Utility of mixed venous vs central venous oximetry following cardiac surgery in infants

Objective

Mixed venous oxygen saturation (PAsat) has been used as a surrogate marker for trends in cardiac output and oxygen utilisation:supply ratio following cardiac surgery in infants. We wished to ascertain whether superior vena cava oxygen saturation (CVsat) would provide similar trend-related information.

Method

Twenty infants were studied following cardiac surgery, median age 2 weeks (range: 2 days-18 weeks). Operations included complete correction of the following: transposition of the great arteries (n = 10), ventricular septal defect (n = 7), atrioventricular septal defect (n = 2), and total anomalous pulmonary venous drainage (n = 1). All patients had absence of postoperative anatomical shunt on colour Doppler echocardiography. Single-lumen pulmonary artery lines were placed intraoperatively, and position of the preoperatively placed percutaneous central venous lines was checked by chest X-ray. Oxygen saturation was measured by co-oximetry. Cardiac index was estimated using a typical oxygen consumption of 9 ml/min/kg, and adjusted by 10% for every degree of temperature change from 37°C. Analysis was by two-way, repeated measures ANOVA.

Results

PAsats were generally higher than CVsats (group effect P = 0.02), with a mean difference of 6.7% 1-hour post operatively (Fig.); however this had dropped to 2.6% at 12 hours, and 0.4% at 24 hours. This did not produce a significant difference in estimated cardiac index (group effect P = 0.16, interaction P = 0.94).

Figure

On sequential readings, PAsats and CVsats trended in the same direction on 75% occasions; however the greatest discrepancy was between the 4 and 8-hour readings, when concordance was only found 55% of the time.

Conclusion

PAsats and CVsats provide similar quantitative and qualitative haemodynamic information in the absence of anatomical shunts following cardiac surgery. Consistent differences between the two readings in the first 12 hours may be due either due to regional perfusion and/or oxygen consumption differences between the upper and lower body, or may reflect a transient, small anatomical leak across sites of shunt correction which is missed by echocardiography.