Truncus Arteriosus

Morphology

Persistence of the truncus arteriosus is a rare lesion in which only one arterial vessel exits the heart and gives rise to the pulmonary, systemic and coronary arteries. There is almost invariably a large ventricular septal defect present, so that the lesion represents incomplete ventricular and great vessel septation.
Four anatomical variants were described by Collette and Edwards and include type I truncus in which the pulmonary trunk arises from the aorta then divides into right and left branches, type II truncus in which the pulmonary vessels arise independently from the posterior part of the ascending vessel, type III truncus in which the pulmonary vessels arise farther laterally from the truncus, and type IV truncus which is now considered to represent a form of pulmonary atresia with ventricular septal defect (tetralogy of Fallot with ventricular septal defect) rather than truncus arteriosus.

Associated cardiac conditions

Associated cardiac conditions include a ventricular septal defect, an abnormal truncal valve, and anomalies of the aortic arch. A ventricular septal defect is almost always present, and is typically large and situated below the truncal valve, cradled by the two limbs of the septomarginal trabeculation. The truncal valve is tricuspid in 70% of cases, quadricuspid in 21%, bicuspid in 9%, and pentacuspid in <1%. The anatomic cause for truncal valve insufficiency is variable and includes thickened and nodular dysplastic cusps, prolapsed of unsupported cusps or conjoined cusps containing a shallow raphe, inequality of cusp size, minor commissural abnormalities, and annular dilatation. Truncal valve stenosis, when present, is usually associated with nodular and dysplastic cusps. A right aortic arch with mirror-image brachiocephalic branching is more commonly associated with truncus arteriosus than with any other congenital cardiac malformation, occurring in 25 - 33% of cases. Interrupted aortic arch, which may be due to inadequate in utero blood flow and is accompanied by ductal continuity of the descending aorta, forms a distinct category in the Van Praagh classification, and occurs relatively frequently (11 - 19% of cases). The arterial duct is absent in approximately half the cases but, in cases in which it is present, remains patent in the postnatal period in nearly two-thirds. The relative sizes of the aorta and the arterial duct tend to vary inversely. Coronary artery abnormalities are not uncommon, and are of surgical importance. Anomalies of coronary ostial origin are common, regardless of the number of truncal valve cusps. In general, the left coronary artery tends to arise from the left posterolateral truncal surface and the right from the right anterolateral surface. In the setting of a single coronary ostium, frequently associated with left coronary dominance, all three major epicardial branches may originate from this common site or the right coronary artery may be absent. High ostial origin, above the truncal sinotubular junction, is common, and when the origin is at or slightly above a truncal valve commissure, the involved ostium may be slit-like and functionally stenotic. Rarely, the left coronary artery originates from the pulmonary trunk.

Hemodynamics

As pulmonary vascular resistance decreases following birth, pulmonary blood flow greatly increases. There is a low systemic diastolic pressure due to pulmonary run-off. This may result in subendocardial myocardial ischemia and a severely limited coronary blood flow reserve. Measures to increase pulmonary vascular resistance, such as decreasing FiO2 and relative hypoventilation, are important measures in order to stabilize diastolic pressure and coronary blood flow (see below).

Clinical presentation and work-up

Patients with truncus arteriosus present within a few days to weeks of life as pulmonary vascular resistance decreases and congestive heart failure develops. Concomitant conditions that may play a role in postoperative outcome include the presence of severe truncal valve regurgitation and interrupted aortic arch.

Most of the important features of truncus arteriosus can be defined by echocardiography. These include the type of truncus, the morphology and function of the truncal valve, the presence of ostial stenosis or branch pulmonary stenosis, defining the type of interventricular communications, estimating the pulmonary blood flow and resistance, and defining associated lesions such as interrupted aortic arch and coronary abnormalities.

Medical & surgical management

Medical therapy is directed towards stabilizing the patient. These measures include diuretics, and positive pressure ventilation with PEEP. Operative therapy for type I and II truncus consists of division of the pulmonary arteries from the aorta, and reconstruction of the outflow tract using a valved homograft conduit. Type III truncus arteriosus poses a specific problem because of the lack of confluence between the left and right pulmonary arteries, and requires extensive pulmonary arterioplasty. Long-term survival appears good and is dependent on the function of the truncal valve and the need for conduit replacement.

Surgical repair of truncus arteriosus