Increased cardiac output increases shunt: Role of pulmonary edema and perfusion

In low-pressure pulmonary edema increased cardiac output [Q̇(T)] increases shunt [Q̇s/Q̇(T)]; we tested whether the mechanism is an increase in extravascular lung water in turn mediated by the accompanying increase in microvascular pressure. In six pentobarbital sodium-anesthetized dogs ventilated with O₂ we administered oleic acid into the right atrium. From base line to 2 h post-oleic acid we measured concurrent significant increases in Q̇s/Q̇(T) (6-29%, O₂ technique) and extravascular thermal volume (ETV, 2.6-7.1 ml/g dry intravascular blood-free lung wt, thermal-green dye indicator technique) that were stable by 90 min. Then, bilateral femoral arteriovenous fistulas were opened and closed in 30-min periods to cause reversible increases in Q̇(T) and associated Q̇s/Q̇(T). When fistulas were open the time-averaged Q̇(T) increased from 5.1 to 6.9 min (P < 0.05), the simultaneous Q̇s/Q̇(T) rose from 30.7 to 38.4% (P < 0.05), but ETV did not increase. We conclude that increasing lung edema does not account for our rise in Q̇s/Q̇(T) when Q̇(T) increased.