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

Abstract

Simultaneous superior vena caval (SCvo ₂) and mixed venous ($S\bar vo_2$) 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 lowSCvo ₂ values, but normal$S\bar vo_2$ values. BothSCvo ₂ and$S\bar vo_2$ increased; the difference between them decreasing significantly during the study period (P<0.001). The results show thatSCvo ₂ is consistently lower than$S\bar vo_2$ 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. BecauseSCvo ₂ was frequently subnormal while$S\bar vo_2$ was in the normal range, monitoring of$S\bar vo_2$ could not be reliably used to rule out oxygen supply/demand imbalance during the early postoperative period in these patients.