Andersons pediatric cardiology 1823

FIG 70.1 Schematic representation of flow through the Fontan circulation The primary long-term goal is to achieve the highest possible cardiac output at the lowest possible central venous pressure (CVP) The CVP is determined by a series of potential resistors through the circuit Optimal outcomes are achieved by minimizing obstruction and impedance to flow from the systemic venous compartment to the functional single ventricle (A) Potential areas of obstruction are shown in a linear fashion (B) Representative anatomically correct illustration Obstruction to flow along any of the pathways (arrows in A or asterisks in B) reduces overall cardiac output and/or elevates central venous pressure Physiology Immediately After Birth Clinical Presentation In general, the majority of neonates with a fUVH can be broadly categorized physiologically with either ductal-dependent pulmonary blood flow (right-sided lesions) or ductal-dependent systemic blood flow (left-sided lesions; see also Chapter 69) The physiology of the neonate with these conditions is discussed in detail further on Uncommonly, a neonate may have (1) no significant obstruction to either the systemic or pulmonary blood flow or (2) no systemic outflow tract obstruction and “just the right amount” of anatomic obstruction to pulmonary blood flow (not causing hypoxemia, pulmonary hypertension, or congestive heart failure) Individualized management plans will have to be developed for these more uncommon types of fUVH (see Chapter 71) However, in general, most neonates with a fUVH require surgery shortly after birth and tend to present in one of four mutually exclusive ways If the diagnosis has been made prenatally, an expectant team of caregivers manages a metabolically stable neonate with prostaglandin and minimal other interventions (see Section 2, Prenatal Congenital Cardiac Disease) Neonates presenting postnatally with a fUVH and ductal-dependent pulmonary blood flow will typically show signs of progressive hypoxemia and respiratory distress upon constriction or closure of the arterial duct Neonates presenting postnatally with a fUVH and ductal-dependent systemic blood flow will typically present with the acute onset of heart failure and, in the worst scenario, shock, with multiorgan system failure upon constriction or closure of the arterial duct There is typically decreased systemic perfusion, with increased flow to the lungs, largely independent of the pulmonary vascular resistance (PVR) The peripheral pulses are weak to absent Renal, hepatic, intestinal, coronary, and central nervous system perfusion is compromised, possibly associated with acute tubular necrosis, necrotizing enterocolitis, white matter injury, cerebral infarction, and/or hemorrhage In patients with aortic atresia, a vicious cycle may also result from inadequate retrograde perfusion of the ascending aorta and coronary arterial supply, with further myocardial dysfunction and continued compromise of flow to the coronary arteries Thus, one has the paradoxic presentation of a profound metabolic acidosis in the face of a relatively high partial pressure of oxygen, occasionally as high as 60 to 70 mm Hg In these neonates, at the initial presentation, sepsis is frequently suspected before the cardiac diagnosis is made.28–30 Fortunately an increasing prevalence of a prenatal diagnosis has made this unfortunate situation increasingly less likely in the current era (see Section 2) Also, routine screening with pulse oximetry has minimized the frequency of shock and/or profound hypoxemia in neonates without a prenatal diagnosis of a fUVH (see Chapter 89) Finally, neonates with a fUVHin whom the ductus remains patent will present with symptoms of mild congestive heart failure and hypoxemia, with or without visible cyanosis or a cardiac murmur and no end-organ dysfunction Fetal and Transitional Circulation An understanding of fetal blood flow patterns and the changes that occur after birth (described in detail in Chapter 15) is crucial to comprehending the physiologic challenges facing the neonate with CHD and particularly the baby with a fUVH A fetus with either left or right heart hypoplasia typically does not show significant intrauterine growth retardation; however, there is growing evidence that univentricular cardiac output may be diminished compared with that of a fetus with a structurally normal heart, with secondary effects to the placenta and developing brain (see Chapters 11 and 76) In left-sided lesions in particular, decreases in fetal cerebral blood flow are partially balanced by a decrease in cerebrovascular resistance, allowing the fetus to maintain oxygen and substrate delivery to the brain Despite these adjustments, brain abnormalities in neonates with CHD are common and well described (see Chapter 76).31–34 Metabolic demands on a fetus are limited, and the fluid-filled high-resistance fetal lungs receive only about 10% of the ventricular output In the absence of significant atrioventricular valve regurgitation, oxygen delivery and growth of the fetus are therefore determined only by the ability of the placenta to provide oxygen-rich blood to the systemic venous atrium and the contractility of the