Dependence of shunt on cardiac output in unilobar oleic acid edema

Objective: In acute respiratory failure, in- creased cardiac output (Qt) increases shunt (Qs/0t). We have tested if this is caused by: 1) a redistribution of blood flow towards edematous regions, or 2) a decrease of regional ventilation in the edematous region. Design: Oleic acid edema was induced in the left lower lobe (LLL) of 11 pigs. Qt was varied with bleeding and infusion of blood and dextran. Blood flow to the LLL was measured at low and high Qt with electromagnetic low probes in 6 animals and with a gamma camera in 5. In the gamma camera pigs regional ventilation was also measured. Measurements and results: 0t was increased by 45% (electromagnetic flow probes) and 7307o (gamma camera). Qs/0t increased from 24.9-31.3~ (/)<0.05) and from 17.6- 28.8% (p < 0.001) respectively. No change in fractional perfusion of LLL could be seen, neither with flow probes nor with gamma camera. A de- crease in ventilation of LLL, 2.6070, was observed when Qt was increased (p< 0.05). Conclusion: Theoretically a small decrease in ventilation can explain the increase in shunt, if regions with low ven- tilation/perfusion (VA/0) ratio are transformed to shunt. This is, however, unlikely since earlier studies have shown that blood flow is distributed either to regions with normal VA/0 ratio or to shunt regions. We conclude that the cardiac output dependent shunt is not caused by redistribution of blood flow between lobes or by de- creased ventilation in the edematous region. We cannot exclude that blood flow is redistributed within the edematous lobe.