was altered by P.hivin.VO2. Approximately 50% of blood flow was diverted away from atelectatic lung when P.hivin.VO2 was low (24 .+-. 2 mm Hg) or normal (46 .+-. 2 mm Hg) (mean left lung blood flow [.ovrhdot.QL%] was 23.2 .+-. 4.6% with low P.hivin.VO2 and 19.0 .+-. 3.4%, with normal P.hivin.VO2). When P.hivin.VO2 was increased to > 100 mm Hg, diversion of blood flow away from atelectatic lung did not occur and .ovrhdot.QL% was nearly the flow expected for normoxic ventilated left lung (mean .ovrhdot.QL% = 40.4 .+-. 5.9%). Shunt (.ovrhdot.QS/.ovrhdot.QT%) was significantly greater when P.hivin.VO2 was high than when it was normal or low (mean .ovrhdot.QS/.ovrhdot.QT% = 51.7 .+-. 5.6%, 31.0 .+-. 3.1%, 26.0 .+-. 3.4% with high, normal and low P.hivin.VO2, respectively). Mean PaO2 [alveolar O2 pressure] was significantly greater when P.hivin.VO2 was high than when P.hivin.VO2 was normal or low despite the increase in .ovrhdot.QL% and .ovrhdot.QS/.ovrhdot.QT% (PaO2 = 327 .+-. 25 mm Hg, 220 .+-. 32 mm Hg, 115 .+-. 21 mm Hg with high, normal and low P.hivin.VO2, respectively). A 40% reduction in cardiac output significantly decreased transmural pulmonary artery pressure but did not affect PaO2, .ovrhdot.QS/.ovrhdot.QT% or .ovrhdot.QL%. The mechanism of blood-flow reduction to atelectatic lung is apparently hypoxic pulmonary vasoconstriction, determined by the P.hivin.VO2. The contribution of mechanical factors in reducing blood flow to atelectatic lung in the open chest is small.