Tolerance of unusually low mixed venous oxygen saturation. Adaptations in the chronic low cardiac output syndrome.

Introduction of a reliable method for continuous monitoring of mixed venous oxygen saturation has focused attention on the therapeutic and prognostic implications of this value. It is generally agreed that mixed venous oxygen saturation in the 40 to 60 percent range is associated with substantial patient morbidity and mortality if not rapidly corrected. However, several patients with chronic low cardiac output syndrome who have had mixed venous oxygen saturation of less than 40 percent for prolonged periods of time and who have not had decompensation were observed. Three representative patients with these findings are described and mechanisms put forth that may account for the observed relations between mixed venous oxygen saturation and mortality in these and other patients.