Balancing the circulation: Theoretic optimization of pulmonary/systemic flow ratio in hypoplastic left heart syndrome

Objectives. This study examined the effects of the pulmonary (Qp)/systemic (Qs) blood flow ratio (Qp/Qs) on systemic oxygen availability in neonates with hypoplastic left heart syndrome, Background. The management of neonates with hypoplastic left heart syndrome to complex and controversial. Both before and after surgical palliation and before ten transplantation, a univentricle with parallel pulmonary and systemic circulations exists. It is generally assumed that balancing pulmonary and systemic blood flow is best to the circulation. Methods. We developed a mathematical model that was based on the simple flow of oxygen uptake in the lungs and whole-body oxygen consumption to study the effect of varying the Qp/Qsratio. An equation was derived that related the key variables of cardiac output, pulmonary venous oxygen saturation and the Qp/Qsratio to systemic oxygen availability. Results. The key findings are 1) as the Qp/Qsratio increases, systemic oxygen availability increases initially, reaches a maximum and then decreases; 2) for maximal systemic oxygen availability, the optimal Qp/Qsratio is ≤ 1; 3) the optimal Qp/Qsratio decreases as Cardiac output or percent pulmonary venous oxygen saturation, or both, increase; 4) the critical range of Qp/Qs, where oxygen supply exceeds basal oxygen consumption, decreases as cardiac output and percent pulmonary venous oxygen saturation decrease; 5) the relation between oxygen availability and Qp/Qsis very steep when Qp/Qsapproaches this critical value; and 6) the percent oxygen saturation of systemic venous blood is very low outside the critical range of Qp/Qsand high within the critical range. Conclusions. This analysis provides a theoretic basis for balancing both the pulmonary and systemic circulation and suggests that evaluating both systemic arterial and venous oxygen saturation may be a useful way to determine the relative pulmonary and systemic flows. When high systemic arterial and low systemic venous oxygen saturation are present, pulmonary blood flow should be decreased; conversely, when both low systemic arterial and venous oxygen saturation are present, more flow should be directed to the pulmonary circulation.