Independent influence of blood flow rate and mixed venous PO2 on shunt fraction.

We have tested the independent and combined effects of changes in mixed venous PO2 (PvO2) and blood flow (QT) on shunt fraction (Qs/QT) in isolated blood-perfused canine left lower lobes with edema. The lobes were ventilated with pure O2. Inflow (Pi) and outflow (Po) pressures always exceeded lobar alveolar pressure. PvO2 was varied by means of a clinical bubble oxygenator with appropriate mixtures of O2 and N2. QT was varied by changes in Pi and Po with care not to produce changes in lobar weight. Changes in QT did not influence Qs/QT. Increasing PvO2 from 40 +/- 6 to 88.4 +/- 40 Torr at constant QT significantly increased Qs/QT from 5.5 +/- 2.0 to 15.6 +/- 7.0%. Combined increases in QT and PvO2 from 66.4 +/- 2.7 to 135.6 +/- 21.5 ml/min and from 38.8 +/- 1.3 to 61.8 +/- 2.2 Torr, respectively, also produced a significant increase in Qs/QT from 7.33 +/- 2.27 to 15.43 +/- 4.45%. However, this combined change was explained exclusively by changes in PvO2. We therefore concluded that, under the conditions of our experiment, changes in PvO2 influence Qs/QT, and this may account for apparent dependence of Qs/QT on cardiac output in pulmonary edema.