functionally single systemic ventricle Blood from the placenta enters the fetus through the umbilical veins and then enters the portal system, inferior vena cava, and ultimately the systemic venous atrium via the ductus venosus In fetuses with a structurally normal heart, umbilical venous blood has an oxygen saturation of about 80% to 85% and a PO2 of about 32 to 35 mm Hg, although this may be diminished in fetuses with CHD.34 The dramatic changes in physiology seen with the transitional circulation in neonates with structurally normal hearts (fall in PVR; increase in pulmonary blood flow; increase in combined ventricular work; increase in systemic ventricular afterload; closure of the ductus venosus, ductus arteriosus, and foramen ovale, among others; see also Chapter 15) result in hemodynamic abnormalities that are quite variable, depending on the individual anatomy of the fUVH and the timing of “transition.” It is beyond the scope of this chapter to define these changes for the myriad of individual structural defects with “single ventricle physiology”; suffice it to say that the initial principles of presurgical management are in most cases directed toward mimicking the fetal circulation: • Ensuring continued patency of the ductus arteriosus • Minimizing restriction at the atrial level (if present) • Manipulating the distribution of the fUVH cardiac output, typically with strategies that keep PVR elevated and systemic vascular resistance low As the PVR continues to fall after birth, a higher proportion of the ventricular output is directed to the pulmonary vascular bed and the volume work of the ventricle increases Although this may be tolerated for days or even weeks, the increase in volume work will eventually lead to signs and symptoms of congestive heart failure As a compensatory mechanism, the systemic vascular resistance rises, further redirecting blood into the pulmonary vascular bed, worsening heart failure, and eventually leading to circulatory failure The baby may show all the classic signs of circulatory insufficiency including tachycardia, pallor, oliguria, and so forth However, clinical experience has shown that the development of heart failure is a slow, gradual process over days to weeks in the majority of neonates with an fUVH during transition, providing there is continued patency of the arterial duct As was learned in the early days of cardiac transplantation as a primary surgical strategy, many babies tolerate this relatively high ratio of Qp to Qs and can maintain their systemic oxygen delivery for weeks or even months.35,36 If the combined cardiac output (Qp + Qs) can be maintained, a high oxygen saturation may result in adequate oxygen delivery, albeit at the expense of a volume overloaded ventricle and possible progressive congestive heart failure In the preoperative neonate with a patent arterial duct, we have found that acute deterioration from increased pulmonary blood flow in isolation is a rare event Clarifications and Proposed Changes to Terminology Used in the Neonate, Infant, and Child With a Functionally Univentricular Heart “Parallel Circulation” Previously, the circulation of the neonate with a fUVH, both before and after surgical palliation, has been described as a parallel circulation We feel that this is an inaccurate term for babies with a fUVH, as a truly parallel circulation should be reserved for neonates with unrepaired transposition of the great arteries (see Chapter 37) More accurately, the fUVH must distribute the ventricular output to both the pulmonary and systemic vascular beds, and if there is atrioventricular valve regurgitation, retrograde to the atrium Thus, rather than a “parallel circulation,” the physiology in the neonate with a fUVH is better described as a multidistribution circulation (Fig 70.2) FIG 70.2 Multidistribution circulation in the neonate with a functionally univentricular heart The various factors affecting oxygen delivery, both