DEPENDENCE OF SHUNT ON CARDIAC-OUTPUT IN UNILOBAR OLEIC-ACID EDEMA - DISTRIBUTION OF VENTILATION AND PERFUSION

Objective: In acute respiratory failure, increased cardiac output (Q(t )) increases shunt (Q(s)/Q(t)). We have tested if this is caused by: 1 ) a redistribution of blood flow towards edematous regions, or 2) a de crease of regional ventilation in the edematous region. Design: Oleic acid edema was induced in the left lower lobe (LLL) of 11 pigs. Q(t) w as varied with bleeding and infusion of blood and dextran. Blood flow to the LLL was measured at low and high Q(t) 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: Q(t) was increased by 45% (electromagnetic flow probes) and 73% (gamm a camera). Q(s)/Q(t) 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 decr ease in ventilation of LLL, 2.6%, was observed when Q(t) was increased (p<0.05). Conclusion: Theoretically a small decrease in ventilation c an explain the increase in shunt, if regions with low ventilation/perf usion (VA/Q) ratio are transformed to shunt. This is, however, unlikel y since earlier studies have shown that blood flow is distributed eith er to regions with normal VA/Q ratio or to shunt regions. We conclude that the cardiac output dependent shunt is not caused by redistributio n of blood flow between lobes or by decreased ventilation in the edema tous region. We cannot exclude that blood flow is redistributed within the edematous lobe.