Simple morphological abnormalities of the right and left atrioventricular valves are uncommon congenital lesions. They include stenosis or absence of the atrioventricular valves, straddling atrioventricular valves, or abnormalities of the morphologically tricuspid right atrioventricular valve or the bicuspid (mitral) left atrioventricular valve. Additionally, a variety of anatomical abnormalities may exist, not only in the setting of situs solitus (usual) but in association with situs inversus or situs ambiguus (right or left atrial isomerism). Such lesions may be detected clinically at the time of birth, in infancy, in adolescence, or late adulthood. Furthermore, they may be associated with abnormalities of atrioventricular or ventricular arterial connections.

In this Section the common abnormalities of the right and left atrioventricular valves primarily in situs solitus are discussed.

The most common abnormality of the right atrioventricular valve is absence or atresia. In others the tricuspid valve may be partially dysplastic and displaced downwards into the right ventricular cavity (Ebstein's anomaly).

In hearts with ventricular septal defects there may be a variance in chordal attachment, leading to straddling of the interventricular septum to a greater or lesser degree.

Complete absence of the right atrioventricular valve occurs in 1 to 3 per cent of all congenital cardiac abnormalities. The vast majority of patients present at birth and the condition is one cause for cyanotic congenital heart disease (so-called blue babies).

In the majority of patients there is complete absence of the right atrioventricular connection (Fig. 1) 1711 with ventriculoarterial concordance (i.e. left ventricle connected to the aorta). In about 30 per cent the ventriculoarterial connection is discordant (transposition); in these patients the presence of an adequate ventricular septal defect and unrestricted pulmonary blood flow prevents cyanosis in the neonatal period. Such patients may present later in life. In the majority however, the absent right atrioventricular valve gives rise to a muscular right atrium, with a large interatrial communication through a secundum defect. There is total absence of the inlet portion of the right ventricle, with the main and outlet chamber of the right ventricle varying in size. The size of the associated ventricular sepal defect may well have a bearing on the capacity of the residual ventricular chamber. The ventricular septal defect is rarely restrictive and there may be associated pulmonary atresia with small main and branch pulmonary arteries.

Occasionally there is considerable valve tissue which on examination is perforated, or more commonly, the valve tissue is represented by a nub of membraneous tissue related to the fibrous body. The atrioventricular node, usually in the triangle of Koch, is thus displaced posteriorly and inferiorly towards the membranous nub.

Most of these children present at the time of birth or a few weeks afterwards. Most of them are desaturated and are therefore clinically cyanotic. In a small proportion with a large ventricular septal defect or in the presence of ventriculoarterial discordance (transposition), progressive cyanosis may be associated with a closing ventricular septal defect. Alternatively, in this small group with transposition of the great vessels, there may be excessive pulmonary blood flow giving rise to tachypnoea, failure to feed, and poor weight gain.

On examination there is a systolic murmur from the apex to the left sternal edge with a palpable left ventricular heave. Children with associated pulmonary atresia invariably have a persistent ductus arteriosus giving rise to a continuous murmur. Left untreated, the majority of children born with cyanosis become progressively more cyanotic and dyspnoeic, and may have syncopal attacks. Most of these children will die within the first 9 months of life if not treated.

In patients with tricuspid atresia and situs solitus there are reduced vascular lung markings reflecting reduced pulmonary blood flow. The heart shadow itself is within normal limits with reduced prominence of the main, left, and right pulmonary arteries. These features are reversed in the small group of patients with tricuspid atresia and ventriculoarterial discordance.

The electrocardiogram shows left axis deviation with signs of left ventricular hypertrophy. The left ventricle, being the recipient of both systemic and pulmonary venous return, tends to be large, dilated, and hypertrophic.

This investigation combined with the present-day availability of colour flow Doppler echocardiography, is the mainstay of investigation and diagnosis of tricuspid atresia. Most features of this condition, as well as any related abnormalities may be clearly seen.

In a small number of patients, cardiac catheterization is needed to delineate the size of the main and branch pulmonary arteries, as well as to confirm the findings of any other associated abnormality. In those patients who undergo initial palliation, catheterization, apart from confirming morphological features, allows the measurement of pulmonary vascular resistance and thereby facilitates corrective surgery.

Following the diagnosis of tricuspid atresia in a cyanosed child at birth, attempts to increase pulmonary blood flow by keeping the arterial duct patent are made. This is achieved in the neonatal period by an infusion of prostaglandin E&sub1;, following which a palliative shunt procedure is performed.

In the smaller group of patients with ventriculoarterial discordance, who invariably present much later after birth, the haemodynamic disturbance is due to excessive blood flow to the lungs. Palliative surgery is performed in the neonatal period or in the first few weeks or months of life. Following such palliation, corrective surgery to separate the systemic and pulmonary circulations permanently is performed once the child is 30 months or older.

Palliative procedures fall into two main categories. The first and most common is the palliation for reduced pulmonary blood flow and cyanosis. Several systemic-to-pulmonary artery shunts have been described over the last 40 years. One of the early shunts described by Blalock and Taussig was in the context of treatment of tetralogy of Fallot; it consists of an anastomosis between the right subclavian artery and the right pulmonary artery. An end-to-side anastomosis between the right pulmonary artery and the superior vena cava was described by Glenn in 1958, specifically for the palliation of tricuspid atresia. In the modern era, the construction of a classical Glenn shunt is felt to compromise the planning of further ‘corrective’ surgery.

Our preference is for the concept of the Blalock Taussig shunt, in which a connection is created between the subclavian artery and the pulmonary artery, using prosthetic materials, such as Dacron, Gore-Tex, or Impra (polytetrafluoroethylene), with the exception of where the shunt in itself is to be considered a permanent treatment. The initial shunt is placed on the same side as the aortic arch, bearing in mind the possible need for a second shunt as the child grows.

Following general anaesthesia and continuous monitoring of arterial and systemic venous pressure, the child is placed in a left lateral position. The chest is entered through a left lateral thoracotomy. After identification of the left subclavian artery, the left lung is gently retracted inferiorly and posteriorly exposing the left pulmonary hilum. The pleura over the left hilum is incised and the left main pulmonary artery and its branches dissected out.

It is not unusual for a large number of thin walled collateral vessels to be present. Careful haemostasis at this stage avoids excessive postoperative bleeding. Distal branches of the pulmonary artery are controlled with circular sutures, while the totally free main pulmonary artery allows a clamp to be placed proximally towards the pulmonary bifurcation. The left subclavian artery is dissected free, after which heparin 1 mg/kg body weight is administered. Following this the Gore-Tex, Impra, or Dacron conduit, varying in size between 3 and 5 mm in diameter, is anastomosed to the site of the origin of the left subclavian, having occluded the latter completely with a side biting clamp. The distal end of the conduit is anastomosed to the left main pulmonary artery, having removed a small button from the superior surface of the arterial wall.

On completion, first the distal and then the proximal clamps are released. On securing haemostasis a single intercostal drain is placed in position and the chest closed in layers.

In 1968 Fontan described the first procedure to separate permanently the systemic and pulmonary circulations in tricuspid atresia.

The original procedure consisted of a Glenn shunt (right pulmonary artery to superior vena cava anastomosis) and further anastomosis between the right atrial appendage and the main pulmonary artery, following division and oversewing of the main pulmonary artery.

In the early experience described, a homograft valve was inserted between the inferior vena cava and the right atrium. In the last 20 years there have been several modifications of the procedure by Fontan himself, as well as others. Today the phrase ‘Fontan’ or ‘modified Fontan’ procedure has been attached to a large variety of procedures that seek to divide the systemic and pulmonary circulations.

Ebstein described the features of this syndrome in 1866, but it was not until the 1950s that Hunter and Lillehei described methods of surgical correction of the morphological abnormalities.

Patients may present soon after birth, in childhood, or in late adulthood. This anomaly may coexist with other associated abnormalities which may have significant impact on prognosis.

The characteristic features of Ebstein's disease are abnormality of the tricuspid valve both in structure and position, and an abnormal right ventricle with resulting gross enlargement of both the right ventricular and atrial chambers. The septal and posterior leaflets of the tricuspid valve may be dysplastic with abnormal attachment of the chordal apparatus to the ventricular wall. There may be a varying degree of tethering of the chordae as well as attachment of the leaflets themselves to the ventricular wall. In addition, the commissure between the septal and posterior leaflets is displaced inferiorly into the body of the ventricular cavity to a greater or lesser extent. The junction between the septal and anterior leaflet, however, remains in a normal position as does the anatomy of the conduction tissue with a normally sited atrioventricular node. The anterior tricuspid leaflet is usually normal in appearance but appears enlarged, tending to billow out into the atrium during systole.

The right ventricular wall proximal to the septal and posterior leaflet tends to become ‘atrialized’ with a smooth appearance and may be thinned out (Fig. 2) 1712. The right ventricle is grossly enlarged, while the only functional parts of the cavity, mainly the apex and the infundibulum, are variably developed.

About 60 per cent of patients have associated secundum atrial septal defect. Other common associated lesions include pulmonary stenosis or pulmonary atresia, persistent arterial duct, and ventricular septal defect. In addition many patients have conduction abnormalities such as Wolff–Parkinson–White syndrome, and paroxysmal and re-entrant supraventricular tachyarrhythmias.

Neonates may present soon after birth with cyanosis and congestive cardiac failure. Often these patients have associated abnormalities such as pulmonary stenosis or pulmonary atresia and a right-to-left atrial shunt. In the absence of such extra anomalies and depending on the degree of tricuspid regurgitation, presentation may be later and indeed patients remain relatively asymptomatic late into adulthood. The potential for supraventricular tachyarrhythmias and other conduction defects can occasionally cause sudden death.

Chest radiography shows a large rounded cardiac shadow with unusually clear lung fields. In extreme cases associated with poor forward blood flow the lungs themselves may be hypoplastic.

Echocardiography, both M-mode and two-dimensional, is used extensively as the diagnostic tool of choice. The size of the right ventricle and its function is easily determined, as is the extent of tricuspid regurgitation and the presence of any associated anomalies.

Cardiac catheterization and angiography is reserved for patients in whom specific haemodynamic data is required prior to planning surgery. The extent of tricuspid regurgitation may be significantly underestimated because of the grossly enlarged right ventricular and atrial cavities.

In patients with reasonable tricuspid valve function the shunt at atrial level may in fact be from left to right. In this event the lung fields may appear to be hyperaemic with clinical signs of high pulmonary blood flow.

Given the spectrum of functional disability the surgical procedures performed vary from complex repairs to simple closure of the secundum type atrial septal defect. In the very sick neonate who is in cardiac failure and is cyanotic following initial resuscitation the Starnes operation is the procedure of choice.

On cardiopulmonary bypass using deep hypothermic circulatory arrest the tricuspid valve orifice is closed with a patch. The atrial septal defect is enlarged and a central aortopulmonary shunt constructed. The procedure essentially converts the anatomy to one of a univentricular heart. Following this initial procedure a Fontan type repair is carried out at a later stage.

In the majority of patients, however, surgery is undertaken late. In over 60 per cent of patients presenting in infancy, adolescence, or late adulthood, the tricuspid valve is repaired with plication of the atrialized portion of the right ventricle in the majority of cases. The coexisting atrial defect is closed with a patch.

There are two commonly described methods of repair. The Danielson repair consists of a transverse plication of the atrialized portion with a posteroinferior suture plication of the valve annulus thereby converting the tricuspid valve to a monocusp valve with functional competence being provided by the large anterior leaflet.

The Carpentier repair consists of a longitudinal plication of the atrialized portion with a annuloplasty using a ring. In a small proportion of patients with good tricuspid valve function and good residual right ventricular function, closure of the atrial septum alone may bring about symptomatic relief.

Surgical repair in adolescence, in the presence of good ventricular function, produces good long-term results. Sudden death from arrhythmias secondary to aberrant conduction pathways still remains a possibility. Such pathways may be ablated either at the time of surgery or prior to surgery, percutaneously in the catheter laboratory.

Abnormalities of the left atrioventricular valve or mitral valve at the time of birth may occur in a variety of settings. The valve may be imperforate, absent, atretic, or stenosed because of fusion of the subvalvar apparatus. Fusion of the papillary muscles produces the so called ‘parachute mitral valve’.

Mitral valve atresia or absence is often associated with the hypoplastic left heart syndrome in which the left ventricle and the aorta are small and hypoplastic. Occasionally the presence of a large ventricular septal defect gives rise to an adequate left ventricular cavity. In such cases ventriculoarterial discordance (transposition) is the usual anatomical feature.

Echocardiography is the mainstay of diagnosis, and many of these children are kept alive by reversed flow through a persistent arterial duct.

Surgical repair in many of these conditions is complex and usually involves early palliation followed by subsequent conversion of the haemodynamics to one of univentricular heart that is right ventricular in type. In a proportion of patients the only morphological abnormality is a cleft in the anterior mitral leaflet (Fig. 3) 1713. Such a cleft lies in the aortic outflow and may give rise to varying degrees of mitral regurgitation. These patients may present early in life or may indeed remain asymptomatic well into adult life.

When a direct surgical approach to the mitral valve is contemplated preservation of the valve is of paramount importance, particularly in infants and children. Even in adults the majority of cleft mitral valves may be repaired successfully. A proportion of patients with connective tissue disorders such as Marfan's syndrome may present early in life with mitral regurgitation due to stretching of the chordal apparatus with prolapse of segments of the cusps into the left atrium. In the early part of life some of these valves may be amenable to repair.

If valve replacement is required mechanical valves are used predominantly in the younger patient in order to avoid the accelerated calcification and early valve dysfunction, associated with tissue valves. The disadvantage of mechanical valves is the need for permanent oral anticoagulation.

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