In hearts with a univentricular atrioventricular connection both atria are connected to a single ventricle; this includes hearts with either atrioventricular valve (AVV) atresia or a double-inlet atrioventricular connection.
Either the tricuspid or mitral valve may be atretic in a heart with AVV atresia. Mitral atresia is one of a heterogeneous group of conditions that comprise the hypoplastic left heart syndrome (HLHS). The incidence of this syndrome is approximately 0.2 per 1000 live births, compared with a rate of only 0.06 per 1000 live births for tricuspid atresia ( Fig. 56.1 ). Nonetheless, the majority of adults with AVV atresia have tricuspid atresia, since, until about 2 decades ago, those with mitral atresia were unlikely to survive. Very rarely, the single AVV connects to what appears to be a solitary ventricle, and the heart can truly be described as univentricular. The hemodynamics are usually similar to those of tricuspid atresia.
A palliative surgical approach is required for both conditions, resulting in a univentricular “Fontan” circulation (see Chapter 12 ). For those with mitral atresia and HLHS, the introduction of a complex three-stage surgical approach culminating in a Fontan circulation means that survivors are now beginning to be seen in adult congenital heart disease clinics.
Tricuspid Atresia
The heart with atrioventricular atresia usually has complete absence (atresia) of the tissue of one AVV; the single remaining valve connects to a dominant ventricle. In so-called classic tricuspid atresia, the floor of the right atrium is muscular and separated from the ventricular mass by the fibrofatty tissue of the atrioventricular groove. The mitral valve connects the left atrium to the left ventricle. Occasionally atrioventricular atresia occurs, in which valve tissue is present but imperforate. In this situation the right atrium is separated from the right ventricle by imperforate valve tissue, and the heart therefore has a biventricular atrioventricular connection.
The left ventricle is dominant in tricuspid atresia; because the right ventricle lacks its inlet portion, it is incomplete (rudimentary). The right ventricle comprises an apical trabecular portion and usually retains its outlet portion, connecting to the pulmonary valve if ventriculoarterial connections are concordant and to the aortic valve if they are discordant. The rudimentary right ventricle lies anterosuperior to the left ventricle.
In all forms of tricuspid atresia, systemic venous blood enters the right atrium, from which the only exit is an atrial septal defect into the left atrium. Thus there is complete mixing of systemic and pulmonary venous blood at the atrial level and the patient is cyanosed. Blood then enters the left ventricle via the single (mitral) AVV. There is usually a large ventricular septal defect leading into a rudimentary right ventricular chamber.
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If the ventriculoarterial connections are concordant, as they are in 70% of cases of tricuspid atresia (classic tricuspid atresia), the pulmonary artery arises from the right ventricle and is usually associated with pulmonary or subpulmonary valve stenosis.
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In the 30% of cases in which ventriculoarterial connections are discordant, the pulmonary artery arises from the dominant left ventricle and is usually associated with pulmonary stenosis. The aorta arises from the right ventricle and there may be obstruction to aortic flow caused by either a muscular infundibulum or a restrictive ventricular septal defect.
Genetics and Epidemiology
Tricuspid atresia was first described in 1817; it accounts for 1% to 3% of congenital heart defects at birth and occurs with a male-to-female ratio of 1.45:1. Most cases of tricuspid atresia are sporadic; however, familial instances have been reported, as have 22q11 microdeletions. The etiology of tricuspid atresia is not yet understood, although mouse studies targeting the transcription factor Gata4 suggest that the protein it encodes may be important in normal cardiac looping and septation and may provide a genetic basis for tricuspid atresia.
Early Presentation and Management
The majority of patients with tricuspid atresia present in infancy with cyanosis. The timing and mode of presentation depend on pulmonary blood flow. In patients with concordant connections there is usually severe pulmonary and subpulmonary stenosis, resulting in deep cyanosis. When ventriculoarterial connections are discordant, there is often only mild pulmonary stenosis; this causes excessive pulmonary blood flow, marked ventricular volume overload, breathlessness, and minimal cyanosis.
Clinical signs usually include cyanosis and clubbing. There is a dominant left and no right ventricular impulse. In the majority of patients who have concordant connections, a loud pulmonary ejection murmur is heard, sometimes associated with a thrill. The second heart sound is usually single.
The electrocardiogram shows left-axis deviation, right atrial hypertrophy, and left ventricular dominance. Two-dimensional (2D) echocardiography demonstrates the anatomy and physiology, but cardiac catheterization is required to assess pulmonary vascular resistance and pulmonary artery anatomy.
Unoperated Survival
The unoperated 10-year survival rate in patients with tricuspid atresia is 46%, with deaths due to hypoxia, cardiac failure, endocarditis, paradoxic emboli, and cerebral abscess. Long-term unoperated survival depends on adequate but not excessive pulmonary blood flow. Such a balanced circulation is rare but occasionally allows unoperated survival into the sixth decade of life.
Operations
All surgical approaches are staged and palliative because a biventricular repair is not possible.
Current management strategies aim for a Fontan-type circulation in all patients with tricuspid atresia (see Chapter 12 ). The Fontan operation is performed in hearts with a univentricular atrioventricular connection in order to abolish cyanosis. The ventricular mass is used to support the systemic circulation by excluding a right-sided “pump” from the circulation. Thus systemic venous blood is directed straight into the pulmonary artery via the right atrium (Fontan operation) or via an intracardiac or extracardiac conduit (total cavopulmonary connection [TCPC]).
Where there is severe pulmonary stenosis, the aim of the initial operation is to improve pulmonary blood flow. If intervention is needed in the neonatal period, before pulmonary vascular resistance has fallen, an aortopulmonary shunt is performed (see Table 47.1 ). Such a shunt further adds to the volume loading of the dominant ventricle. If the pulmonary vascular resistance is low, a cavopulmonary shunt (bidirectional Glenn anastomosis) is performed; the superior vena cava is disconnected from the right atrium and anastomosed to the pulmonary artery. This procedure has the advantage of offloading the ventricle and perfusing the lung at low pressure in preparation for a Fontan-type operation (if in conjunction with transection of the pulmonary trunk or takedown of the aortopulmonary shunt). However, as the child grows, the relative contribution of the superior vena cava to total systemic venous return diminishes. As a result, the child becomes increasingly cyanosed; a Glenn anastomosis is inadequate as the sole source of pulmonary blood supply in an adult. The subsequent definitive operation abolishing cyanosis involves the completion of a Fontan-type procedure.
In patients with ventriculoarterial discordance who have excessive pulmonary blood flow, it is necessary to place a pulmonary artery band to reduce flow and prevent pulmonary vascular disease so that a Fontan-type operation can be performed later.
Mitral Atresia and Hypoplastic Left Heart Syndrome
The HLHS was first described in 1851 and comprises a spectrum of malformations that share an underdevelopment of the left side of the heart and aortic structures. This broad spectrum of malformations means that an exact definition of the term hypoplastic left heart syndrome is difficult and has been the subject of much discussion.
HLHS may be defined simply as a spectrum of cardiac malformations that share common atresia or stenosis of the aortic or mitral valves and hypoplasia of the left ventricle, ascending aorta, and arch. Abnormalities of the mitral valve are common, and an abnormal aortic valve appears to be universal. The great majority of patients with HLHS have an intact interventricular septum.
In practice, a broader morphologic range of conditions may be considered part of HLHS, since they share a similar physiology and a left ventricle that is unable to support the systemic circulation. They include unbalanced atrioventricular septal defect—a condition that is likely to have a different etiology.
Because all hearts with mitral atresia form part of HLHS, the discussion in this chapter is focused on the management and outcomes for HLHS in general.
Genetics and Epidemiology
HLHS is common, with a prevalence of about 0.162 per 1000 live births. It is a heritable condition linked to other left-sided anomalies, including bicuspid aortic valve and aortic coarctation, which may share a common etiology.
Associated noncardiac abnormalities are rare, but the condition does occur in association with Turner syndrome, which is also linked with bicuspid aortic valve and coarctation.
Recent research shows that HLHS has high heritability and is almost entirely caused by genetic effects. HLHS has been shown to be present in 8% of siblings and 3.5% of first-degree relatives of index patients with HLHS; other cardiovascular malformations are evident in 22% of siblings and up to 27% of first-degree relatives of these patients. Although left-sided valve lesions are most strongly associated in affected families, conotruncal anomalies and thoracic aortic aneurysms also occur. Genes that cause HLHS may be involved in valve development and include transcription factors, signaling molecules, or extracellular proteins.
Morphology
In mitral atresia there is usually a univentricular atrioventricular connection to a dominant right ventricle via a tricuspid valve. The mitral valve is atretic—either imperforate or absent—and there is a posteroinferior incomplete left ventricle. The interventricular septum is usually intact. There is considerable morphologic heterogeneity, which influences the hemodynamic picture. Thus if a ventricular septal defect is present and the aortic root is patent, the physiology may be similar to that of tricuspid atresia (described earlier). If mitral atresia is associated with an intact ventricular septum, it forms part of HLHS.
Early Presentation
Mitral atresia may present prenatally, detected by midtrimester anomaly imaging. In societies where termination is an option, the live birth rate is dependent on attitudes to treatment options and termination.
The circulation in the neonate depends on the pulmonary venous return passing through an interatrial communication and mixing with systemic venous return. Mixed blood then passes into the right ventricle, into the pulmonary artery, and into the systemic circulation via a large patent arterial duct ( Fig. 56.2 ). The aortic arch and coronary arteries receive their supply retrogradely via the arterial duct if there is coexistent aortic atresia. The size of the interatrial communication influences the efficiency of the circulation.
For those diagnosed postnatally, presentation occurs early because without intervention the condition is almost universally fatal within the first few weeks of life. In general a large arterial duct is open at birth, so the child presents in congestive heart failure with increasing tachypnea because of uncontrolled pulmonary blood flow through the duct and a volume-loaded circulation. Alternatively, if the duct closes, presentation is in extremis, with collapse and acidosis. Similarly, the neonate presents with collapse and acidosis soon after birth if the atrial septum is intact or the foramen ovale is restrictive.
After initial assessment there are three management options: compassionate care, a three-staged approach to a Fontan circulation, and cardiac transplantation. Some families choose the option of compassionate care after considering the demanding surgical options.
Staged Surgical Management
The Stage 1 operation (Norwood procedure) ( Fig. 56.3 ) is performed in the neonate, the second stage (involving a cavopulmonary shunt) is performed between 3 and 6 months of age, and the third stage (Fontan procedure) ( Fig. 56.4 ) is done between 18 months and 5 years of age. Survival has improved in recent years, but there is still considerable attrition. Some 60% to 80% of patients survive 5 to 10 years.
An initial balloon atrial septostomy may be necessary before the stage 1 procedure is performed. The principle is to place the right ventricle in the systemic circulation so as to ensure an unobstructed systemic outflow tract, free mixing of pulmonary venous and systemic venous blood, and a balanced pulmonary blood flow. The main pulmonary artery is divided, and the pulmonary blood supply is provided either by a Blalock-Taussig shunt (subclavian artery to pulmonary artery) or by a conduit from the right ventricle to the pulmonary artery (Sano modification). The pulmonary valve becomes the neoaortic valve by means of a Damus procedure, anastomosing the proximal pulmonary artery to the ascending aorta and augmenting it. The ascending aorta and aortic arch are reconstructed as necessary using the main pulmonary artery and homograft tissue. An atrial septectomy is then performed. A number of modifications have been described, aiming to improve survival after the first-stage operation and to reduce distortion of the pulmonary arteries.
Mortality from the stage 1 procedure was 30% to 35% in early series, but refinements in technique and postoperative care have reduced mortality to 10% to 15% in large centers. There is no room for complacency, however, with significant interstage attrition. The greatest hazard period is during the first year after the stage 1 procedure, with survivors facing a risk of death of up to 15% before reaching the second stage. Many of these deaths are unexpected and sudden. Various mechanisms have been proposed, including residual aortic arch obstruction, restrictive atrial septal defect, pulmonary-systemic flow imbalance, shunt thrombosis, and coronary ischemia due to diastolic runoff. There is some evidence that intensive home monitoring between stages 1 and 2 may contribute to improved survival.
The stage 2 operation (a cavopulmonary or Glenn anastomosis) is usually performed at 3 to 6 months of age, once pulmonary vascular resistance has fallen. The Blalock shunt or right ventricular to pulmonary artery conduit is taken down, and a cavopulmonary shunt is performed.
The stage 3 operation (Fontan procedure) is performed at 3 to 5 years of age, usually by means of a TCPC (see Fig. 56.4 ). Mortality from the stage 2 operation is 1% to 4%, and that from the stage 3 Fontan completion is about 3% to 4%.
In a further attempt to reduce the neonatal insult of the stage 1 procedure, a hybrid approach has recently been developed. It aims to produce the same physiology as the Norwood procedure by placing bilateral pulmonary artery bands, stenting the arterial duct, and performing an atrial septostomy or septectomy. The technique requires a midline sternotomy and the skills of both surgeon and interventional cardiologist, but it avoids the need for cardiopulmonary bypass. In the largest series, stage 1 mortality is similar or better than that for an entirely surgical approach, but interstage mortality and reinterventions are higher. The advantage of the hybrid approach lies in the avoidance of a prolonged initial operation with cardiopulmonary bypass in a sick neonate. However, disadvantages of the hybrid approach include the fact that the ascending aorta is not reconstructed, so the coronary supply relies on retrograde flow from the duct down the small ascending aorta; a steal phenomenon may occur, leading to ischemia and sudden death. Furthermore, great care must be taken with stent placement to avoid encroachment over the ascending aorta. In addition, the second-stage operation is more extensive than after a conventional surgical approach, involving reconstruction of the aorta and pulmonary arteries, removal of the stent, and creation of the cavopulmonary anastomosis. Nonetheless, a hybrid approach is likely to be useful in high-risk cases such as those with intact interatrial septum, low birth weight (<2 kg), and additional noncardiac problems. It is not suitable for patients in whom the aorta is diminutive.
Neonatal cardiac transplantation is not widely available and is a realistic option in only a few centers worldwide. Dedicated centers performing neonatal transplantation for HLHS have achieved midterm results comparable to those of the staged Fontan approach. However, multicenter studies show a 25% mortality for neonates awaiting transplantation. Transplantation is a more widespread option for children who have embarked on the staged Fontan approach but have developed cardiac failure. High waiting-list mortality and poor donor organ availability are major limitations. Those who do receive a transplant have the advantage of a biventricular circulation, albeit at the expense of requiring lifelong immunosuppression.
Late Outcome
Long-term survival in tricuspid atresia is expected after aortopulmonary shunt or Fontan-type surgery, but life expectancy remains limited. A child with a Fontan circulation who survives to age 18 years has a 60% chance of surviving to age 40 years. Although a Fontan circulation is the goal for most patients with tricuspid atresia, it carries long-term complications, which have been appreciated increasingly over the past decade; they are discussed later and in Chapter 13 .
The chronic low cardiac output, preload starvation, and high systemic venous pressure state of the Fontan circulation results in end-organ damage with detrimental effects on liver, renal, ventricular, and lymphatic function, pulmonary vascular hemodynamics, and skeletal muscle mass ( Box 56.1 ).
