PaO2 change during progressive pulmonary edema in dogs

We observed the changes of PaO2, pulmonary shunt (Qsp/Qt), effective compliance, and direct lung impedance after induction of hydrostatic pulmonary edema in five mongrel dogs. Edema was induced by elevating left atrial pressure in a stepwise manner while the dogs were ventilated with continuous positive pressure. Corresponding morphologic changes in the lung were examined by light and electron microscopy. PaO2 changed little or slightly until mean left atrial pressure reached 40 to 60 mm Hg. PaO2 then rapidly decreased from 411 +/- 61 to 89 +/- 19 torr, accompanied by an increase in Qsp/Qt from 33 +/- 6% to 57 +/- 14%. Effective compliance decreased gradually and direct lung impedance decreased rather steeply at first and then gradually. Lung specimens taken before the abrupt fall of PaO2 revealed froth on the cut surface, indicating that alveolar pulmonary edema preceded the PaO2 decrease. Electron microscopy showed widening and splitting of the thin portion of the alveolar-capillary interstitium when PaO2 fell abruptly. This widening of the thin portion may have been one of the causes of an abrupt PaO2 fall in the dog subjected to volume overload and increased afterload.