How does increased cardiac output increase shunt in pulmonary edema?

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Abstract
In pulmonary edema, the relationship between cardiac output (.ovrhdot.QT) and shunt (.ovrhdot.Qs/.ovrhdot.QT) may be due to a diffusion barrier for O2 transfer (incomplete alveolar-capillar equilibration) or to redistribution of increased pulmonary blood flow toward edematous units. Transfer of O2 and multiple inert gases in the left (LLL) and right (RLL) lower lobes and in the whole lungs of 8 dogs having oleic acid edema in LLL. When mean .ovrhdot.QT was increased from 3.0 to 5.5 l .cntdot. min-1 during O2 ventilation, relative perfusion of LLL did not increase, but .ovrhdot.Qs/.ovrhdot.QT increased because LLL shunt increased from 56 to 78%. Increased pulmonary blood flow is apparently not redistributed toward edematous regions, but such redistribution within LLL and other slightly edematous lobes cannot be excluded. In LLL, inert gas shunt and O2 shunt were not systematically different during O2 ventilation and lobar venous PO2 [partial pressure of O2] measured during air ventilation was not different from that predicted by inert gas transfer. Diffusion limitation for O2 does not contribute to .ovrhdot.Qs/.ovrhdot.QT or to the increase in .ovrhdot.Qs/.ovrhdot.QT when .ovrhdot.QT increases. Conceivably, increased .ovrhdot.QT increased .ovrhdot.Qs/.ovrhdot.QT by increasing edema or hematocrit in edematous regions.