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





  • Minimize the cumulative mortality risk of surgical and catheter interventions



  • Minimize the cumulative morbidity of perioperative care to all organs, particularly the heart, brain, and kidney



  • Minimize the risk and frequency of unplanned reinterventions



  • Minimize the risk of interstage mortality and morbidity



  • Maximize growth, nutrition, neurodevelopment, psychosocial adaptation, and cardiovascular fitness during staged reconstruction and beyond



  • Maximize patient and family quality of life



  • 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


  • Cavopulmonary anastomoses



  • Extracardiac conduit or lateral tunnel

Hypoplastic and/or narrowed central pulmonary arteries


  • Congenital



  • Surgically related



  • Asymmetric flow related

Elevated pulmonary vascular resistance


  • Long-standing increased pulmonary blood flow



  • Long-standing increased pulmonary artery pressure



  • Long-standing pulmonary venous hypertension



  • High altitude



  • Airway obstruction with chronic hypoventilation, chronic lung disease, ongoing long-term mechanical ventilation



  • Lung hypoplasia

Pulmonary vein stenosis


  • Congenital



  • Acquired

Elevated pulmonary venous atrial pressure


  • Pulmonary venous obstruction of the atrial outlet



  • Restrictive atrial septal defect



  • Atrioventricular valve stenosis



  • Elevated ventricular end-diastolic pressure (multifactorial, see below)

Elevated end-diastolic pressure


  • Hypertrophy



  • Persistent obstruction of ventricular outflow or aortic arch



  • Long-standing pressure or volume load



  • Hypertension



  • Ventricular scarring



  • Atrioventricular valve regurgitation



  • Semilunar valve regurgitation



  • Aortopulmonary collaterals



  • Sinus node dysfunction with junctional escape rhythm and cannon waves



  • Tachyarrhythmias



  • Ventricular pacing secondary to atrioventricular block




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.



  • 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 :




  • 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. 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 before and after surgical interventions, are shown. This multidistribution circulation is inherently inefficient and results in progressive congestive heart failure over time. As can be seen, the variability in efficiency is dependent on anatomy, ventricular function, valvar function, pulmonary parenchymal function, electrophysiologic status, systemic and pulmonary vascular resistance, ductal patency (prior to surgery) and the status of surgical connections (after surgery). Perioperative management is tailored to the assessment and provision of adequate oxygen delivery. When oxygen delivery is insufficient, targeted interventions toward the cause(s) are undertaken, specifically geared toward the individual anatomy and physiology present. Given the complexity and individual variation of this multidistribution circulation, commonly described treatment strategies such as “balancing systemic and pulmonary blood flow” and “avoiding overcirculation” may be too simplistic. Increased pulmonary blood flow in isolation does not generally result in inadequate systemic oxygen delivery over the short term and, if present, additional causes should be sought. Pulmonary alveolar function must be adequate for effective gas exchange, and total ventricular output must be sufficient in volume to provide enough systemic venous return to mix with pulmonary venous return. See text for details.


Both before and after surgical management in the neonate, the fUVH is volume-overloaded, as it supplies the pulmonary blood flow (Qp) plus the systemic blood flow (Qs) plus the regurgitant volume (Qr) if present. Additionally, although the term cardiac output has frequently been used clinically to describe the systemic blood flow in children with a biventricular circulation, this is an incorrect term in a neonate with a fUVH. The true “cardiac” output is distributed in two to three different compartments. Clinically, it is important in the neonate with a fUVH to be clear regarding the difference between the ventricular output (Qp + Qs + Qr) and that fraction of the ventricular output that delivers oxygen to the tissues—the systemic blood flow.


In addition, a number of terms have come into common usage over the past decades but are neither sensitive nor specific in the assessment of physiologic stability and the management of adequate oxygen delivery in a neonate with a fUVH. As our understanding of the fUVH circulation has improved, we suggest that the frequently used terms listed here are outdated and should be modified.


Terminology suggested to be modified/eliminated include:




  • Parallel circulation (discussed earlier)



  • Balancing the pulmonary and systemic blood flow



  • Pulmonary overcirculation



  • Single-ventricle physiology



“Balancing the Pulmonary and Systemic Blood Flow”


In vitro studies have suggested that the optimal ratio of Qp:Qs is approximately unity (balanced), and in some clinical scenarios this may be true. However, recent in vivo studies suggest a wide range in this ratio, which did not affect the systemic delivery of oxygen or hospital survival. In this study by Li and associates, systemic delivery of oxygen was highly correlated with the absolute value of Qs, and much more so than the ratio of Qp:Qs. In addition, there are situations in the neonate with a fUVH that, while the circulation is “balanced,” represent a precarious clinical scenario. For example, a patient with a mixed venous oxygen saturation of 25%, a pulmonary venous oxygen saturation of 95%, and a systemic oxygen saturation of 60% indeed has a balanced circulation—the Qp to Qs ratio is 1.0—but nonetheless has severely impaired systemic oxygen delivery. The term “balanced circulation” is thus neither a sensitive nor specific term to define the goals of management.


“Pulmonary Overcirculation”


Technically, this term describes increased pulmonary blood flow to the lungs compared to the body, a clinical scenario seen in many children with septal defects, a patent arterial duct, and so on. However, “pulmonary overcirculation” has unfortunately become synonymous colloquially with decreased systemic blood flow, with an implied imbalance of systemic and PVR as the cause of potential clinical deterioration.


In the preoperative neonate, as stated earlier and confirmed with the clinical experiences waiting for organ transplantation, slowly progressive congestive heart failure—over days to weeks—is the more common clinical scenario during the transitional circulation, rather than acute deterioration. Should more acute clinical changes take place, prompt investigation of the patency of the arterial duct should be undertaken. In the postoperative neonate, low systemic oxygen delivery may be due to preferential maldistribution of flow into the pulmonary vascular bed (“high Qp:Qs”) from a low PVR combined with an anatomically large shunt, but as stated above, a number of additional causes must be investigated before attributing the clinical scenario solely to “pulmonary overcirculation” (e.g., arch narrowing, low global univentricular output, low oxygen content or multiple combinations of these factors, see Fig. 70.2 ). As in the preoperative state, the converse is true: not all babies with a “high” oxygen saturation are at risk of clinical deterioration. In postoperative patients, both hyperventilation and significantly increased supplemental oxygen did not result in clinical deterioration. During hyperventilation, there were no changes in systemic or mixed venous saturation, arteriovenous saturation difference, oxygen excess factor (Ω), or blood pressure. Importantly, high levels supplemental oxygen produced significant increases from baseline in systemic saturation (90% ± 1% vs. 80% ± 1%; P < .01), mixed venous saturation (54% ± 3% vs. 44% ± 2%; P < .01), and Ω (2.6% ± 0.2% vs. 2.3 ± 0.2%; P < .01), with no change in arteriovenous saturation difference or blood pressure.


This study, as well as our clinical experience, suggests that increased pulmonary blood flow per se in the face of normal systemic blood flow and normal pulmonary parenchyma may have no physiologic consequences. For example, a patient with a mixed venous oxygen saturation of 65%, a pulmonary venous oxygen saturation of 95% and a systemic oxygen saturation 85% has a Qp:Qs ratio of 2 : 1; however, systemic oxygen delivery is maintained, as evidenced by a normal mixed venous oxygen saturation and a narrow difference between the arterial and venous oxygen saturations. Fundamentally and more accurately, circulatory maldistribution is a more inclusive term, which can be used to describe some of the causes of a state of decreased oxygen delivery, in particular, redirection of ventricular outflow to the pulmonary vascular bed and/or significant atrioventricular valve regurgitation. However, the clinical scenario of low oxygen delivery may also be due to low univentricular output, low oxygen content, or multiple combinations of these factors, as shown in Fig. 70.2 . Each of these causes of decreased systemic oxygen delivery has different management strategies; therefore an accurate and complete assessment of the multidistribution circulation must be undertaken.


In conclusion, although “pulmonary overcirculation” may cause clinical instability—particularly if it contributes to pulmonary congestion, inadequate alveolar gas exchange, tachypnea, or respiratory distress— undercirculation of either the pulmonary or systemic vascular beds is always an unstable clinical scenario. Simplifying the complex physiology shown in Fig. 70.2 —with terminology such as high Qp:Qs, pulmonary overcirculation , and balancing flow —limits a full understanding of the nuances of the complex circulation and may negatively affect assessment and management at the bedside. In fact, as our understanding of this complex physiology improves, increasingly complex models have recently been proposed (using a combination of local identifiability, Bayesian estimation and maximum a posteriori simplex optimization) as well as an e-simulation model.


“Single-Ventricle Physiology”


The term “single-ventricle physiology” has occasionally been used interchangeably before and after all three planned stages of surgical management. However, the circulatory patterns in the neonate and infant before and after the neonatal palliation, superior cavopulmonary connection (stage II or Glenn procedure) and total cavopulmonary (stage III or Fontan operation) are all extremely different (see later). Using a single term such as single-ventricle physiology across the surgical spectrum in all of these situations is both irrational and inaccurate. Instead, we propose using a more precise shorthand nomenclature, such as the (1) multidistribution circulation, (2) the Glenn circulation, and (3) the Fontan circulation. The Glenn circulation is a series circulation , albeit with an obligate 40% to 60% right-to-left shunt from the inferior caval vein to the ventricle, and the Fontan circulation is also a series circulation , with the great majority of systemic venous return entering the pulmonary vascular bed in series with the systemic vascular bed. However, in contrast to the series circulation in patients with two ventricles, there are unique features of both the Glenn and Fontan circulations, particularly with respect to the central venous pressure, which is discussed later.




Physiologic Effects of Staged Reconstruction in the Patient With a Functionally Univentricular Heart Undergoing Superior Cavopulmonary Connection and Subsequent Fontan Procedure


The general physiologic aims and management strategies of the multidistribution circulation in the neonate and infant detailed in Table 70.2 and Fig. 70.2 pertain to postoperative care as well and are discussed in more detail in Chapter 71 . During each of the three planned stages of surgical management, there are a number of important physiologic changes that follow surgery, some of which are temporary, whereas others are planned consequences of the surgical procedure itself ( Table 70.3 ).



Table 70.2

General Principles of Surgical Palliation in the Newborn and Beyond
































Fundamental Management Principles Short Term Longer Term


  • 1.

    Provide unobstructed systemic blood flow

Preserve systemic oxygen and nutrient delivery Minimize the severity of ventricular hypertrophy
Preserve ventricular function


  • 2.

    Provide limited/restricted pulmonary blood flow without pulmonary artery distortion

Provide adequate gas exchange Minimize the risk of elevated pulmonary vascular resistance
Minimize pulmonary stenosis and hypoplasia


  • 3.

    Ensure unobstructed pulmonary venous return

Minimize risk of hypoxemia
Avoid pulmonary edema
Minimize the risk of pulmonary venous hypertension and elevated pulmonary vascular resistance


  • 4.

    Ensure unobstructed systemic venous return

Minimize risk of central venous hypertension leading to chylothorax, effusions, and thrombosis
Minimize risk of postoperative low cardiac output
Lower central venous pressure
Improve cardiac output


  • 5.

    Minimize atrial incisions

Reduce risk of perioperative arrhythmias and sinus node dysfunction Reduce risk of long-term supraventricular arrhythmias and sinus node dysfunction


  • 6.

    Minimize the duration and severity of volume and pressure load

Reduce risk of ventricular dysfunction and atrioventricular valve regurgitation Minimize severity of ventricular hypertrophy
Minimize inefficient circulatory effects of regurgitant fraction
Preserve ventricular function

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Jan 19, 2020 | Posted by in CARDIOLOGY | Comments Off on Physiologic Principles to Maximize Outcome in Patients With a Functionally Univentricular Heart
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