Continuous monitoring of mixed venous oxygen saturation in hemodynamically unstable patients

A balloon-tipped catheter has recently become available which, when placed in the pulmonary artery, in addition to enabling the usual measurements of pulmonary arterial pressure, pulmonary capillary wedge pressure, and cardiac output by thermodilution, measures mixed venous oxygen saturation (SvO2) by spectrophotometry. Unlike the measurement of cardiac output by thermodilution, which is done intermittently, the continuous measurement of SvO2 is an effective method of monitoring hemodynamically unstable patients, since changes in cardiac output will immediately become apparent via a corresponding change in SvO2. This is of particular benefit in patients in whom knowledge of the immediate effects of therapy is important. It is also of value in assessing the time of onset of action and duration of action when a cardioactive drug is given to increase cardiac output. We suggest that monitoring SvO2 will provide an earlier indication of the effect of both diagnostic and therapeutic interventions and, therefore, will improve our management in such patients.