Influence of cardiac output on intrapulmonary shunt.

The effect of changing cardiac output on intrapulmonary shunt was studied in the setting of hemorrhagic pulmonary edema induced by intravenous oleic acid in 17 alpha-chloralose-anesthetized dogs. The dogs were mechanically ventilated and cardiac output was alternately depressed and augmented using either pharmacologic means or mechanical alteration of venous return. Following oleic acid, the measured ventilation-perfusion distributions, as measured by the multiple inert gas elimination technique, demonstrated a two-compartment blood flow distribution consisting of shunt and normal units. Changes in cardiac output were not associated with change in the shape of the distributions but there was a significant linear correlation between the level of cardiac output and the shunt fraction. The shunt fraction also varied directly with the mixed venous partial oxygen pressure and inversely with the pulmonary vascular resistance. These data suggest that shunt and nonshunt vessels behave differently in response to alterations of blood flow and emphasize that any interpretation of changes in shunt in the setting of diffuse lung injury must be interpreted in light of changes in cardiac output.