How does increased cardiac output increase shunt in pulmonary edema?

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