A wide variety of congenital cardiac lesions lack two well-developed ventricles; from a functional standpoint, they are characterized by a single ventricular chamber that supports both systemic and pulmonary circulations. It is well established that the definitive palliation for these univentricular hearts is the Fontan circulation, whereby the pulmonary and systemic blood flow are placed in series with the single ventricle connected to the systemic circulation. To achieve the Fontan state, many patients need preliminary procedures, either to reduce or to augment pulmonary blood flow.

A truly morphologic univentricular heart is rare; more often there is an additional rudimentary chamber. The most common type of single ventricle is tricuspid atresia, with an incidence of 1 to 3 percent of all congenital heart lesions. In tricuspid atresia there is no direct communication between the right atrium and the right ventricle, and often a dimple or localized fibrous thickening is found at the expected site of the tricuspid valve. The right atrium is large and an interatrial communication is always present. The right ventricle is hypoplastic and lacks an inflow portion, but its trabecular and outflow portions may be well developed. Tricuspid atresia is uniformly associated with other cardiac malformations. A ventricular septal defect (VSD) is usually present. The ventriculoarterial connection can be concordant or, less commonly, discordant (transposition of the great arteries); rarely, there is double-outlet right or left ventricle. Infundibular obstruction to pulmonary blood flow is often present, but this can also be observed at the level of the VSD (restrictive VSD) or the pulmonary valve, which may be atretic. In patients with transposition of the great arteries, there may be subaortic stenosis and/or coarctation. The associated cardiac malformations determine the balance between pulmonary and systemic blood flow. Other forms of univentricular hearts include left atrioventricular valve atresia, double-inlet right and left ventricle, hearts with a common atrioventricular valve and only one well-developed ventricle, hearts with heterotaxy syndrome and only one fully developed ventricle, and pulmonary atresia with an intact ventricular septum and hypoplastic right ventricle. In addition, some complex hearts with two well-formed ventricles cannot be septated and become candidates for a single-ventricle strategy as the only viable surgical option. Examples of the latter morphologic group are hearts with major straddling of the atrioventricular valves, or certain forms of double-outlet right ventricle, or transposition of the great arteries with remote VSD. Hypoplastic left heart syndrome, a common form of univentricular heart, is detailed in Chapter 76.

With the advent of the Blalock–Taussig shunt in 1944,¹ palliation became possible for patients with single-ventricle physiology and cyanosis. Pulmonary artery banding was introduced in 1952 by Muller to alternatively reduce excessive pulmonary blood flow.² Following extensive laboratory work, Glenn showed that partial right heart bypass was possible in a patient with tricuspid atresia by anastomosing the superior vena cava (SVC) (or azygos vein) to the divided right pulmonary artery.³ This procedure evolved to the bidirectional cavopulmonary shunt, in which the SVC is anastomosed end to side to the undivided pulmonary artery, thus shunting blood into both lungs.⁴ In 1968, Fontan and Baudet performed the first total right heart bypass operation by establishing an atriopulmonary connection in a patient with tricuspid atresia.⁵ The operation was based on the concept that the hypertrophied right atrium could replace the right ventricle as the driving force for the pulmonary circulation. In addition, separating the systemic and pulmonary circulations and placing them in series improved arterial oxygen saturation and reduced volume load on the single ventricle. Over the years, there have been several modifications of the Fontan procedure. It became apparent that the atrial “kick” was not essential to cardiac output, and the atriopulmonary connection has now been largely replaced by the more energy-efficient total cavopulmonary anastomosis.⁶ In this palliative configuration, the SVC drains directly into the pulmonary artery and an intra-atrial tunnel or, more recently, an extracardiac conduit⁷ is used to baffle the inferior vena cava (IVC) to the pulmonary artery. For patients with subaortic obstruction undergoing the Fontan procedure, Waldman described the concurrent use of a modified Damus–Kaye–Stansel procedure to achieve unobstructed systemic outflow.⁸ Staging of Fontan palliation was introduced in an attempt to reduce the operative risk of the Fontan operation. Initial performance of a superior cavopulmonary shunt was followed at a later stage by completion of the total cavopulmonary connection.⁹ Another modification in technique was the creation of a fenestration in the Fontan baffle to allow a right-to-left shunt to maintain systemic ventricular filling. This proved to be particularly important in patients with impaired pulmonary blood flow, albeit at the cost of variable degrees of cyanosis. The fenestration can be closed percutaneously.¹⁰ Improved understanding of the Fontan physiology and evolution of the surgical techniques have extended the original indication of the Fontan operation from tricuspid atresia to many other forms of complex univentricular hearts.¹¹

In tricuspid atresia, the systemic venous return crosses the interatrial communication to the left atrium, where it mixes with pulmonary venous blood. As a result, oxygen saturations in the ventricles, aorta, and pulmonary artery are virtually identical (Fig. 77-1). The presence of an unrestrictive atrial septal defect (ASD) is obligatory. Systemic arterial desaturation is always present but is worse when the pulmonary-to-systemic blood flow ratio is low (in case of associated pulmonary stenosis or restrictive VSD). In case of severe hypoxemia, metabolic acidosis will eventually develop. In tricuspid atresia with normally related great vessels, blood reaches the pulmonary artery indirectly through a VSD and hypoplastic right ventricle. Pulmonary blood flow may be unrestricted initially but usually diminishes over time due to restriction of the VSD or the development of infundibular stenosis. In tricuspid atresia with transposition of the great arteries, the left ventricle is connected directly to the pulmonary artery and, unless pulmonary stenosis is present, pulmonary blood flow is high and pulmonary vascular disease may develop. In this anatomic subgroup, the aorta receives blood indirectly via the VSD and the hypoplastic right ventricle. Progressive restriction of the VSD will produce the effect of developing subaortic stenosis. Similar issue may present after pulmonary artery banding. In patients with functionally single ventricles with other morphologic features, the pathophysiology will be determined by the balance between pulmonary and systemic blood flow, the effects of intracardiac mixing and streaming, and the presence of other cardiovascular pathology (such as abnormal pulmonary venous drainage or atrioventricular valve regurgitation). In most single-ventricle circulations, systemic ventricular dysfunction and failure can develop as the result of cyanosis and/or volume overload.

The natural history of single-ventricle circulations is predominantly related to the degree of pulmonary blood flow and any associated cardiac lesions. There may also be associated noncardiac abnormalities, which will have an independent impact on outcome. Early onset of cyanosis is a predictor of worst survival, leading to death in 90 percent of untreated cyanotic patients with tricuspid atresia in the first year of life.¹² Patients with unobstructed pulmonary blood flow also fare poorly, often dying in infancy from congestive heart failure or early onset of pulmonary vascular disease. This clinical course may be accelerated in the presence of left-sided obstruction, such a subaortic stenosis or coarctation. A small subset of patients has a balanced circulation with unobstructed systemic blood flow and adequate restriction to prevent pulmonary overcirculation. These patients have a more favorable prognosis, with a mean survival of 8 years.¹³ Without surgical intervention, only 10 percent of infants will be alive by the age of 10 years.¹⁴

The clinical features are determined by the degree of pulmonary blood flow and associated cardiac malformations (Fig. 77-2). Most patients with tricuspid atresia present with cyanosis in the first days of life. Some may have a duct-dependent pulmonary circulation and will become rapidly cyanotic as the ductus arteriosus closes after birth. In the majority of patients, cyanosis is progressive and due to increasing infundibular stenosis and/or restriction of the VSD. Hypoxic spells may occur and are characterized by cyanosis, dyspnea, and occasionally syncope. Clubbing may develop in older children. In a minority of patients, however, there is no obstruction to pulmonary blood flow, and these patients present with congestive heart failure from overcirculation. With falling pulmonary vascular resistance, symptoms will grow worse in the first weeks of life. In most univentricular hearts there is mixing of circulations, but streaming may occur, particularly in complex hearts, resulting in differential saturation in the great arteries.

Chest x-ray usually shows signs of low pulmonary blood flow, with reduced pulmonary vascular markings and diminutive hilar shadowing. In cases of high pulmonary blood flow (usually tricuspid atresia with discordant ventriculoarterial connection), there is pulmonary plethora and cardiomegaly. Electrocardiography in most patients will demonstrate left-axis deviation and, frequently, a tall (>2.5 mm) notched p-wave consistent with right atrial enlargement.

Echocardiography provides both detailed anatomic diagnosis and functional assessment. The echocardiogram will usually show a hyperechoic shelf at the expected site of the tricuspid valve, in association with reduced right ventricular dimensions. The following information is essential to allow proper surgical planning: size and course of the pulmonary arteries, relationship of the great vessels, degree of right ventricular outflow tract obstruction, size of the interatrial communication, size and position of the VSD, presence of patent ductus arteriosus, aortic coarctation or other structural cardiovascular abnormalities and, lastly, effectiveness of ventricular function. Particularly in infants (who typically have excellent echocardiographic windows), enough information can usually be acquired to proceed directly to operative intervention. Cardiac catheterization is not routinely performed but remains an important diagnostic adjunct if there is doubt about the size and distribution of the pulmonary arterial supply and a need for direct pressure measurements. In addition, catheter interventions can be performed to supplement or sometimes avert initial surgical intervention. Magnetic resonance imaging and computed tomography are being utilized with increasing frequency to investigate patients with congenital heart disease, allowing for three-dimensional reconstruction of the cardiovascular structures and noninvasive assessment of hemodynamic function. When available, magnetic resonance imaging should serve as the initial screening procedure for preFontan assessment (branch pulmonary artery size and flow, ventricular function, cardiac index, atrioventricular valve function, and residual outflow obstruction) prior to any further diagnostic or interventional catheterization.

The initial selection criteria for the procedure, described by Fontan and colleagues for patients with tricuspid atresia, were very strict and known as the “ten commandments” (Table 77-1).¹⁵ With increasing experience, these criteria have been relaxed and, at the same time, the indication for the operation has been extended to many forms of complex single ventricle. However, owing to the unique physiology associated with the Fontan procedure, appropriate patient selection remains of paramount importance.

The minimal age of patients undergoing the Fontan operation has been progressively lowered in an attempt to try to minimize the deleterious effects of long-standing cyanosis and volume overload. The optimal and minimal ages are not known, but successful Fontan operations have been performed in patients as young as 7 months who presented with increasing cyanosis and suitable hemodynamics.¹⁶ It remains to be seen whether a very early Fontan operation has a better long-term outcome, particularly in patients with well-balanced circulations. Preoperative sinus rhythm, normal caval drainage, and normal right atrial volume are not absolute requirements, but unobstructed pulmonary venous drainage is obligatory. The pulmonary vasculature remains an important selection criterion for the Fontan procedure. Mean pulmonary arterial pressure (<15 mm Hg) and pulmonary arteriolar resistance (<4 U/m²) should be low and any pulmonary distortion related to previous shunts should be corrected. Adequate size of the pulmonary arteries is also important. There have been various attempts to standardize pulmonary artery measurements, such as the McGoon ratio¹⁷ and the Nakata index,¹⁸ but the usefulness of these indices has been questioned, as they do not account for the compliance of the pulmonary vascular bed and they have been established for a biventricular circulation. The McGoon ratio is obtained from the sum of the diameters of the immediately prebranching portions of the left and right pulmonary arteries divided by the diameter of the descending aorta just above the diaphragm. The Nakata index is derived from the sum of the diameters of the left and right pulmonary arteries (measured just before the origin of the upper-lobe branches) divided by the body-surface area. Adequate ventricular function is also crucial for a successful Fontan procedure. Ventricular impairment, however, is not always an absolute contraindication if it can be related to volume overload in the presence of an aortopulmonary shunt that will be taken down at the time of second-stage palliation. The same is true for mild or moderate atrioventricular valve regurgitation in the setting of a volume-loaded heart. More recently, ventricular hypertrophy has also been recognized as an important risk factor for Fontan failure.¹⁹ Predisposing conditions for ventricular hypertrophy, such as subaortic obstruction or coarctation, should be corrected as soon as possible. In the absence of uniform selection criteria, most centers will use a combination of the above criteria and grade patients as being at low, intermediate, or high risk for the Fontan procedure.