Double Inlet Ventricle

Morphology & Embryology

Double inlet ventricle is a congenital cardiac malformation in which both atria connect to only one ventricular chamber by either two separate atrioventricular valves or a common atrioventricular valve[198, 196, 195]. The ventricle to which both atrioventricular valves or common atrioventricular valve connects is usually well formed, whereas the ventricle not receiving the great majority of the venous return to the heart is often rudimentary. By definition, the term double inlet ventricle is used only if more than 50% of the overriding valve lies over the dominant ventricle. Recent evidence indicates that a ring of specialized tissue encircles the junction of the right and left ventricles in the developing heart tube. Part of this ring is also an atrioventricular junctional structure, rightward expansion of which forms the right atrioventricular junction. Partial or complete failure of rightward expansion is thought to be responsible for maldevelopment of the right atrioventricular junction, and link together the lesions of straddling tricuspid valve, double inlet left ventricle, and tricuspid atresia. The essence of double inlet left ventricle is that, although there has been expansion of the right atrioventricular junction, the right atrial myocardium retains its connection with the dominant left ventricle, while the right ventricle is incomplete and rudimentary. Hence, the left ventricle receives both atrioventricular valves or a common atrioventricular valve. When both atrioventricular valves are present, they often cannot be designated as either mitral or tricuspid, and are commonly straddling or stenotic. The rudimentary chamber is separated from the main chamber by a septum that does not extend to the crux of the heart, and is connected to the main chamber via a ventricular septal defect. The second chamber is of right ventricle morphology, is always anterior, and is located either to the left or the right. The size of the outlet chamber is related to the degree of development and straddling of the tricuspid valve, in addition to the size of the ventricular septal defect. The ventriculoarterial connections are most commonly discordant (Lambert heart) or may be concordant (Holmes heart). Unusual forms of ventriculoarterial connections in double inlet ventricle include double or single (pulmonary atresia) outlet.

Outflow obstruction to the pulmonary artery is common, and is the most important determinant of the clinical course. The obstruction may be subvalvar, valvar, or both, or may be complete (pulmonary atresia). Obstruction to systemic outflow also occurs, usually at the level of the ventricular septal defect in hearts with discordant ventriculoarterial connections, and the presence of aortic coarctation is a strong marker for the presence of a restrictive ventricular septal defect.

The conduction tissues, as seen in relationship to the ventricular septal defect, have a directly comparable arrangement to that seen in tricuspid atresia. Unlike tricuspid atresia, however, the AV node is situated anteriorly within the right atrioventricular orifice rather than within the atrial septum. This arrangement is seen irrespective of whether the rudimentary right ventricle is right or left-sided. The position of the rudimentary ventricle affects only the relationship of the atrioventricular bundle to the outflow tract from the dominant left ventricle. This disposition of conduction tissue is readily explained on the basis of its formation from the ring of tissue initially developed between the inlet and outlet components of the ventricular loop.

Associated Cardiac Anomalies

Associated cardiac anomalies occur in about one-third of cases with double inlet ventricle. atrioventricular valve malformations are common, and include straddling, leaflet dysplasia, leaflet cleft and tags and annular hypoplasia. The pulmonary valve may be stenotic due to either annular hypoplasia or leaflet thickening, or may be atretic. Subvalvar pulmonary stenosis may be due to a restrictive ventricular septal defect in cases in which the pulmonary artery arises from the rudimentary chamber, or may less commonly be due to infundibular narrowing. Aortic arch anomalies such as coarctation, interrupted aortic arch and hypoplastic aortic arch are strongly associated with a restrictive ventricular septal defect in cases in which the aorta arises from the rudimentary chamber.

Diagnostic Work-up

Most of the main features of double inlet ventricle can be defined by echocardiography or angiocardiography. Important left atrioventricular valvar stenosis should be sought, as its presence necessitates either atrial septostomy or septectomy. Pulmonary arterial pressure is important, since the choice of operative therapy is largely dictated by the presence of pulmonic stenosis and its severity.

Hemodynamics

There are several important hemodynamic variables in double inlet ventricle, including the completeness of intracardiac mixing at the atrial level, the degree of outlet obstruction, and the presence of atrioventricular valve malfunction. In double inlet ventricle, mixing of systemic and pulmonary venous blood occurs in the main ventricular chamber. This may result in nearly equal oxygen saturation in the aorta and the pulmonary artery. In some cases, streaming of blood within the ventricle results in a substantial difference in oxygen saturation between the aorta and the pulmonary artery. The streaming may be favorable, with aortic saturation being greater than pulmonary arterial saturation, or unfavorable, with pulmonary saturation being greater than aortic. The degree of saturation is also influenced by the severity of pulmonic stenosis, the single most important determinant of the clinical course. Aortic stenosis is also important, although it is much less commonly a hemodynamic factor, and is almost always due to subvalvar stenosis at the level of the ventricular septal defect. Subaortic stenosis often follows palliative pulmonary artery banding, in which the resultant hypertrophy of muscle around the ventricular septal defect may play a contributing role. Atrioventricular valve stenosis is hemodynamically important if the atrial septum is intact or the atrial septal defect is restrictive, in which case venous return may be restricted or obstructed.

Clinical Course

There are two modes of presentation, occurring in the neonatal period or during early infancy, and is dependent on the status of pulmonary blood flow. Patients with reduced blood flow due to pulmonic stenosis or atresia typically present within the first week of life, whereas those with increased pulmonary blood flow usually present within the first few months of life. Patients with balanced pulmonary blood flow may present much later in life. Cyanosis is more striking in cases with obstructed pulmonary blood flow, while congestive cardiac failure is more common in those with unrestricted pulmonary blood flow.

Atresia of the left-sided atrioventricular valve, when combined with a restrictive intra-atrial communication, results in severe pulmonary venous hypertension with its typical chest radiographic appearance and severe respiratory distress in early life. This situation may be masked initially by pulmonary stenosis, only to become unmasked following the creation of a systemic-pulmonary artery shunt. Severe atrioventricular valve regurgitation results in elevated atrial pressure and the early appearance of congestive heart failure.

Medical & Surgical Management

The estimated overall survival without treatment of patients born with double inlet ventricle is about 57% at 1 year and 45% at 5 years, although some subsets have reasonably favorable prognosis[198, 196, 195]. Medical management is largely restricted to the management of congestive cardiac failure in cases with unrestricted pulmonary blood flow, or to the use of PGE1 in neonates with severely restricted pulmonary blood flow or with pulmonic atresia. Occasionally, percutaneous balloon or blade atrial septostomy is required to relieve pulmonary venous obstruction in cases with left sided atrioventricular valve stenosis and an intact atrial septum or restrictive atrial septal defect.

Surgical therapy is usually directed with the ultimate goal of achieving a Fontan type of physiology, often requiring a plan of staged-reconstruction. Ventricular septation and cardiac transplantation are also viable options dependent on local practice and philosophy. Ventricular septation may be an option in cases with an enlarged dominant ventricle into which two reasonable atrioventricular valves with little or no overriding or straddling. The ventriculoarterial connection must be amenable to repair with the appropriate ventriculoarterial connection, and there should be little or no pulmonary or systemic outflow obstruction[346]. Stage septation[421] consists of placing an apical patch and a second patch at the superior portion between the atrioventricular valves using widely spaced interrupted sutures, and completing the septation 6 - 18 months later with a third patch.

Staged-reconstruction consists of three stages, the first being a palliative procedure in which the systemic and pulmonary circulations are usually placed in parallel, the second stage consisting of a superior cavopulmonary anastomosis, and the final stage being conversion to a total cavopulmonary connection (Fontan physiology). The Qp:Qs for the three stages are typically 2-3:1, 0.5:1, and 1:1.

Patients with restricted pulmonary blood flow and no subaortic stenosis or left ventricular outflow tract obstruction are treated with a systemic-pulmonary artery shunt[18, 192, 244, 684]. Pulmonary artery reconstruction may or may not be required depending on the presence of discontinuous pulmonary arteries, pulmonary artery stenosis or distortion, or significant aorto-pulmonary collaterals[889].

Patients with a univentricular atrioventricular connections and left ventricular outflow tract obstruction consists of a modified Damus-Kaye-Stansel procedure[216, 221, 393, 394, 395, 396, 397, 490, 499, 540, 613, 677, 732, 818, 896, 902], in which the two great vessels are attached proximally, and pulmonary blood flow is supplied by an systemic-pulmonary shunt, a superior cavopulmonary anastomosis, or a total cavopulmonary connection. If the left ventricular outflow tract obstruction extends into the aortic arch or descending aorta, a Norwood procedure is required in order to relieve the distal obstruction. Finally, the arterial switch operation[216] has also been utilized, transforming the subaortic stenosis to sub-pulmonary stenosis and thereby theoretically protecting the pulmonary vascular bed. Many of the cases undergoing the arterial switch operation for a univentricular atrioventricular connection ultimately require a systemic-pulmonary shunt for insufficient pulmonary blood flow, raising the possibility of increasing the risk of an ultimate Fontan operation.

Patients with unrestricted pulmonary blood flow and no subaortic stenosis or left ventricular outflow tract obstruction may initially be palliated with pulmonary artery banding[499, 689, 644, 393]. Drawbacks to this procedure is the potential for pulmonary artery distortion, developing pulmonary valvar regurgitation or damage to the pulmonary valve, resultant ventricular hypertrophy, decreased ventricular compliance, and development of subvalvar aortic stenosis[499]. The latter may result from both ventricular hypertrophy and geometric changes to the left ventricle. Pulmonary artery banding may therefore increase the risk of a subsequent Fontan procedure[611].

Ventricular septal defect enlargement or resection of the infundibular septum may be required depending on the degree of restriction of the subaortic stenosis[796]. Atrial septectomy may be required in cases with stenosis of the left atrioventricular valve or those with an intact or nearly intact atrial septum.

The second stage operation, the bi-directional Glenn procedure, consists of division and anastomosis of the superior vena cava to the pulmonary artery and division and closure of the pulmonary trunk below its bifurcation. A bilateral bi-directional Glenn procedure is required when there is persistence of the left superior vena cava. The bi-directional Glenn procedure can usually be performed in the first 8 - 12 weeks of life, at which time pulmonary vascular resistance is usually sufficiently low to accommodate approximately half of the cardiac output. Insufficient pulmonary blood flow may be augmented by the addition of a small systemic-pulmonary shunt. A persistently high pulmonary vascular resistance results in insufficient pulmonary blood flow and severe cyanosis (systemic saturation less than 60% and a PaO2 less than 25 - 30 torr), manifested by high "Glenn pressures" (superior vena cava pressure over 18 - 20 mm Hg). A high Glenn pressure may significantly decrease cerebral perfusion by decreasing the pressure gradient across the cerebral bed. This may be clinically manifested as fullness of the fontanel, persistently irritability, systemic hypertension and relative bradycardia. In not medically reversible, the bi-directional Glenn may need to be taken down and a systemic-pulmonary shunt reconstructed. Unusual following the bi-directional Glenn procedure is persistent pleural effusions, although the rare late development of pulmonary arteriovenous malformations may cause severe cyanosis. The latter result in right to left shunting, and, if severe, precludes successful conversion to Fontan operation. Conversion to a Fontan operation completes the single-ventricle pathway in cases with unrestricted pulmonary blood flow.