For many years, the pulmonary artery catheter has been used to monitor cardiac filling pressures and to determine cardiac output in hemodynamically unstable patients. Recently, a new pulmonary artery catheter with fiberoptic capabilities, which provides continuous mixed venous O2 saturation (SvO2) measurements, has become available and has been found to be helpful in managing unstable patients. To determine the efficacy of this device in predicting early changes in cardiac output, we studied 46 patients catheterized with the opticath and 25 with the standard pulmonary artery catheter; we compared changes in the SvO2 with associated cardiac index changes. We found that small changes (5%) in SvO2 did not correlate well with changes in cardiac output, yet larger changes (10%) in SvO2 seemed to correlate better. More importantly, we found that only 50% of the SvO2 changes predicted anticipated changes in cardiac output. As can best be determined from the limitations of a nonrandomized study, the value of continuous SvO2 monitoring as an early predictor of cardiac output change remains questionable.