Relationship between oxygen uptake and mixed venous oxygen saturation in the immediate postoperative period

Abstract

Background: During muscular exercise, a negative correlation has been demonstrated between the value of mixed venous oxygen saturation (Sv̇(O2)) and the level of muscular work, expressed at each level as the ratio of oxygen uptake (V̇(O2)) to each subject's maximal oxygen uptake (V̇(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: V̇(O2)max was determined in 11 patients 3-5 days before coronary artery bypass surgery. During the first 2 postoperative h, V̇(O2) and Sv̇(O2) were monitored. V̇(O2) was measured by indirect calorimetry and Sv̄(O2) by a fiberoptic pulmonary arterial catheter. Results: The highest postoperative value of V̇(O2) was most often associated with visible shivering and ranged among patients from 19% to 53% of preoperatively measured V̇(O2) max. There was a highly significant negative correlation between Sv̄(O2) and the ratio V̇(O2)/V̇(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, V̇(O2) rarely exceeds 50% of preoperative V̇(O2)max. Assuming a stable state of myocardial function, Sv̄(O2) measurement may provide an indirect means of assessment of the 'exercise test' imposed on patients recovering from general anesthesia.