Continuous monitoring of mixed venous oxygen saturation

Continuous monitoring of arterial oxygen tension is not sufficiently accurate for practical use in adults. Because of the shape of the hemoglobin dissociation curve, fluctuations at higher levels of oxygen tension are not reflected by corresponding changes in saturation; consequently, measurement of saturation at these levels also lacks sensitivity in detecting physiologic instability. However, at the lower levels of oxygen present in venous blood, a linear relationship exists between saturation and tension. The use of improved fiberoptic oximetry systems in conventional pulmonary artery flotation catheters has made the bedside application of this relationship of practical value in the continuous assessment of mixed venous oxygen saturation. Our own experience with it extends to more than 630 patients. Changes predictive of cardiorespiratory instability and instantaneous response to nursing or therapeutic maneuvers make this technique highly effective in the care of the critically ill.