Lack of Correlation between Rales and Arterial Oxygen Saturation in Patients with Pulmonary Congestion and Edema

It is generally believed that pulmonary edema impairs the diffusion of oxygen from alveoli to pulmonary capillary blood and produces anoxemia. However, some patients who apparently have widespread pulmonary edema, as judged by the extent of rales heard over the lung fields, still have normal arterial oxygen saturation. Explanations are advanced for the lack of correlation between extent of rales and arterial oxygen saturation.