Atrioventricular Valve Atresia

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.

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.
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.

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