Truncus Arteriosus and Aortopulmonary Window

Embryology

During the fifth week of gestation, paired lateral ridges appear in the cephalic portion of the truncus. These truncal swellings ultimately form the aorticopulmonary septum. The right superior truncus swelling, located on the right superior wall of the truncus, grows distally and leftward, whereas the left inferior truncus swelling, located on the left inferior wall, moves distally and rightward. The opposing movements of the swellings as they grow toward the aortic sac result in the spiraling of the aorticopulmonary septum. The most proximal portions of the swellings form parts of the infundibular or conal septum. Hence, variable deficiency of these truncal swellings results in the various forms of truncus arteriosus and the commonly present ventricular septal defect (VSD).

Pluripotent neural crest cells play an important role in the development of the conotruncus and aortic arch. Deletion of Cdc42 (cell division cycle 42), a GTP-binding protein that regulates cytoskeleton remodeling and is essential for neural crest cell migration, results in persistent truncus arteriosus and interrupted aortic arches in murine models. Selective ablation of neural crest cells in chick embryos before migration results in a number of congenital heart defects. These anomalies include truncus arteriosus and interrupted aortic arch, explaining their coexistence in approximately 10% to 20% of patients with truncus arteriosus.

Animal models provide further insight into the genetic basis for truncus arteriosus. The mouse mutant Splotch, which has a mutation in the homeobox gene Pax3, has a phenotype with truncus arteriosus and aortic arch abnormalities. In humans, monoallelic microdeletion of chromosome 22q11 is associated with multiple defects of neural crest origin, including typical facies, cleft palate, thyroid and parathyroid gland aplasia, and conotruncal and aortic arch abnormalities. The resulting phenotypic syndromes include DiGeorge, velocardiofacial, and Shprintzen syndromes, which are associated with truncus arteriosus. One candidate gene identified in the area of the microdeletion is HIRA . HIRA interacts with Pax3, and thus may be integral to Pax3 regulation of neural crest cells. Environmental risk factors for the development of truncus arteriosus include maternal diabetes and exposure to retinoic acid.

Anatomy

In truncus arteriosus, there is generally situs solitus and d -looping of the ventricles. A single great vessel arises from the base of the heart, giving origin to the pulmonary, systemic, and coronary arteries. Classifications of truncus arteriosus were proposed by Collett and Edwards in 1949 and by Van Praagh and Van Praagh in 1965 ( Fig. 121-1 ). The system described by Collett and Edwards ( Table 121-1 ) is based on the site of origin of the pulmonary arteries, whereas the Van Praagh classification ( Table 121-2 ) is based on the degree of septation of the trunk and the presence or absence of a VSD. The Van Praagh scheme also requires that at least one of the pulmonary arteries arise from the common trunk. This stipulation appropriately relegates Collett-Edwards type IV, or pseudotruncus, to the spectrum of pulmonary atresia with aortopulmonary collaterals. The Van Praagh classification also provides for the inclusion of the relatively common association of hypoplastic or interrupted aortic arch. The term hemitruncus is frequently encountered in the literature to describe the anomalous origin of the right pulmonary artery from the ascending aorta with a normal origin of the left pulmonary artery from the main pulmonary artery, usually in the absence of a VSD. This lesion is distinct from Van Praagh type B3 and should not be considered a form of truncus arteriosus.

A VSD is nearly always present. It results from the deficiency of the infundibular septum and is generally nonrestrictive. The VSD is cradled by the two limbs of the septal band and bounded superiorly by the truncal valve ( Fig. 121-2 ). The posterior (inferior) limb of the septal band usually inserts into the parietal band, resulting in discontinuity of the tricuspid and truncal valves, maintaining a muscular rim between the conduction system and the VSD. When there is failure of insertion, the VSD extends into the membranous septum, with the conduction system running along the posterior-inferior rim of the defect.

The truncal valve can be tricuspid (69%), quadricuspid (22%), bicuspid (9%), or, rarely, unicuspid or pentacuspid. The valve is usually in continuity with the mitral valve and infrequently with the tricuspid valve. The truncus equally overrides both ventricles in 68% to 83% of cases, is deviated over the right ventricle in 11% to 29%, and is deviated to the left in 4% to 6%. The valve may be stenotic or regurgitant, which can complicate management of the patient with truncus arteriosus. A moderate or greater degree of truncal insufficiency is present in 20% to 26% of patients. Mild stenosis is generally detected on preoperative evaluation because of the increased flow across the truncal valve. Significant stenosis is present in only 4% to 7% of patients. Gradients of greater than 30 mm Hg are concerning for residual stenosis after complete repair.

The branch pulmonary arteries generally originate from the left posterolateral aspect of the truncus, just distal to the truncal valve. They are usually of good size without ostial or branch stenosis. Collett and Edwards type I is most frequently encountered, occurring in 48% to 68% of cases, followed by type II (29% to 48%) and type III (6% to 10%). In practice, most cases seem to fall into a category of “type ,” with the branch pulmonary arteries arising not from a main pulmonary artery but in proximity from the posterior aspect of the truncus. Origin of one pulmonary artery from a systemic artery other than the truncus (Van Praagh type A3/B3) is relatively rare, with an incidence of 2% to 5%.

Pathophysiology

The pathophysiology of truncus arteriosus is one of a total admixture lesion, with mixing occurring at the level of the VSD and proximal truncus. Although there is cyanosis, systemic oxygen saturations are frequently 85% to 90% in the newborn period because of elevated pulmonary blood flow. In the absence of pulmonary artery stenosis or systemic outflow obstruction, the amount of pulmonary blood flow is mainly affected by the pulmonary vascular resistance (PVR). In the first few days of life, PVR remains relatively high, limiting pulmonary blood flow. As PVR decreases, the amount of pulmonary blood flow increases, leading to pulmonary overcirculation and signs and symptoms of congestive heart failure. The unrestricted left-to-right shunt results in both pressure and volume overload to the pulmonary circuit. In addition, truncus arteriosus is distinguished from other left-to-right shunt lesions by both systolic and diastolic shunting. These factors lead to the early development of irreversible pulmonary vascular occlusive disease in patients with truncus arteriosus.

Truncal valve regurgitation and, less frequently, stenosis can exacerbate the hemodynamic stresses placed on the heart in truncus arteriosus. Regurgitation adds an additional volume overload to the ventricles, worsening the signs and symptoms of congestive heart failure. The diastolic runoff, which occurs not only because of the insufficient valve but also as a result of the low-resistance pulmonary vascular bed, can lead to poor systemic perfusion, most notably to the coronary arteries. Stenosis increases the afterload on the ventricles and thereby increases myocardial oxygen demand. Significant truncal valve stenosis can limit systemic perfusion, again compounded by the runoff into the pulmonary circuit.

Diagnosis

Diagnostic Studies

The chest radiograph generally shows moderate cardiomegaly with increased pulmonary vascular markings. The arch is rightward in approximately one third of patients, and the thymus gland may be absent in those with 22q11 microdeletion. The combination of a right arch and increased pulmonary vascular markings is strongly suggestive of truncus arteriosus. The two-dimensional and Doppler echocardiography examinations are the diagnostic modalities of choice. The echocardiogram can define the anatomy of truncus arteriosus at birth or in utero. Prenatal echocardiography is increasingly identifying congenital heart anomalies; however, because of the challenges of imaging the branch pulmonary arteries, truncus arteriosus remains one of the more commonly misdiagnosed heart defects (78.6% accuracy). A parasternal long-axis view will demonstrate the large truncal valve overriding the VSD ( Fig. 121-3 A ). The addition of Doppler interrogation will reveal truncal valve stenosis or regurgitation (see Fig. 121-3 B ). Suprasternal notch views can further define the anatomy of the pulmonary arteries and aortic arch ( Fig. 121-4 ). Cardiac catheterization is generally reserved for the delineation of the anatomy in complex forms of truncus arteriosus, such as truncus arteriosus with a single pulmonary artery (Van Praagh type A3/B3). Cardiac catheterization is also indicated to assess PVR in the patient presenting late with truncus arteriosus. Magnetic resonance imaging (MRI) is a useful alternative or adjunct to cardiac catheterization for defining the anatomy of complex truncus arteriosus. MRI increasingly plays a role in the postoperative assessment of these patients in evaluating ventricular function as well as conduit and branch pulmonary artery anatomy.

Natural History

The typical natural history of truncus arteriosus is characterized by early demise because of congestive heart failure. Death rates are approximately 40% at 1 month, 70% at 3 months, and 90% at 1 year. Patients who survive infancy generally succumb by childhood or early adolescence because of congestive heart failure or, more commonly, pulmonary vascular obstructive disease. Rarely, patients survive infancy without developing pulmonary vascular obstructive disease, although those who do so are estimated to be less than 5% of all patients.

Treatment

Because of the inherent high early mortality, truncus arteriosus warrants early intervention. Initially, the surgical treatment of truncus arteriosus was limited to the banding of one or both of the branch pulmonary arteries. The first successful intracardiac repair was accomplished by Sloan's group at the University of Michigan in 1962 using an unvalved polytetrafluoroethylene (PTFE) conduit for the pulmonary reconstruction. In 1967, McGoon and colleagues performed the first valved conduit repair, using an aortic allograft. During this period, complete repair was often undertaken as a staged procedure after initial pulmonary artery banding. However, complications of pulmonary artery banding, including pulmonary artery distortion, band migration, and failure to prevent the development of pulmonary vascular obstructive disease, resulted in a continued high mortality with this strategy. Ebert and coworkers published the first series of patients undergoing repair of truncus arteriosus in infancy in 1984. With continued improvements in neonatal operative techniques, as well as perioperative care, management has evolved to earlier complete repair. After the early reports of neonatal repair from the University of Michigan and the Children's Hospital of Boston, neonatal repair has become the treatment of choice for truncus arteriosus.