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Physiologic Principles to Maximize Outcome in Patients With a Functionally Univentricular Heart
Introduction
The advances made in the management of newborns and infants with all forms of critical congenital heart disease (cCHD) over the past 3 decades have truly been among the triumphs of cardiac care. In all forms of cCHD, nearly simultaneous, cumulative, and synergistic advancements have been made in surgical strategies, anesthesia and bypass techniques, critical care, bedside nursing care, imaging, and catherization; importantly also, an in-depth understanding of the complex and variable physiology seen in neonates with cCHD has been achieved. In particular, these advances have greatly improved the outcome of babies born with a functionally univentricular heart (fUVH). Specifically, management strategies (e.g., phosphodiesterase inhibitors, nitric oxide, flow-triggered mechanical ventilation, noninvasive monitoring, point-of-care testing) initially studied in and applied to infants with a biventricular circulation have been extended to those with a fUVH, and techniques originally specific to fUVH management have subsequently been applied to neonates and infants with other conditions (e.g., arch repair from a midline sternotomy, hybrid catheterization/surgical strategies, prolonged alpha blockade, interstage monitoring).
This chapter focuses on management principles and practices for neonates and children with a fUVH who are undergoing staged reconstruction, with emphasis on the physiologic consequences of the underlying CHD and the changes that occur in the transitional circulation (see Chapter 15 ), surgical interventions, and patient growth along the Fontan pathway depicted in Chapter 68 . These physiologic principles are the underpinnings of surgical and perioperative management discussed in Chapter 71 .
Guiding Principles of Staged Reconstruction
Surgical and medical management in the first few years of life is undertaken to achieve a singular goal: a successful Fontan operation with maximum durability and quality of life . Such a “successful” outcome is achievable in many and has been improving in recent decades. Nonetheless, successful outcomes for staged reconstruction lag behind the surgical and medical advances in nearly every other form of CHD, both for mortality and morbidity. Improvement in this fundamental goal is most likely to be achieved by incremental and cumulative advancements in management during staged reconstruction and beyond. The specific goals outlined in Box 70.1 are discussed in more detail in other chapters of Section 6, Functionally Univentricular Heart, as well as elsewhere throughout this text.
Box 70.1
Strategies to Improve Overall Outcome During Staged Reconstruction
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Minimize the cumulative mortality risk of surgical and catheter interventions
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Minimize the cumulative morbidity of perioperative care to all organs, particularly the heart, brain, and kidney
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Minimize the risk and frequency of unplanned reinterventions
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Minimize the risk of interstage mortality and morbidity
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Maximize growth, nutrition, neurodevelopment, psychosocial adaptation, and cardiovascular fitness during staged reconstruction and beyond
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Maximize patient and family quality of life
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Improve collaboration between centers, and involve patients and families, to share and validate best practices
Strategies and Physiologic Goals to Obtain the Optimal Outcome of Staged Reconstruction
Rather than beginning this section with a review of the physiologic and management strategies governing the care of newborns and infants, we have chosen to start this review with the physiologic and surgical principles above that contribute to a successful Fontan operation. We discuss the tenets of a successful Fontal procedure before the discussion of newborn management because–although the neonatal procedures occur first or at high risk—it is necessary to understand the rationale of the higher risk procedures to achieve the primary goal. To achieve this goal, long-term follow-up data and our clinical experience suggest that the optimal Fontan outcome will most consistently be achieved by providing the highest possible cardiac output, at rest and with exertion, at the lowest possible central venous pressure . The well-described risk factors for suboptimal outcomes are described in Table 70.1 and pictured in Fig. 70.1A and B .
Table 70.1
Risk Factors for Mortality and Morbidity Following the Fontan Operation
| Systemic venous obstruction |
|
| Hypoplastic and/or narrowed central pulmonary arteries |
|
| Elevated pulmonary vascular resistance |
|
| Pulmonary vein stenosis |
|
| Elevated pulmonary venous atrial pressure |
|
| Elevated end-diastolic 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.
- 1
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).
- 2
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.
- 3
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. 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 ).
- 4
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 ).
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 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 PO 2 of about 32 to 35 mm Hg, although this may be diminished in fetuses with CHD.
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 :
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Ensuring continued patency of the ductus arteriosus
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Minimizing restriction at the atrial level (if present)
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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. 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.
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