Patients with hypoplastic left heart syndrome (HLHS) have some combination of aortic and mitral atresia or stenosis, which due to decreases in flow, leads to underdevelopment of the left ventricle. The single right ventricle (RV) provides systemic blood flow from the pulmonary artery (PA) to the aorta via a patent ductus arteriosus. Prostaglandin E1 (alprostadil) is used to maintain PDA flow prior to creation of PA-to-systemic shunt. A key principle relates to balance of the systemic (Qs) and pulmonary (Qp) blood flows, represented by the ratio Qp/Qs. In a parallel circulation, a Qp/Qs ratio of 1 represents the lowest total cardiac output (Qp + Qs) required to provide systemic oxygen delivery. Another consequence of the parallel circulation is that increased oxygen extraction reduces not only SvO2 but also SaO2. Therefore for any given Qp/Qs, increased tissue oxygen demand can be met only by increased cardiac output. For any given decrease in cardiac output, DO2 and Svo2 are disproportionately reduced because Sao2 also falls. Thus, changes in oxygen supply and demand are interdependent and destabilizing in patients with parallel univentricular physiology.