Pulmonary/systemic flow ratio in children after cavopulmonary anastomosis

Objectives. This study attempted to provide formul for calculation of the pulmonary/systemic flow ratio in children after bidirectional cavopulmonary anastomosis. Background. With the bidirectional cavopulmonary anastomosis, only the superior ven cav blood is oxygenated by the lungs. The inferior ven cav flow recirculates into the systemic circulation. The ratio of these flows will determine systemic arterial saturation. Methods. According to the Fick principle, 1) Systemic cardiac output (liters/min) = Pulmonary venous flow + Inferior ven cav flow; 2) Systemic blood oxygen transport (ml/min) = Pulmonary venous blood oxygen transport + Inferior ven cav blood oxygen transport. By substituting the first equation into the second, Pulmonary/systemic flow ratio = (Systemic saturation — Inferior ven cav saturation)/(Pulmonary venous saturation — Inferior ven cav saturation). Results. We applied the third formul to dat obtained from 34 catheterizations in 29 patients after bidirectional cavopulmonary anastomosis. Mean [±SD] age at operation was 1.70 ± 1.43 years, and mean age at catheterization was 2.95 ± 1.65 years. The pulmonary/systemic flow ratio calculated for all 29 patients was 0.58 ± 0.09. Of 17 patients with aortography, 10 had systemic to pulmonary collateral vessels. Patients with collateral vessels had significantly higher pulmonary/systemic flow ratio (0.61 ± 0.07 vs. 0.53 ± 0.07, respectively, p < 0.02) and systemic saturation (88 ± 4% vs. 82 ± 4%, respectively, p < 0.002) than those without collateral vessels. The pulmonary/systemic flow ratio in those patients with no collateral vessels was similar to the previously reported echocardiographically derived superior ven cava/ systemic flow ratio in normal children. Conclusions. The pulmonary/systemic flow ratio after bidirectional cavopulmonary anastomosis can be calculated. Pulmonary blood flow in these patients determines systemic saturation and accounts for the majority of venous return in young children.