BALANCING THE CIRCULATION - THEORETIC OPTIMIZATION OF PULMONARY SYSTEMIC FLOW RATIO IN HYPOPLASTIC LEFT-HEART SYNDROME/

Objectives. This study examined the effects of the pulmonary (Q(P))/sy stemic (Q(S)) blood flow ratio (Q(P)/Q(S)) on systemic oxygen availabi lity in neonates with hypoplastic left heart syndrome. Background. The management of neonates with hypoplastic left heart syndrome is comple x and controversial. Both before and after surgical palliation and bef ore heart transplantation, a univentricle with parallel pulmonary and systemic circulations exists. It is generally assumed that balancing p ulmonary and systemic blood flow is best to stabilize the circulation. Methods. We developed a mathematical model that was based on the simp le flow of oxygen uptake in the lungs and whole-body oxygen consumptio n to study the effect of varying the Q(P)/Q(S) ratio. An equation was derived that related the key variables of cardiac output, pulmonary ve nous oxygen saturation and the Q(P)/Q(S) ratio to systemic oxygen avai lability. Results. The key findings are 1) as the Q(P)/Q(S) ratio incr eases, systemic oxygen availability increases initially, reaches a max imum and then decreases; 2) for maximal systemic oxygen availability, the optimal Q(P)/Q(S) ratio is less than or equal to 1; 3) the optimal Q(P)/Q(S) ratio decreases as cardiac output or percent pulmonary veno us oxygen saturation, or both, increase; 4) the critical range of Q(P) /Q(S), where oxygen supply exceeds basal oxygen consumption, decreases as cardiac output and percent pulmonary venous oxygen saturation decr ease; 5) the relation between oxygen availability and Q(P)/Q(S) is ver y steep when Q(P)/Q(S) approaches this critical value; and 6) the perc ent oxygen saturation of systemic venous blood is very low outside the critical range of Q(P)/Q(S) and high within the critical range, Concl usions. This analysis provides a theoretic basis for balancing both th e 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 system ic arterial and low systemic venous oxygen saturation are present, pul monary blood flow should be decreased; conversely, when both low syste mic arterial and venous oxygen saturation are present, more flow shoul d be directed to the pulmonary circulation.