Point: hypoxia is the optimal means of reducing pulmonary blood flow in the preoperative single ventricle heart.

Patients with single ventricle are often quite ill and present complex management dilemmas. The distribution of blood flow to the systemic and pulmonary circulations, which in this patient population are in parallel rather than series, depends primarily upon the relative resistances of the respective vascular beds. The entire cardiac output is managed by the single ventricle and must somehow divide itself between the systemic and pulmonary circuits. As pulmonary vascular resistance decreases after birth, many of these children will suffer from overcirculation into the pulmonary circuit. In patients with ductal dependent systemic circulation, such as hypoplastic left heart syndrome, this can result in a paucity of systemic circulation with concomitant acidosis (Figure 1A).