Single Ventricles

Congenital Heart Defects Associated With Single-Ventricle Physiology

Most patients with single-ventricle physiology have two ventricles, one large (i.e., dominant) and the other small (i.e., codominant). The term single-ventricle circulation is best used to describe the physiologic state rather than the anatomic condition because a true single ventricle is rare. Congenital heart defects associated with single-ventricle physiology are listed in Table 46.1 . Tricuspid atresia, hypoplastic left heart syndrome (HLHS), double-inlet left ventricle, and double-outlet right ventricle (DORV) account for most of the anatomic defects leading to single-ventricle physiology.

In tricuspid atresia ( Fig. 46.1 ), there is no connection between the right atrium (RA) and the right ventricle (RV) because the atrioventricular (AV) valve is imperforate or absent. This appears on the echocardiogram as a fibrous band or plate with no connection between the RA and RV. Tricuspid atresia is typically associated with an atrial septal defect (ASD) with right-to-left shunting, a hypoplastic RV, and a ventricular septal defect (VSD). The great vessels are usually normally related (70% of patients) or transposed (30%), often with obstruction to pulmonary and/or systemic blood flow. If the great vessels are normally related, the pulmonary artery (PA) arises from the RV and is usually associated with pulmonary or subpulmonary stenosis. In transposition of the great arteries, the PA arises from the left ventricle (LV), and the aorta arises from the RV. There may be obstruction to aortic outflow due to a muscular infundibulum or a restrictive VSD.

Patients with tricuspid atresia typically require a modified Blalock-Taussig shunt (subclavian-to-PA anastomosis using a synthetic tube), which is followed by a bidirectional Glenn shunt and a Fontan operation.

HLHS encompasses a spectrum of defects associated with a severely underdeveloped left heart and aortic arch. The mitral and aortic valves have severe stenosis and/or atresia. Typically, the ascending aorta is hypoplastic, and coarctation exists ( Fig. 46.2 ). The goal of surgical palliation in HLHS is to establish an RV-based systemic circulation, in which the RV becomes the dominant ventricle supporting the systemic circulation. Children born with HLHS typically undergo a series of three operations, beginning with a Norwood operation in the neonatal period (stage 1) and followed by a bidirectional cavopulmonary anastomosis (stage 2, around 6 months) and a Fontan operation completion between 2 and 3 years of age (stage 3) ( Fig. 46.3 ).

In the Norwood operation, the pulmonary valve becomes the new or neoaortic valve, and the aorta is reconstructed using the trunk of the PA, the original hypoplastic aorta, and graft tissue. The RV becomes the systemic chamber responsible for pumping blood to the brain and body. The PAs are disconnected from the RV, and pulmonary blood flow is provided by a Blalock-Taussig shunt or an RV-to-PA conduit (i.e., Sano modification). HLHS and its management are reviewed in several excellent articles.

Double-inlet LV refers to hearts in which both atria connect to one dominant LV. Double-inlet LV is often associated with transposition of the great arteries. When double-inlet LV occurs with normally related great arteries, it is called the Holmes heart. VSDs are usually detected, and they connect the dominant LV to a small hypoplastic RV. Depending on whether the great arteries are normally related or transposed, a restrictive VSD may cause obstruction to systemic or pulmonary blood flow. Other associations include pulmonary stenosis (i.e., valvular and subvalvular), coarctation of the aorta, and interrupted aortic arch ( Fig. 46.4 ).

Patients with double-inlet LV and transposition of the great arteries usually require a Norwood-type operation in the neonatal period. This is followed by a bidirectional cavopulmonary anastomosis and a Fontan operation.

DORV describes hearts in which the aorta and PA originate from the RV. The three types of DORV are a less complex DORV with a single VSD, normal-sized ventricles, and no major PA anomalies (group 1); a DORV with transposition of the great arteries and a subpulmonary VSD (i.e. Taussig-Bing anomaly; group 2); and a complex DORV that includes AV septal defects (which may or may not straddle the VSD) and/or a hypoplastic valve or ventricle (group 3).

Complete surgical repair (i.e., biventricular repair) is achievable in most DORV patients in groups 1 and 2. The Rastelli operation is performed for group 1 DORV patients. It comprises an intraventricular tunnel or baffle from the VSD to the aorta with or without placement of a conduit from the RV to the main PA. Group 2 DORV patients undergo the arterial switch operation, with tunneling of the VSD to the neoaorta. Because of the complexity, group 3 patients are not suitable for complete repair and usually require a Fontan operation ( Fig. 46.5 ).

Surgical Evolution After The Fontan Operation

First reported in 1971, the Fontan operation has given rise to a new generation of children born with very complex forms of congenital heart disease who survived to adulthood. The Fontan operation is the most common surgery for children with complex congenital heart defects not suitable for biventricular repair.

The Fontan operation directs the systemic venous return into the pulmonary circulation, usually without an interposing RV. The objectives of surgical palliation are to provide adequate pulmonary and systemic blood flow while alleviating cyanosis and ventricular volume overload. In the absence of a subpulmonary chamber or pump, systemic venous flow to the pulmonary circulation is passive. Postcapillary energy and systemic venous pressure are the driving forces for pulmonary blood flow, augmented by respirophasic changes in intrathoracic pressure. In 2019, the American Heart Association’s Scientific Statement for Evaluation and Management of the Child and Adult With Fontan Circulation reported that there may be up to 70,000 patients alive today with a Fontan circulation, and that this population is expected to double in the next 20 years.

Over time, the Fontan operation has evolved to facilitate more efficient venous flow from the systemic circulation to the pulmonary circulation. The first Fontan connections were RA-to-PA connections, called the atriopulmonary Fontan procedure ( Fig. 46.6 ). Valves and conduits were used in some patients but were found to impede flow through Fontan connections. The Bjork modification comprised a conduit (often valved) between the RA and RV in cases of tricuspid atresia.

The elevated RA pressures associated with atriopulmonary Fontan connections resulted in progressive RA enlargement along with arrhythmia, thrombus formation, and pulmonary venous obstruction. Consequently, the atriopulmonary Fontan connection was abandoned in favor of a circuit that largely excluded the RA from the Fontan connection. Introduced in 1988 and still used today, the lateral tunnel Fontan connection comprises a synthetic tunnel sutured within the RA, extending from the inferior vena cava (IVC) to the PA ( Fig. 46.7 ). ^,

Because the intraatrial conduit was still associated with RA dilation, scarring, and atrial arrhythmia, it was superseded by the extracardiac Fontan operation, which bypasses the RA altogether. IVC blood flows through a synthetic conduit located external to the RA ( Fig. 46.8 ). The simplicity of the extracardiac Fontan operation, reduced suture burden, and avoidance of aortic cross-clamping led to its becoming the procedure of choice in many congenital heart centers.

A 2019 meta-analysis included 3300 patients, 1729 with an extracardiac Fontan operation and 1601 with a lateral tunnel Fontan procedure. Those with an extracardiac Fontan operation had improved survival (93% vs. 89% at 20 years, respectively; P = 0.007) and greater freedom from tachyarrhythmia (92% vs. 83% at 15 years; P < 0.0001). The extracardiac Fontan operation was also associated with a lower thromboembolic risk than the lateral tunnel and atriopulmonary Fontan types.

A staged approach to the Fontan operation completion is preferred. Typically, patients with a single ventricle undergo a shunt or PA banding first, followed by a cavopulmonary anastomosis or Glenn shunt. The Glenn shunt is a surgical anastomosis between the superior vena cava (SVC) and the pulmonary circulation. Older patients undergoing echocardiographic assessment may have a classic Glenn shunt ( Fig. 46.9 ), whereas younger patients are more likely to have a bidirectional Glenn shunt ( Fig. 46.10 ). The classic Glenn shunt refers to a unilateral anastomosis of the SVC to the right PA; the left and right PAs are disconnected. Pulmonary arteriovenous malformations (AVMs) are common in patients with a classic Glenn procedure; they are thought to arise from the absence of hepatic venous flow to the right lung because regression of AVMs can occur after hepatic flow is restored. The bidirectional Glenn shunt comprises end-to-side anastomosis of the SVC to the undivided PA. These connections ensure that systemic venous return from the upper half of the body flows passively into the lungs.

The bidirectional Glenn shunt typically is performed in patients between 4 and 6 months of age. Fontan completion connecting the IVC to the PAs is performed in patients between 2 and 4 years of age.

The Kawashima operation is a type of bidirectional Glenn connection that is performed in patients with left atrial (LA) isomerism and an interrupted IVC with azygous continuation to the right SVC or hemiazygos continuation to a persistent left SVC. Those with a persistent left SVC have bilateral Glenn shunts. This routes all of the systemic venous return, except hepatic and mesenteric flow, to the PAs ( Fig. 46.11 ). As in patients with a classic Glenn shunt who do not receive hepatic venous flow, a significant percentage of patients who have undergone the Kawashima operation develop pulmonary AVMs as a cause of progressive cyanosis.

The Fontan operation separates systemic venous and pulmonary venous blood flow, resulting in normal arterial saturations. The flow through the pulmonary circulation is passive and therefore limits any increase in cardiac output, especially during exercise. The Fontan circulation results in chronic volume unloading of the single ventricle at the expense of increased systemic venous pressures. Short- and medium-term outcomes after the Fontan procedure are excellent, with operative mortality rates approaching 1% and reported transplantation-free survival rates of 95% and 90% at 5 and 10 years, respectively. ^,

Late complications in adulthood are common, including systemic venous hypertension, reduced cardiac output, and limited cardiac reserve during exercise. Other complications include atrial arrhythmias, atrial thrombus, hepatic congestion, liver cirrhosis, and ascites. Protein-losing enteropathy, which affects 4% of Fontan operation survivors, is characterized by chronic diarrhea and gastrointestinal protein loss, which leads to peripheral edema, pleural effusions, ascites, impaired immunity, and increased mortality rates. Another rare complication is plastic bronchitis, in which bronchial casts of a gelatinous consistency obstruct the tracheobronchial tree.

By creating a right-to-left shunt between the Fontan circulation and the RA, a fenestration can relieve elevated Fontan pressures, improve ventricular preload, and increase cardiac output ( Fig. 46.12 ). This is most relevant in the postoperative period, when the risk of early Fontan failure and chronic pleural effusions is high. In later years, fenestrations may be closed to reduce the cyanosis associated with the right-to-left shunting.

Ventricular Function Before and After A Fontan Operation

During fetal life, the dominant ventricle is responsible for the combined output of the systemic and pulmonary circulations. The parallel circulations cause a persistent, chronic volume load, leading to prenatal eccentric remodeling and ventricular dilation in the single ventricle at birth. The bidirectional Glenn shunt procedure performed at 4 to 8 months of age reduces the chronic volume load, leading to reverse remodeling and improved ejection fraction. After the Fontan operation, further volume unloading occurs, which in the face of unchanging ventricular mass leads to an acquired hypertrophy that impairs ventricular diastolic performance.

The incidence of heart failure increases with age. Up to 40% of adult Fontan patients meet the Framingham criteria for a diagnosis of heart failure. Male sex, a common AV valve, older age at the time of a Fontan operation, elevated preoperative and early postoperative PA pressures, concomitant surgery at the time of the Fontan operation, and prolonged pleural effusions after Fontan completion have been associated with worse late survival rates. ^,

Early recognition of heart failure in Fontan patients remains a major clinical challenge, as does accurate assessment of ventricular function. The accuracy of echocardiographically derived measurements has been questioned because assumptions of uniform geometry do not apply in those with single-ventricle physiology. Reduced ventricular preload after the Fontan operation also raises questions about the relevance of ejection fraction as a load-dependent measure of ventricular function. Echocardiographers face the additional challenges of limited views, chest wall deformities, unpredictable orientation of the heart in the thorax, and abnormal chamber geometry. Given these challenges, a range of techniques, including cardiac magnetic resonance imaging (MRI), are being used to better understand ventricular function in patients with single-ventricle physiology.

Terminology

Segmental analysis refers to the stepwise assessment of the connections between the three segments of the heart: atria, ventricles, and great arteries. Variations in these segmental arrangements are common in congenital heart disease.

Concordance describes situations in which one anatomic segment connects appropriately with the next. A concordant AV connection describes an appropriate connection of the morphologic RA to the morphologic RV and the morphologic LA to the morphologic LV. Concordant ventriculoarterial connections describe an appropriate origin of the pulmonary trunk from the morphologic RV and of the aorta from the morphologic LV.

A discordant AV connection involves connection of the morphologic RA with the morphologic LV. Discordant ventriculoarterial connections describe mismatched great arteries and ventricles such that the PA arises from the LV and the aorta arises from the RV.

Depending on its relation to the thoracic (spine) midline, the cardiac position may be in the left chest (normal or levoposition), in the right chest (dextroposition), or in the midline (mesoposition). Levoposition is confirmed when the heart is identified 30 to 45 degrees to the left of the midline (i.e., midline marked posteriorly by the vertebral body). Mesoposition indicates that the heart occupies a midline position. Dextroposition is seen when the heart is 30 to 45 degrees to the right of the central plane.

Cardia denotes the location of the apex. Levocardia indicates a normal leftward apex, whereas dextrocardia means the apex points to the right.

Situs refers to the right-left position of the body’s organs . Situs solitus denotes normal position (i.e., the right lung and liver are to the right, and the left lung, heart, spleen, and stomach are to the left). In situs inversus (1 case in every 5000 to 10,000 individuals), the right-left orientation of the unpaired viscera (including the heart) is inverted.

Using this terminology, the finding of a normal cardiac position with normal visceral arrangements is classified as levocardia with situs solitus. Alternatively, if the heart occupies the right chest with reversal of the body’s organs (i.e., liver present on the left and stomach on the right), the classification is dextrocardia with situs inversus.

Dextrocardia has three causes: (1) situs inversus; (2) loss of normal lung volume in the right chest leading to mediastinal shift; and (3) dextroversion, in which the apex fails to rotate normally during fetal development. Situs inversus is distinguished by the reversed position of the right and left heart structures. When dextrocardia occurs due to mediastinal shift or dextroversion, the RA and RV are still on the right, and the LA and LV are on the left. Most people with dextrocardia and situs inversus have otherwise normal hearts.

In patients with single-ventricle physiology, the congenital heart defect most commonly associated with dextrocardia and situs inversus is congenitally corrected transposition of the great arteries. Situs ambiguous refers to situations in which situs cannot be clearly defined and commonly coincides with atrial isomerism (i.e., mirror-image atria). LA isomerism is associated with paired morphologically left structures: bilateral left bronchi, bilateral bilobed lungs, and bilateral spleens. RA isomerism is associated with paired morphologically right structures; bilateral right bronchi, bilateral trilobed lungs, and congenital absence of the spleen.