Dependence of shunt on cardiac output in unilobar oleic acid edema. Distribution of ventilation and perfusion.

OBJECTIVE

In acute respiratory failure, increased cardiac output (Qt) increases shunt (Qs/Qt). 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.

METHODS

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.

RESULTS

Qt was increased by 45% (electromagnetic flow probes) and 73% (gamma camera). Qs/Qt increased from 24.9-31.3% (p < 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 decrease in ventilation of LLL, 2.6%, was observed when Qt was increased (p < 0.05).

CONCLUSIONS

Theoretically a small decrease in ventilation can explain the increase in shunt, if regions with low ventilation/perfusion (VA/Q) 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/Q 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 decreased ventilation in the edematous region. We cannot exclude that blood flow is redistributed within the edematous lobe.