Abstract
The PAO2-PaO2 relationship was studied for the entire therapeutic range of oxygen in patients and animals in acute respiratory failure. The method is based on the assumption that the steady state values of PaO2 may be obtained as a mean of the two PaO2 values at an identical FIO2, one obtained 6 min after the FIO2 was raised from a lower level, the other obtained 6 min after the FIO2 was reduced from the higher level. We found that the shunts were large in the low FIO2 range (170 mmHg and below), took a minimum value in the moderate PAO2 range (170 to 300 mmHg), and increased again in the high PAO2 range (300 to 700 mmHg). A similar pattern was observed in the animal experiments, two or more hr following experimentally produced aspiration pneumonitis. In contrast, the dogs with bilateral pneumothorax showed a pattern which followed the isoshunt line closely. It was concluded that patients with acute respiratory failure requiring artificial ventilation have two components of the pulmonary shunt, one parallel with and the other inversely related with the PAO2. Possible mechanisms for the former were discussed.
