RELATIONSHIP BETWEEN OXYGEN-UPTAKE AND MIXED VENOUS OXYGEN-SATURATION IN THE IMMEDIATE POSTOPERATIVE PERIOD

Background: During muscular exercise, a negative correlation has been demonstrated between the value of mixed venous oxygen saturation (S (v) over bar(O2)) and the level of muscular work, expressed at each level as the ratio of oxygen uptake (V-O2) to each subject's maximal oxygen uptake (V(O2)max). Because the immediate postoperative period is associated with an increase in whole body oxygen demand, and in this regard resembles the effects of muscular exercise, a similar correlation may exist during this period. Methods: V(O2)max was determined in 11 patients 3-5 days before coronary artery bypass surgery. During the first 2 postoperative h, V-O2 and S (v) over bar(O2) were monitored. V-O2 was measured by indirect calorimetry and S (v) over bar(O2) by a fiberoptic pulmonary arterial catheter. Results: The highest postoperative value of V-O2 was most often associated with visible shivering and ranged among patients from 19% to 53% of preoperatively measured V(O2)max. There was a highly significant negative correlation between S (v) over bar(O2) and the ratio V-O2/V(O2)max. This correlation was observed when data were examined collectively (136 simultaneous determinations of the two variables) and at the individual level (10-18 determinations for each patient). The slopes and the y intercepts of individual lines of correlation were within a narrow range. Conclusions: During the first 2 postoperative h after coronary artery bypass surgery, V-O2 rarely exceeds 50% of preoperative V(O2)max. Assuming a stable state of myocardial function, S (v) over bar(O2) measurement may provide an indirect means of assessment of the ''exercise test'' imposed on patients recovering from general anesthesia.