Mixed venous oxygen saturation. Its role in the assessment of the critically ill patient

Except in the presence of left to right shunts, slow aspiration from the distal lumen of a catheter placed in the pulmonary artery can provide a sample of mixed venous blood. The oxygen saturation of such a sample (SvO2) is dependent on arterial oxygen saturation, hemoglobin concentration, cardiac output, and tissue oxygen demands. A fall in SvO2 usually implies anemia, arterial oxygen desaturation, and/or decreased cardiac output; however, a normal or high value does not exclude such disturbances. When employed in conjunction with the other indicators of tissue oxygenation available in an intensive care unit, SvO2 can be useful as a guide for both prognosis and urgency of therapy.