Oxygen uptake and mixed venous oxygen saturation during aortic surgery and the first three postoperative hours

This study was designed to determine the significance of changes in mixed venous oxygen saturation (SVO2) associated with aortic surgery. In 12 patients undergoing aortic aneurysm repair, SVO2 was monitored using a fiberoptic pulmonary arterial catheter, and oxygen uptake (VO2) was measured at 2-min intervals by a mass-spectrometer system. Excluding the phase of aortic clamping, VO2, hemoglobin, and arterial oxygen saturation were moderately stable during anesthesia, and changes in SVO2 were correlated with changes in cardiac output (CO). SVO2 remained stable during infrarenal aortic clamping, but increased during supraceliac aortic clamping. During the first three postoperative hours, changes in SVO2 were opposite to changes in VO2 and CO. They were especially marked in the patients whose preoperative left ventricular ejection fraction was less than 50%. We conclude that SVO2 changes are an indicator of same-direction changes in CO during general anesthesia except during periods of aortic clamping. The interpretation of SVO2 changes is more complex during aortic clamping and during the immediate postoperative period, two critical periods during which simultaneous changes in VO2 and CO occur.