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Dorchester Center, MA 02124
Tricuspid atresia was described in 1817, but almost a century elapsed before the great arterial relationships were defined. Because of the morphologic heterogeneity of the malformation, “manifold anatomic combinations can result in this haemodynamic arrangement.” ,
Incidence has been estimated at 0.06 per 1000 live births with a prevalence of 1% to 3% of congenital heart disease. , Substantial variation exists in the anatomy of the atretic tricuspid valve. This chapter is concerned with hearts in situs solitus without ventricular inversion in which a physiologic or anatomic connection does not exist between the morphologic right atrium and the morphologic right ventricle. In 95% of patients, absence of the right atrioventricular connection is the result of fibro-fatty tissue that is interposed between the muscular floor of the right atrium and the parietal wall of the ventricular mass. In the remaining 5%, atresia is produced by an imperforate tricuspid valvular membrane. , Systemic venous return cannot directly reach the ventricular portion of the heart, but instead crosses the atrial septum from a morphologic right atrium into a morphologic left atrium where it mixes with pulmonary venous return before traversing a solitary atrioventricular valve into a morphologic left ventricle, which is the only pumping chamber for the pulmonary and systemic circulations ( Fig. 22.1 , and ) . This arrangement has been referred to as a “functionally” univentricular heart. Atresia of the tricuspid valve with Ebstein’s anomaly and atresia of the right atrioventricular valve with single ventricle are dealt with in Chapters 11 and 23 .
Tricuspid atresia as just defined has certain anatomic features that consistently recur, and certain features that are variable. , , Consistent features include: (1) physiologic and anatomic absence of a connection between the morphologic right atrium and the morphologic right ventricle, (2) hypoplasia of the morphologic right ventricle, (3) an interatrial communication, and (4) a morphologic left ventricle equipped with a morphologic mitral valve. The variable features provide the rationale for a clinical classification based on three gross morphologic features: , , (1) transposed or non-transposed great arteries, (2) the presence or absence of pulmonary stenosis ( Figs. 22.2 and 22.3 ), and (3) the size of the coexisting ventricular septal defect. An embryologic classification is based on two microscopic features: (1) rudiments of an atretic tricuspid apparatus that create a dimple on the floor of the right atrium that can be localized with transillumination by a light source placed within the hypoplastic right ventricle, , , and (2) a fibrous atrioventricular remnant that forms a microscopic tract from the right atrium to a tiny inlet component of the subjacent right ventricle. Failure of expansion of this exceedingly small inlet component during early embryogenesis is believed to be the pathogenetic mechanism responsible for the majority of cases of tricuspid atresia. Congenital tricuspid stenosis is a less severe form of the malformation in which a well-formed tricuspid valve joins the small inlet portion of the right ventricle ( Fig. 22.4 ).
In approximately three fourths of cases, an interatrial communication exists in the form of a restrictive patent foramen ovale. The valve of the foramen ovale occasionally protrudes aneurysmally as the obstructed right atrium vainly seeks an exit. , The aneurysmal protrusion can obstruct left atrial flow. A much less common form of interatrial communication is an atrial septal defect that is almost always ostium secundum (see Fig. 22.1 ; see and ). ,
The great arteries are non-transposed in approximately 90% of cases ( Fig. 22.5 ). , Pulmonary blood flow then depends on the condition of the ventricular septum (see Fig. 22.2 ). The arrangement at birth is usually a restrictive ventricular septal defect (see Fig. 22.2 B) that constitutes a zone of subpulmonary stenosis which is physiologically advantageous when blood flow to the lungs is adequate but not excessive. The advantage is lost in about 40% of cases because the defect decreases in size or closes altogether—acquired pulmonary atresia (see Figs. 22.2A and 22.6 ). , The time course of spontaneous closure is similar to that of isolated peri-membranous ventricular septal defects (see Chapter 14 ), with the majority that are destined to close doing so in the first year of life. , Rarely, the ventricular septum is congenitally intact and the pulmonary valve is atretic, an arrangement that completely denies the left ventricle access to the pulmonary circulation (see Fig. 22.2 A). Also rarely, obstruction is exclusively at valve level because a bicuspid pulmonary valve is stenotic. A non-restrictive ventricular septal defect (see Figs. 22.2C and 22.7 ) permits unobstructed flow from left ventricle to main pulmonary artery. The right ventricle is well-developed, and the pulmonary valve is normally formed. Pulmonary blood flow is regulated by pulmonary vascular resistance.
Tricuspid atresia with complete transposition of the great arteries typically occurs with a non-restrictive ventricular septal defect without pulmonary stenosis (see Fig. 22.3 ). , Left ventricular blood has unobstructed access to the transposed aorta through a well-developed right ventricle. Pulmonary blood flow is regulated by pulmonary vascular resistance because the transposed pulmonary trunk originates from the left ventricle (see Fig. 22.3 ). Pulmonary vascular disease usually develops in the first year of life ( Figs. 22.8 and 22.9 ). A decrease in size or spontaneous closure of the ventricular septal defect constitutes a zone of subaortic stenosis because the transposed aorta arises from the right ventricle. Pulmonary stenosis is infrequent and pulmonary atresia is rare (see Fig. 22.3 ).
Distribution of the coronary arteries in tricuspid atresia is analogous, if not identical, to that of univentricular hearts with a single morphologic left ventricle and an outlet chamber (see Chapters 23 and 29 ). The rudimentary right ventricle of tricuspid atresia and the right ventricular remnant of single ventricle are both delimited by coronary arteries.
Additional anatomic variables associated with tricuspid atresia involve the mitral valve, the ductus arteriosus, the ascending aorta, the aortic isthmus, the atrial appendages, and the pulmonary valve. Abnormalities of the mitral valve are represented by myxomatous, redundant, or prolapsing leaflets ( Fig. 22.10 ; and ), a cleft anterior leaflet, and direct attachment of leaflets to papillary muscles. When the great arteries are non-transposed and the ventricular septum is congenitally intact—which is physiologic pulmonary atresia—the fetal ductus arteriosus functions as a small malformed aortic tributary. The ascending aorta and isthmus are large because the aorta receives the entire cardiac output. When the great arteries are transposed and the ventricular septal defect is restrictive, left ventricular blood is diverted into the pulmonary trunk, so the ductus arteriosus enlarges while the ascending aorta and isthmus are underfilled and hypoplastic. , Very rarely, the pulmonary valve is absent. Juxtaposition of the atrial appendages, a condition in which both appendages lie on one side of the great arteries, , almost always means that the great arteries are transposed (see Chapter 24 ). Juxtaposition is present in about 50% of patients with tricuspid atresia and complete transposition.
The physiologic consequences of tricuspid atresia begin with the obligatory right-to-left shunt at the atrial level. The left atrium receives the normal pulmonary venous return together with the systemic venous return across the interatrial communication. The left atrial mixture flows across a morphologic mitral valve into a morphologic left ventricle, which is the sole pumping chamber for the systemic and pulmonary circulations. When the great arteries are not transposed, pulmonary blood flow is reduced because a restrictive ventricular septal defect constitutes a zone of subpulmonary stenosis (see Figs. 22.2 and 22.6 ). This arrangement accounts for about 90% of cases. Left ventricular volume overload is curtailed at the price of increased cyanosis. When the ventricular septal defect is non-restrictive and pulmonary vascular resistance is low, pulmonary blood flow and left ventricular volume overload are excessive, and cyanosis is mild (see Fig. 22.7 ). When the great arteries are transposed, the ventricular septal defect is usually non-restrictive and pulmonary stenosis is usually absent (see Fig. 22.3 ). Low pulmonary vascular resistance results in increased pulmonary blood flow, mild cyanosis, and left ventricular volume overload. , The degree of pulmonary vascular resistance that achieves adequate but not excessive pulmonary blood flow is a delicate balance that is seldom realized (see Figs. 22.8 and 22.9 ).