The malformation of tricuspid atresia consists of a complete agenesis of the tricuspid valve with an absence of a direct communication between the right atrium and right ventricle. Tricuspid atresia is the third most common form of cyanotic congenital heart disease. It is also the most common cause of cyanosis with left ventricular hypertrophy.
The natural history of this condition is such that, without surgical intervention, only one third of patients survive to age 1 year and only 10% live to age 10 years.  The Fontan procedure, which was first performed in 1968 and then described in 1971, has changed the natural history dramatically and allowed survival into the third and fourth decades of life. 
History of the Procedure
The Fontan operation is named for Fontan, who was the first to describe an operation for patients with tricuspid atresia that could result in separate systemic and pulmonary circulations despite the absence of a ventricle, in this case, the right ventricle. Until that description, a pumping chamber was assumed to be essential to move the blood through the lungs. Although the Glenn operation, which involves the end-to-end anastomosis of the superior vena cava (SVC) to the right pulmonary artery, was described in 1958, it was primarily used as palliative surgery.  It was used to augment pulmonary blood flow as an alternate to the Blalock-Taussig shunt procedure. Patients remained cyanotic because the return from the inferior vena cava (IVC) still entered the heart and mixed with the pulmonary venous return. Moreover, the left ventricle continued to have the additional volume load of the IVC return.
Fontan was the first to completely bypass the right heart in a human subject and to channel both the IVC and SVC blood to the pulmonary arteries. He unsuccessfully attempted to bypass the right heart in healthy dogs. However, he reasoned that it may work in patients with tricuspid atresia because the right atrium is thicker and more muscular in humans than in canines and hence better able to perform a contractile function. He also believed that valved conduits were essential to prevent the blood from going in the opposite direction (ie, into the IVC). Therefore, he incorporated one valved homograft between the IVC and the right atrium and another valved homograft between the right atrium and the left pulmonary artery.
The final operation consisted of the classic Glenn SVC–to–right pulmonary artery operation as well as the right atrium–to–left pulmonary artery connection with the 2 homografts, as described. He published his results from the first three patients in 1971. 
Kreutzer et al (1973) offered a modification almost immediately.  They described a connection between the right atrial appendage and the main pulmonary artery accomplished by means of a pulmonary homograft or the patient's own pulmonary annulus but without a valved homograft at the IVC–right atrial junction. In this situation, the continuity of the pulmonary arteries is maintained.
Since these original articles were published, several other modifications have been described. These have greatly reduced the mortality and morbidity previously associated with tricuspid atresia. In addition, the Fontan-Kreutzer operation is applicable to all cardiac conditions where only a single functional ventricle is present.
The morphology of the atretic tricuspid valve is most often muscular but is occasionally an imperforate membrane. In rare cases, it is Ebstenoid, or it has no connection between the tricuspid valve and the right ventricle. Systemic venous blood entering the right atrium can exit the atrium only through an interatrial communication. In 80% of patients, this interatrial communication is a stretched patent foramen ovale. In most of the remainder, it may be a secundum atrial septal defect.
The right ventricle is small and hypoplastic when no ventricular septal defect is present. In the presence of a ventricular septal defect, the right ventricle may be better developed than it is without the defect, but even then it is smaller than normal. The most common ventricular septal defect in tricuspid atresia is muscular, but it is occasionally perimembranous.
Almost 100 years ago in 1906, Kuhne proposed a classification scheme for tricuspid atresia based on the great artery orientation.  Tandon and Edwards reevaluated and redefined this scheme in 1974, and this is the classification in general use at this time.  As mentioned before, this system is primarily based on associated anomalies.
The relationship of the great artery defines the type, as follows: Type I involves normally related great arteries, type II is D-transposition of the great arteries, and type III is L-transposition of the great arteries. The subtypes a, b, and c depend on the degree of pulmonary obstruction and, to a lesser degree, the presence of a ventricular septal defect. Subtype a is pulmonary atresia, b is pulmonic stenosis, and c is no pulmonary obstruction.
Type Ib is the most common; this is tricuspid atresia with normally related great arteries, a small ventricular septal defect, and pulmonic stenosis. Type I accounts for 80% of all cases, and type III is the rarest and responsible for 3-7% of all cases.
About 60% of patients with transposition of the great arteries may have additional cardiovascular abnormalities.  A persistent left superior vena cava (SVC) is seen in 15% of patients. Patients with tricuspid atresia, transposition of the great arteries, and large pulmonary flow (type IIc) are most likely to also have a coarctation of the aorta, hypoplasia of the aortic arch, and a patent ductus arteriosus. This is in contrast to patients with tricuspid atresia and normally related great vessels, an intact ventricular septum, or a small ventricular septal defect (type Ia or Ib). These patients tend to have a small patent ductus arteriosus and a large ascending aorta and isthmus. Juxtaposition of the atrial appendages is reported in 40% of patients with tricuspid atresia and transposition of the great arteries. 
The basic problem arises from the absent connection between the right atrium and pulmonary artery due to the absent tricuspid valve. Hence, the only egress to the blood entering the right atrium is into the left atrium through an interatrial communication. This combined venous return then enters the left ventricle and is pumped out into the aorta. Some of this blood finds its way into the pulmonary artery through a ventricular septal defect or a patent ductus arteriosus.
Because of the absence of the tricuspid valve and the lack of continuity between the right atrium and the right ventricle, venous blood returning to the right atrium can exit only by means of an intra-atrial connection. Because this situation leads to an obligate right-to-left shunt at the level of the atria, saturation of the left atrial blood is diminished, and hypoxia and cyanosis result.
The classification is discussed below. However, in the most common type (type Ib), left atrial blood enters the left ventricle and is pumped out into the aorta. Some of this blood may reach the lungs through a small ventricular septal defect or through a patent ductus arteriosus if the ventricular septum is intact. In most cases of tricuspid atresia, pulmonary blood flow is restricted, leading to reduced pulmonary venous return and hence additional cyanosis. In this scenario, the single left ventricle can easily handle the added volumes of the systemic and the pulmonary venous return.
On the contrary, in patients with tricuspid atresia, a large ventricular septal defect, and no pulmonic stenosis (type IIc or rare cases of type Ic), pulmonary blood flow is excessive. This can manifest as congestive cardiac failure early and predispose the patient to the pulmonary vascular disease late. Because of the large amount of pulmonary venous return, cyanosis may be difficult to detect. Also, this excessive pulmonary venous return can pose a volume burden on the single ventricle and contribute to congestive cardiac failure.
The cause of tricuspid atresia is unknown. Although specific genetic causes of the malformation have not been determined in humans, data indicate that the FOG2 gene may be involved in the process. Mice in which the FOG2 gene is knocked out are born with tricuspid atresia.  The significance of this finding and its applicability in humans has yet to be defined.
The true prevalence of the Fontan procedure for tricuspid atresia is unknown, but a rate reported in clinical studies was approximately 1.5%. An autopsy series revealed a prevalence of approximately 3%.  The calculated incidence is 1 case per every 10,000 live births. No sex dominance is known, and girls and boys are almost equally affected.
Tricuspid atresia is usually detected in the neonatal period. Cyanosis is the most prominent feature, and this is due to the obligate intracardiac right to left shunt. However, the amount of pulmonary venous return modulates this sign. Cyanosis is most prominent in patients with restricted pulmonary blood flow (type Ib) and least noticeable in those with excessive pulmonary blood flow (types I and IIc). In the latter group, symptoms of congestive heart failure and pulmonary edema predominate. Examples are feeding and breathing difficulties, frequent respiratory infections, and growth failure. Clinical signs include tachypnea, tachycardia, and an enlarged liver.
Hypoxic-cyanotic spells may occur in infants with tricuspid atresia and diminished pulmonary blood flow, similar to those seen in tetralogy of Fallot. These episodes are potentially life-threatening and are indications for surgery.
Polycythemia is a consequence of the cyanosis and, as in other conditions with cyanosis, cerebrovascular accidents are a risk in infants with iron deficiency anemia. In older, nonsurgically treated children, brain abscesses may occur because the filtering function of the lungs is no longer operational in the presence of a right-to-left intracardiac shunt. These are uncommon at this time because children with tricuspid atresia undergo the Fontan procedure typically around age 2-4 years; this effectively separates the systemic and the pulmonary circulations.
The combination of cyanosis, polycythemia, and the artificial material used for modified Blalock-Taussig shunts predispose older patients receiving surgical palliation to bacterial endocarditis.
The treatment for tricuspid atresia is predominantly surgical. Because one of the requirements for a Fontan procedure is low pulmonary vascular resistance, this operation cannot be performed in neonates. Hence, an interim palliative procedure may be needed depending on the amount of pulmonary blood flow. Most infants with tricuspid atresia have restrictive pulmonary blood flow. Therefore, after initial medical stabilization is achieved with intravenously administered prostaglandin 1 (PGE1), the first surgical intervention is palliative pulmonary-to-systemic anastomosis to increase pulmonary blood flow and to improve systemic oxygenation. These aims are achieved by using a modified Blalock-Taussig shunt, which is a small polytetrafluoroethylene (PTFE) graft to connect the subclavian artery and a pulmonary artery.
In those few patients with no pulmonary obstruction, a pulmonary artery band may initially be required to prevent the onset of pulmonary vascular disease.
In patients with a balanced circulation secondary to pulmonary obstruction that is not severe, no immediate intervention may be required at birth. However, they may undergo a bidirectional Glenn procedure after about age 3 months, by which time the pulmonary vascular resistance decreases to low levels. This procedure is essentially an anastomosis between the SVC and the pulmonary artery. One of the major advantages of this procedure is that it results in volume unloading of the left ventricle. Early volume unloading improves exercise capacity in preadolescents with the Fontan procedure.  Furthermore, this may serve as a first stage for subsequent surgery involving total cavopulmonary anastomosis (ie, modified Fontan procedure).
Choussat et al (1977) delineated selection criteria to define an ideal candidate for a Fontan procedure.  They described the 10 following criteria, which are occasionally and facetiously referred to as the 10 commandments for an ideal Fontan operation.
Age older than 4 years
Normal systemic venous return
Normal right atrial volume
Mean pulmonary artery pressure less than 15 mm Hg
Pulmonary arteriolar resistance less than 4 Wood units/m 2
Pulmonary artery–aorta ratio more than 0.75
Left ventricular ejection fraction more than 0.60
Competent mitral valve
Absence of pulmonary artery distortion
Due to the several modifications of the operation that have followed its initial description, the absence of most of the above criteria is regarded as only relative contraindications. Current absolute contraindications are a pulmonary vascular resistance above 4 Wood units/m2, severe hypoplasia of the pulmonary arteries, and severe diastolic dysfunction of the left ventricle.
The Fontan operation has only a few absolute contraindications at this time: pulmonary vascular resistance more than 4 Wood units/m2, severe hypoplasia of the pulmonary arteries, and severe diastolic dysfunction of the left ventricle.
The age at which a Fontan is performed has been steadily lowered, and several centers perform it at age 2 years or even younger in patients with good anatomy and physiology.  The advantage of performing a Fontan operation at an early age is improving oxygenation, which may enhance somatic growth and neurodevelopmental outcomes. It also reduces the volume load on the single ventricle.
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