Interrupted Aortic Arch

Interrupted aortic arch (IAA) is the archetype of complex congenital cardiac anomalies that, before the advent of prostaglandin El (PGE1) carried an extremely high mortality rate during the neonatal period. Not surprisingly, various simple palliative surgical procedures have emerged in an attempt to minimize this high mortality. Through the 1980s, as knowledge increased regarding the preoperative resuscitation of these children, there was an impressive decline in the mortality rate. By the end of the decade, one-stage repair, including direct anastomosis of the arch, emerged as the procedure of choice in most specialized centers. Interrupted aortic arch is characterized by a lack of luminal continuity between the ascending aorta and the descending thoracic aorta. A large patent arterial duct feeds the descending thoracic aorta in almost all cases.
Characteristically, there is significant aortic hypoplasia, so that ascending aorta is about half the normal diameter. The pulmonary arteries are large. Cardiac anomalies are frequent and a large ventricular septal defect is nearly always present, the majority of which are subpulmonic. With posterior malalignment of the ventricular septum, subaortic stenosis is also present. Frequently associated lesions include truncus arteriosus and aortopulmonary window.

PGE1 has revolutionized the management of interrupted aortic arch. Complete resuscitation should be maintained over several days, if necessary, before operative intervention is undertaken. Operative repair is nearly always indicated. Direct anastomosis of the arch with closure of the ventricular septal defect is generally the preferred surgical approach, and although this procedure is physiologically corrective, it should not be viewed as fully corrective due to the high incidence of significant late obstruction of the left ventricular outflow tract. Such obstruction may respond to a simple surgical intervention such as subaortic resection, but in some cases may require an extensive procedure to enlarge the left ventricular outflow tract. Procedures to enlarge or bypass the subaortic arch at the time of the initial surgical procedure are rarely, if ever, indicated.

Embryology & Morphology

The embryology and morphology of interrupted aortic arch is closely related to that of coarctation of the aorta, and is described more completely in the preceding section. The maldevelopment of the aortic arch and isthmus is probably related to inadequate flow through the proximal aorta in utero. The large ventricular septal defect, which is almost invariably present, shunts the great majority of blood ejected from both ventricles into the pulmonary artery, through the arterial duct, and into the distal aorta. This duct-dependent left-to-right-to-left again in utero shunting, especially if concomitant subaortic stenosis is present, results in decreased blood flow through the aortic isthmus and subsequent maldevelopment of the aortic arch.

Pathophysiology

For the patient with the most common form of interrupted aortic arch (i.e., with an associated patent arterial duct and ventricular septal defect), there may be little suspicion of serious heart disease during the early neonatal period until ductal closure begins. If this occurs abruptly or is not recognized rapidly, the child will soon become profoundly acidotic and anuric as perfusion of the lower body becomes entirely dependent on collateral communication between the two separate aortic systems. The distribution of palpable pulses depends on the anatomic subtype. For example, with type B interrupted aortic arch, the pulse in the right arm remains palpable when the left arm and femoral pulses become impalpable secondary to ductal closure. Ischemic injury to the liver is reflected in a marked elevation of hepatic enzymes. and ischemic injury to the gut may be followed by evidence of necrotizing enterocolitis. Renal injury can be quantitated to some extent by the elevation observed in serum creatinine levels. Ultimately, a severe degree of systemic acidosis (prolonged pH less than 7.0) results in injury to all tissues of the body, including the brain and the heart itself. The child may have seizures and become flaccid and poorly responsive. Myocardial injury becomes apparent from decreased contractility and the low cardiac output state, despite normalization of other parameters. Because pulmonary blood flow is preserved during ductal closure, it is rare to see evidence of pulmonary insufficiency, although the child will hyperventilate in an attempt to compensate for the metabolic acidosis. Occasionally, the arterial duct does not close during the neonatal period, and the diagnosis may be delayed for several weeks. As pulmonary resistance falls, there will be an increasing left-to-right shunt, and the child will show evidence of congestive heart failure, including failure to thrive.

Preoperative Evaluation

Currently, an accurate anatomic diagnosis can be made using echocardiography alone. This is an important advantage to the critically ill neonate, because the additional insult of an invasive cardiac catheterization can be avoided. In addition to localizing the site of the interruption, the echocardiographer should provide the following information:

1. The distance between the discontinuous aortic segments.
2. The narrowest dimension of the left ventricular outflow tract (generally related to posterior displacement of the infundibular septum), the diameter of the aortic annulus, and the diameter of the ascending aorta. It is unusual for the segments of the arch that are present to be so hypoplastic that they cause hemodynamic compromise.
3. The features of associated anomalies must be carefully defined. For example, the location of an associated ventricular septal defect should be defined in relation to its margins. The infundibular septum is often severely hypoplastic, rendering approach to the superior margin of the defect through the tricuspid valve difficult. Additional ventricular septal defects are rare.

Hemodynamic assessment

Because the diagnosis is generally made when ductal patency has been re-established using PGE1, pressure data are of little use in formulating a plan for surgical management. The issue that most commonly arises concerns the adequacy of the left ventricular outflow tract. Attempts to quantitate the degree of obstruction by measuring a pressure gradient are hampered by a lack of information regarding the amount of flow passing through this area. The ventricular septal defect is almost always nonrestrictive. There is no evidence that multiple ventricular septal defects are more accurately identified by angiography than by color flow Doppler echocardiography.

Medical and interventional management

The introduction of PGE1 in 1976 revolutionized the management of interrupted aortic arch. Before this time, which also predated the introduction of two-dimensional echocardiography, it was necessary to manage acidotic neonates symptomatically as they underwent emergency cardiac catheterization and were then rushed from the catheterization laboratory to the operating room. PGE1 must be infused through a secure intravenous line. If ductal patency does not become apparent in any neonate less than 1 week old within 1 hour after the administration of PGE1, it should be assumed, until proven otherwise, that there is a dosage error or a technical problem with delivery of the medication into the central blood stream. Establishing ductal patency represents just the first step in medically resuscitating the neonate with interrupted aortic arch. Because the lower half of the body is dependent on perfusion through the arterial duct and because blood in the arterial duct has the choice of passing into the pulmonary circulation or the systemic circulation, it is important that pulmonary resistance be maximized. This can be achieved by avoiding a high level of inspired oxygen and by avoiding respiratory alkalosis through hyperventilation. In fact, control of ventilation is best accomplished by intubating the neonate, sedating him or her with a fentanyl infusion, and inducing paralysis with pancuronium. A peak inspiratory pressure and ventilatory rate that will achieve a Pco2 level of 40 to 60 mm Hg should be selected. Metabolic acidosis must be aggressively treated with sodium bicarbonate. Because myocardial function is likely to be depressed when the child is first seen and because it may be necessary for the heart to handle a moderate volume load, an inotropic agent such as dopamine is almost routinely employed. Dopamine has the added advantage of maximizing renal perfusion in this context of an ischemic renal insult. It is not uncommon to persist with medical resuscitation for 2 to 3 days before operative intervention is undertaken. It is very unusual for a child to be taken to the operating room with any acid-base, renal, or hepatic abnormalities.

Surgical Management

Successful surgical repair was first accomplished by Samson in 1955 in a patient with a short-segment type A interrupted aortic arch. A direct anastomosis was possible. However, the associated ventricular septal defects were not closed at the time of the arch repair. One-stage repair was first reported by Barrett-Boyes and associates. In this procedure, arch continuity was established using a synthetic conduit. One-stage repair, including direct anastomosis of the arch, was first described by 1975. Interrupted aortic arch carried an extremely high mortality risk until the introduction of PGE1 by Elliott. Over the ensuing 5 to 10 years, it became apparent that careful resuscitation of the neonate before proceeding to surgery resulted in a significant improvement in surgical outcome. Because the presence of interrupted aortic arch is incompatible with life unless ductal patency is maintained, the diagnosis alone is the indication for surgery. Surgery should be undertaken as soon as complete metabolic resuscitation has been achieved.

See volume 1, Procedures.

Postoperative Management

After biventricular repair of simple interrupted aortic arch with ventricular septal defect, separation from bypass and early postoperative management should be routine. Failure to progress appropriately may be due to residual hemodynamic lesions, which need to be ruled out. A residual ventricular septal defect can be excluded by oxygen saturation data collected with the pulmonary artery line on the first postoperative morning, while an anastomotic gradient can be excluded both intraoperatively and early postoperatively by blood pressure determinations. Echocardiography, cardiac catheterization, and magnetic resonance imaging are useful diagnostic tools to assess the possible presence of residual obstruction to left ventricular outflow. A left-to-right shunt at the atrial level should also be excluded. If an important residual hemodynamic lesion is identified, the child should be expeditiously returned to the operating room for correction of the problem.

Results

Complete preoperative resuscitation is an important factor in decreasing postoperative morbidity and mortality following repair of interrupted aortic arch with complex associated anomalies such as functional single ventricle and truncus arteriosus, as well as for patients with ventricular septal defect as the only associated anomaly.

Obstruction of the Left Ventricular Outflow Tract. Although obstruction of the left ventricular outflow tract is rarely sufficiently severe to justify an alteration in surgical strategy at the time of the neonatal reparative procedure, it is, not uncommon for surgical intervention to be required for left ventricular outflow tract obstruction occurring late postoperatively. Of 33 patients who underwent repair of a ventricular septal defect as the only associated anomaly with interrupted aortic arch, only 58% were free of evident obstruction of the left ventricular outflow tract (defined as a gradient greater than 40 mm Hg) 3 years after operation. The morphology of left ventricular outflow tract obstruction with interrupted aortic arch varies, and therefore, surgical management also varies according to the specific circumstances. In some cases it is possible to resect the posteriorly deviated infundibular septum, working through the aortic valve. An aortic valvotomy may also be required if there is valvar stenosis. If there is annular hypoplasia and a longer tunnel subaortic stenosis, our approach is to perform an annular enlarging procedure such as the extended aortic root replacement, using an aortic allograft with either an anterior incision into the ventricular septum (for very severe stenosis) or a posterior incision into the anterior leaflet of the mitral valve (when stenosis is less severe). Alternative procedures that have been used in the past include the Konno procedure and the use of a conduit from the left ventricular apex to the descending aorta.

Di George’s Syndrome. Absence or severe hypoplasia of the thymus is not uncommon in patients with interrupted aortic arch, especially those with type B interruption. Although a large calcium requirement is often seen during the early postoperative period, it is rarely necessary for children to receive oral calcium supplements when they leave the hospital. Occasionally, vitamin D supplements are useful to maintain serum calcium levels during the first few postoperative weeks. The long-term immune status of survivors of interrupted aortic arch surgery requires further assessment and follow up.

Left Bronchial Obstruction. The left main bronchus usually passes under the arch of the aorta. If a direct anastomosis is performed without adequate mobilization, a bowstring effect over the left main bronchus may result. This is manifested by air trapping in the left lung with hyperexpansion, as seen on the plain chest radiograph. The diagnosis can be confirmed by bronchoscopy along with a computed tomographic or magnetic resonance imaging scan. Surgical management may entail placement of an ascending-to-descending aortic conduit after division of the arch. With adequate initial mobilization of the ascending and descending aorta, this should almost never be necessary.