The heart is exposed with a median sternotomy incision. The presence of thymic tissue is confirmed because many of these children have athymia associated with DiGeorge syndrome. The clear presence of thymic tissue, therefore, may have implications for later management and prognosis. The heart is suspended in a pericardial cradle and the great vessels examined. If the patient is severely overcirculated with congestive heart failure, it may be advisable to promptly encircle one of the pulmonary arteries with a snare to limit the total pulmonary blood flow. On occasion, this can stabilize the patient while dissection is performed around the arch vessels.

The operation can be performed either with the patient on continuous cardiopulmonary bypass or by utilizing circulatory arrest. We have elected to use both of these techniques, and in simple truncus arteriosus, the complete operative repair can be performed during a single period of circulatory arrest of <40 minutes in most cases. The right and left pulmonary arteries are mobilized and encircled with tourniquets for control and the origins of the pulmonary arteries examined. In addition, on opening the pericardium, it is important to determine the location of the anterior descending coronary artery and confirm that it does not cross the right ventricular wall in the area of anticipated ventriculotomy. Heparin is administered and the aorta cannulated as distally as possible. Distal cannulation is particularly important because the pulmonary bifurcation may come off relatively distally and there is little room for cross-clamping of the aorta between the anticipated division of the pulmonary bifurcation and the distal aorta. Either the venae cavae are cannulated if continuous bypass is used, or a single right atrial cannula is used with circulatory arrest. If mobilization of the pulmonary artery confluence behind the aorta is considered to be too close to the anticipated cross-clamp application site, the brachiocephalic vessels are encircled with tourniquets and circulatory arrest is used, with clamping of the aorta more distally on the arch during the reconstruction of the back of the truncus arteriosus after excision of the pulmonary bifurcation. The infant is placed on cardiopulmonary bypass and the snares on the right and left pulmonary arteries tightened as shown in Figure 87.1 to prevent pulmonary overcirculation during cooling. The child is cooled to 18°C for circulatory arrest or 34 to 37°C for continuous bypass repair. In the presence of significant truncal valve insufficiency, venting of the left ventricle is important during cooling and may be accomplished by placing a vent through the right superior pulmonary vein across the mitral valve into the ventricle, or if truncal insufficiency is severe, by compressing the heart to maintain emptying of the ventricle during cooling.

The aorta is then cross-clamped and cardioplegic solution is administered into the truncal root with the pulmonary artery snares applied to force the cardioplegic solution into the coronary arteries. Alternatively, cardioplegia can be induced in a retrograde manner or the aorta opened and the coronaries directly cannulated. After cardioplegia is achieved, the snares are removed from the pulmonary arteries, and an incision is made near the takeoff of the pulmonary bifurcation anteriorly. Through this initial incision, the truncal valve is examined and the origin of the left coronary artery carefully identified. The left coronary can arise high from above the truncal sinuses and be intimately associated with the origin of the pulmonary bifurcation. Once the coronary arteries are identified, the pulmonary bifurcation is excised from the back of the truncus arteriosus. The aorta is then repaired using a
patch of pulmonary homograft. Primary suture closure of the defect in the truncus may be used; however, we believe that the placement of a patch allows for less tension on the anastomosis and therefore a decreased risk of bleeding from behind the aorta. This is important after the right ventricular reconstruction has been performed because exposure of the back of the aorta is difficult after the completion of the repair. In addition, suturing bleeding sites in this area may be hazardous because the origin of the left coronary artery may be very close to the margins of the excision of the pulmonary bifurcation. Although pericardium can also be used for a patch, we have elected to use a portion of the pulmonary or aortic homograft that will be used for right ventricular outflow tract reconstruction because it results in a hemostatic suture line and is easier to work with than nontanned pericardium.

After the repair of the truncus arteriosus is complete and the pulmonary bifurcation is mobilized to bring the bifurcation to the left for reconstruction, additional cardioplegic solution is injected into the aortic root to test the suture line of the truncal patch, and additional sutures are placed if necessary. Next, an incision is made in the right ventricle, avoiding major conal branches of the coronary arteries. This incision is made at the base of the truncal valve, and care must be taken not to carry the incision too far superiorly into the truncal valve annulus. The absence of infundibular septum in this area results in the possibility of damage to the truncal valve if the incision is started too far superiorly. The orientation of the incision is made so that the takeoff of the conduit from the right ventricle will be directed toward the pulmonary bifurcation.

As shown in Figure 87.2, the ventricular septal defect is exposed through the incision in the right ventricle. The defect is closed with a patch of polyester (Dacron) material using a running technique. Superiorly, the absence of the infundibular septum mandates that the patch be more ovoid in shape, and the patch is secured to the epicardial margin of the ventriculotomy incision superiorly to avoid interference with the truncal valve (Fig. 87.3). In the majority of cases, there is a bridge of muscle between the margin of the ventricular septal defect and the tricuspid valve septal leaflet, and in these patients, suturing in this area can be performed without risk to the conduction
tissue. In a minority of cases, there is absence of this muscle tissue, and suturing of the ventricular septal patch to the base of the septal leaflet of the tricuspid valve is necessary.

After closure of the ventricular septal defect is complete, the right ventricular outflow tract is reconstructed using a pulmonary or aortic homograft. Although in the past we elected to use larger pulmonary homografts to allow for the maximum possible growth of the infant before conduit change is necessary, more recently we have used aortic allografts preferentially, because the high pulmonary vascular resistance seen in the early postoperative period often causes dilation in the pulmonary allografts. Another advantage for the initial use of an aortic homograft is the curved nature of the ascending aorta that can be positioned to the left, curving around the enlarged truncal root in patients in whom the truncus is particularly enlarged.

The use of an aortic homograft in the initial repair of truncus arteriosus prevents pulmonary homograft dilation early. At the time of the conduit replacement after truncus repair which typically occurs between 3 and 6 years after the operation, a pulmonary homograft is preferentially selected since pulmonary vascular resistance is now low and placement of a large pulmonary homograft may obviate the need for additional conduit replacements in the future, and the development of pulmonary insufficiency in the homograft conduit can be treated later with a stent mounted on pulmonary valve with a percutaneous approach.