Superior vena caval and mixed venous oxyhemoglobin saturations in children recovering from open heart surgery.

Simultaneous superior vena caval (Scvo2) and mixed venous (Svo2) oxyhemoglobin saturation values in 15 children recovering from open heart surgery were compared to assess the value of superior vena caval blood samples in monitoring systemic oxygen supply/demand balance. Samples were obtained immediately following the operation and postoperatively every morning for 4 days. During the 4-day study period, the patients' cardiopulmonary functions improved, allowing partial weaning from respiratory and cardiovascular support. The lowest values of superior vena caval (46.7 +/- 8.4%) and mixed venous (63.7 +/- 10.9%) oxyhemoglobin saturation were measured immediately after the operation. At this time, 6 patients had abnormally low Scvo2 values, but normal Svo2 values. Both Scvo2 and Svo2 increased; the difference between them decreasing significantly during the study period (P less than 0.001). The results show that Scvo2 is consistently lower than Svo2 in children recovering from open heart surgery. This difference may be secondary to residual intracardiac left-to-right shunting of blood or to altered distribution of systemic blood flow. The saturation difference between the two venous samples decreases during postoperative recovery, making a superior vena caval blood sample an inadequate substitute for a mixed venous blood sample in calculating derived cardiopulmonary variables intended to reflect the function of the body as a whole. Because Scvo2 was frequently subnormal while Svo2 was in the normal range, monitoring of Svo2 could not be reliably used to rule out oxygen supply/demand imbalance during the early postoperative period in these patients.