[Determination of heart time volume using the Fick principle in the early postoperative phase after correction of congenital heart defects. Comparison of the calculation of arterio-mixed venous oxygen differences and pulmonary oxygen uptake with the calculation of arterial-central venous oxygen differences and pulmonary oxygen uptake]

Abstract

Comparison of the calculation by means of the arterio-mixed venous oxygen difference and the oxygen uptake with the calculation by means of the arterio-central venous oxygen difference and the oxygen uptake.

Objective: How reliable is the measurement of cardiac output on Fick's principle without a pulmonary artery catheter?

Setting: PICU in an University hospital.

Design: In the postoperative period following complete repair of congenital heart disease we carried out 91 simultaneous measurements of blood gases in 45 infants and children (mean age 18.6 months, mean body weight 8.9 kg) from a systemic artery, the A. pulmonalis, and the V. cava superior. We also determined the pulmonary oxygen uptake in 24 patients (48 measurements). Cardiac output was calculated on Fick's principle using the arterio-mixed venous oxygen difference and the pulmonary oxygen uptake (HZV a-pa) and compared to the cardiac output derived from the central venous values (HZV a-zv). We differentiated between patients with a left to right shunt of 10% or more postoperatively (group A, n = 18) and all others (group B, n = 27).

Results: In both groups the correlation coefficient between HZV a-zv and HZV a-pa was high (group A: r = 0.97, group B: r = 0.94). In group A HZV a-pa (mean: 1958 ml/min) was higher than HZV a-zv (mean: 1340 ml/min), group B showed the opposite situation (mean HZV a-pa: 1136 ml/min, mean HZV a-zv: 1373 ml/min). With the Wilcoxon signet-rank test we found significant differences between the partial pressure of oxygen and the saturation of central venous and mixed venous blood samples in both groups, but HZV a-zv and HZV a-pa were different significantly on a level of p < or = 0.01 only in group A.

Conclusions: In both groups HZV a-pa and HZV a-zv correlated well. Therefore, if a pulmonary artery catheter is not inserted; the course of the cardiac output can be calculated with acceptable reliability from the central venous blood gases. By means of Fick's principle the pulmonary blood flow is determined, which is higher than the systemic blood flow in cases of left to right shunting, because of the recirculation in the pulmonary blood circuit. Interpreting the results this has to be taken into account.