ANATOMY

The anatomic malformation that is common to all forms of AV canal defects is a deficiency in the AV septum caused by incomplete embryonic development of superior and inferior endocardial cushion tissue. The common AV canal is normally present during the early tubular stage of fetal life and constitutes the sole connection between the primitive common atrium and the primitive common ventricle. After cardiac looping, the valves of the heart develop in the embryo from precursor structures, called endocardial cushions. These endocardial swellings become populated by valve precursor cells formed by a transformation from endothelial to mesenchymal tissue, and they undergo directed growth and remodeling to form the valvular structures and the membranous septa of the mature heart. (Fig. 116-1). Abnormal differentiation and remodeling of the cushion mesenchyme into valvuloseptal tissue is thought to be a mechanism for the development of AV canal defects.⁴ The resultant anatomic defect involves the abnormal development of AV valves and the persistence of interatrial and interventricular communications.⁵

Figure 116–1 Schematic of complete atrioventricular (AV) canal defect during morphogenesis, with the AV canal region shown in dark gray. Projections from the superior and inferior endocardial cushions normally differentiate into the AV valves and AV septum. AS, atrial septum; IC, inferior endocardial cushion; LA, left atrium; LV, left ventricle; SC, superior endocardial cushion; Sept 1, septum primum; VS, ventricular septum.

(Modified from Van Praagh R, Litovsk S. Prog Pediatr Cardiol 1999;10:115-27.)

The wide variability in the degree of development of the endocardial cushions explains the variability in size and extent of the septal defects and the degree of involvement of the AV valves. Nevertheless, several anatomic features are shared between all types of AV canal defects. These include the following:

In addition to these fundamental concepts, anatomic features of significance to the surgeon include the following:

The AV valve in a canal defect has an area of apposition of leaflets that, unlike a normal commissure, is not supported by a papillary muscle. This area of apposition is called a cleft, and it is the apposition of the superior and inferior bridging leaflets. This cleft is not a commissure,⁶ as was once thought,^7–9 for two reasons. First, a commissure is generally supported by chordae on either side of the defect, whereas a cleft is unsupported, with paucity of chordae at the edges. Second, the chordae that arise from the two adjacent leaflets in a commissure usually attach to a single papillary muscle, which promotes coaptation and prevents regurgitation. The chordae that arise from the left superior and inferior leaflets attach to two different papillary muscles. This increases the distracting force during ventricular systole and predisposes to regurgitation through the cleft. The extent of regurgitation is generally mild to moderate, although severe regurgitation can be present.

In 1966, Rastelli and colleagues¹⁰ described a classification of complete AV canal defect based on the extent of bridging of the left superior leaflet (LSL) across the interventricular septum (Fig. 116-2). In complete AV canal defects, the left inferior leaflet (LIL) is not well developed; it is usually short and immobile, with rolled edges. The Rastelli classification does not take into consideration the LIL, which displays greater anatomic variation. No clear morphologic relationship exists between the LSL and the LIL.

Figure 116–2 The Rastelli classification for complete atrioventricular canal defects. A, In the Rastelli type A defect, the superior bridging leaflet is divided into two leaflets at the crest of the interventricular septum, corresponding to the right superior leaflet (RSL) and the left superior leaflet (LSL). B, In the Rastelli type B defect, the LSL bridges across the septum and attaches to a papillary muscle in the right ventricle. C, In the Rastelli type C defect, there is marked bridging of the superior bridging leaflet (SL), making it free floating and unattached to the underlying interventricular septum. LIL, left inferior leaflet; LLL, left lateral leaflet; RIL, right inferior leaflet; RLL, right lateral leaflet.

(Modified from Jacobs JP, Burke RP, Quintessenza JA, Mavroudis C. Ann Thorac Surg 2000;69;S36-43.)

The most common Rastelli type, present in approximately 55% of patients, is the Rastelli A defect. There is no bridging of the LSL; instead, its chordae attach to the crest of the interventricular septum. The superior bridging leaflet can be divided into left and right components over the crest of the interventricular septum. The inferior bridging leaflet is rarely divided and is often attached by short dense chordae to the crest of the interventricular septum.

In the rare Rastelli B defect, the superior bridging leaflet is often partially divided into right and left components, with associated mild to moderate bridging of the LSL. Its chordae attach either to the right of the crest of the interventricular septum or to a prominent papillary muscle in the right ventricle, depending on the extent of bridging. The moderator band is often short. The Rastelli B subtype is often associated with unbalanced AV canal defect.

In the Rastelli C defect, there is extensive bridging of the LSL. Its chordae are free floating and are not attached to the underlying crest of the interventricular septum but rather to the anterolateral papillary muscle of the right ventricle. The Rastelli C type of complete AV canal defect can be seen in association with other complex heart lesions such as tetralogy of Fallot (TOF), transposition of the great arteries (TGA), and double-outlet right ventricle (DORV).

The development of the Rastelli classification facilitated the widespread application of corrective surgery for this defect. Although it is an oversimplification and represents categorization of a continuum of leaflet abnormalities, it may still have a limited role in the repair of complete AV canal defects using the classic single-patch technique. In that setting, recognition of whether the bridging leaflets are undivided over the crest of the interventricular septum is important because undivided leaflets need to be surgically incised into right and left components to allow placement of a patch to close the interventricular and interatrial communications. Even in those settings, however, the Rastelli classification may have limited applications because neither the extent nor the location of naturally occurring divisions in the bridging leaflets is taken into account. The Rastelli classification is of less importance in the double-patch or modified single-patch techniques that are described later in the chapter.

Lev’s¹¹ description of the location of the conduction tissue was another important contribution in the field, because it facilitated the development of surgical techniques that avoided injury to the His bundle and the AV node. The coronary sinus ostium and the AV node are displaced inferiorly in AV canal defects. The AV node lies at the junction of the interatrial septum and the hinge point of the AV valve. This places the AV node between the coronary sinus ostium and the crest of the interventricular septum, in the so-called nodal triangle, which is not at the tip of Koch’s triangle. The nodal triangle is bound by the inferior extent of the right AV valve anulus, the coronary sinus orifice, and the inferior edge of the interatrial septum (Fig. 116-3). The bundle of His then passes superiorly and anteriorly from the AV node to the crest of the interventricular septum, reaching it where the crest is fused posteriorly with the AV valve anulus. The bundle then travels along the crest of the interventricular septum, in an inferior-to-superior direction, giving rise to the left bundle branches. Before reaching the midpoint of the interventricular crest, it becomes the right bundle branch and heads toward the moderator band and the muscle of Lancisi.¹² The superior aspect of the interventricular septum is devoid of conduction fibers. The only exception is that, in patients with complete AV canal defects and heterotaxy syndrome, two AV nodes may be found.¹³

Figure 116–3 The location of the atrioventricular (AV) node and the conduction tissue. A, Normal heart. Note the location of the AV node at the tip of the triangle of Koch. B, AV canal heart. The AV node is now located in the nodal triangle, not at the tip of the triangle of Koch. The coronary sinus, AV node, and bundle of His are displaced inferiorly compared with the normal heart. RA, right atrium; RBB, right bundle branch; RV, right ventricle.

(Modified from Kertesz NJ. Prog Pediatr Cardiol 1999;10:153-9.)

The relationship between the common AV valve and the ventricles varies. If the AV valve is situated primarily over one ventricle, associated hypoplasia of the contralateral ventricle frequently occurs, which in extreme cases precludes a two-ventricle repair. Abnormal papillary muscle development, such as two closely spaced papillary muscles or a single papillary muscle, is seen in right ventricle–dominant forms of complete AV canal defect. In left-dominant forms, often the papillary muscles are closely spaced and fused to the septum, giving the appearance of chordal attachments to the septum. In fact, the papillary attachments to the septum can usually be divided to achieve a two-ventricle repair in many of these patients.

Approximately 50% to 75% of patients with complete AV canal have Down syndrome, whereas Down syndrome is rare in patients with partial AV canal defects,¹ occurring in less than 10% of these patients. AV canal defects make up approximately 3% of all major congenital cardiac defects and are seen in approximately 25% to 30% of patients with Down syndrome. This represents a 1000-fold higher rate than in the general population.² AV canal defects are the most common congenital heart anomaly in patients with Down syndrome. The presence of Down syndrome is thought to accelerate the development of pulmonary vascular obstructive disease (PVOD), especially in the setting of complete AV canal. An additional 15% to 20% of fetuses with AV canal syndrome have heterotaxy syndrome: AV canal defects are present in almost all patients with asplenia and in a high number of patients with polysplenia. Although an underlying genetic mechanism has not been identified, some intriguing evidence exists in that regard. First, there is a strong association between AV canal defects and Down syndrome. High levels of endostatin, which is known to regulate endocardial cushion development, have been found in patients with Down syndrome. Second, a weaker association exists with heterotaxy syndrome. Third, familial clustering of AV canal defects has been reported.¹⁴ Fourth, the incidence of recurrent congenital heart disease in offspring of patients with AV canal defects is approximately 10%.¹⁵ In another study, approximately 14% of offspring of mothers with isolated AV canal defects had congenital heart disease, usually either TOF or AV canal defects.¹⁶ This contrasts with the 2% to 4% probability of congenital heart disease in children of parents with other congenital cardiac lesions.

PATHOPHYSIOLOGY

A left-to-right shunt is present in patients with AV canal defects, unless PVOD or coexistent right ventricular (RV) outflow tract obstruction is present. Patient presentation depends on the extent of left-to-right shunting, which in turn depends on pulmonary vascular resistance. The amount of left-to-right shunting increases in the first few weeks of life, paralleling the fall in pulmonary vascular resistance. At this stage, symptoms of congestive heart failure with poor weight gain, tachypnea, and sweating begin to appear.

In partial AV canal defects, where an interventricular communication is absent, the shunt is located only at the atrial level. Patients with partial AV canal defects with little or no regurgitation through the AV valves have a clinical presentation similar to that of patients with secundum ASDs. The degree of shunting depends on the size of the ostium primum ASD defect and the relative diastolic compliance of the two ventricles. RV stroke volume is increased, whereas RV systolic pressure may be normal or only slightly increased. Patients may remain asymptomatic for years. An exception is the presence of significant left AV valve regurgitation in the setting of an ostium primum ASD, which is the case in 10% to 15% of patients. The regurgitation increases left-to-right shunting considerably; a left-ventricle-to-right-atrium shunt may also be present. This results in increased RV and LV stroke volume, cardiomegaly, tachypnea, tachycardia, poor feeding, and failure to thrive, leading to progressive heart failure in infancy. Without treatment, patients with partial AV canal defects and large interatrial communications rarely survive beyond 40 years of age, although older patients have been reported.¹⁷ Atrial arrhythmias are common, increase in frequency with advancing age, and are a poor prognostic sign.

Patients with complete AV canal defects exhibit left-to-right shunting because the VSD is large and nonrestrictive. The extent of the shunt at the ventricular level depends primarily on pulmonary vascular resistance. The pathophysiology in complete AV canal defects is similar to that of a large nonrestrictive VSD with two additional features. First, an atrial level shunt is present, with the potential for a left-ventricle-to-right-atrium shunt and increased volume loading of the ventricle. Second, AV valve regurgitation may be present, further increasing the volume load. These factors accelerate the development of heart failure symptoms in early infancy.

In many patients, particularly those with Down syndrome, pulmonary vascular resistance remains elevated after birth, which decreases the magnitude of left-to-right shunting. In other infants, pulmonary vascular resistance drops in the first few weeks of life, resulting in a large left-to-right shunt. Patients typically display tachypnea, failure to thrive, cardiomegaly, and diminished distal perfusion in the first few months of life. If AV valve regurgitation is coexistent, the extent of left-to-right shunting is increased because of shunting at both atrial and ventricular levels, resulting in biventricular volume overload. This accelerates the development of heart failure. In one study, moderate AV valve regurgitation was present in approximately 20% of complete AV canal defects, whereas severe regurgitation was present in 15% of such lesions.¹⁸ In our experience,¹⁹ mild AV valve regurgitation is common, occurring in approximately 35% of patients in the first year of life, although the incidence of severe AV valve regurgitation is less frequent, occurring in approximately 4% of patients.

Irreversible changes in the pulmonary vasculature are seen as early as 6 months in patients with unrepaired complete AV canal defects.²⁰ The presence of Down syndrome accelerates the development of PVOD.²¹ This may result partly from an underlying pulmonary vascular defect predisposing to PVOD and partly from increased bronchial secretions, decreased number of distal bronchi, inherent abnormalities of the lung parenchyma, and the thick tongue that predisposes to oropharyngeal airway obstruction, hypoventilation, sleep apnea, tracheobronchomalacia, and CO₂ retention. These factors accelerate the development of respiratory infections and PVOD. If PVOD progresses, eventually the shunt reverses to a right-to-left shunt and the Eisenmenger complex develops, with progressive cyanosis in later stages.

Without surgery, patients with complete AV canal defects and Down syndrome have an 80% survival rate at 10 and 15 years.²² The primary causes of death within the first several years of life are heart failure and recurrent pulmonary infections. Subsequently, PVOD becomes an increasingly prevalent cause of death. Toward the end of the second decade of life, exercise tolerance begins to decrease progressively, leading to premature death from PVOD by the third to fourth decades of life. One of the more dramatic causes of death in patients with PVOD is massive hemoptysis, which can occur as early as the third decade of life.

TIMING OF SURGERY

The development of surgical techniques to treat AV canal defects parallels the evolution of cardiac surgery for congenital hearts defects. Initial attempts at repair were complicated by limited knowledge of the anatomic features, particularly with respect to the conduction tissue.

The first cardiac operation using a pump oxygenator was attempted by Dennis and Varco in 1952 in a patient with a preoperative diagnosis of an ASD. The patient did not survive surgery. Postmortem examination revealed a partial AV canal defect. In 1955, Lillehei and colleagues²³ were the first to report repair of complete AV canal using cross-circulation. Successful repair of an ostium primum defect using cardiopulmonary bypass was first reported by Kirklin and colleagues in 1955.²⁴

PREOPERATIVE EVALUATION

Because of the nearly continuous spectrum of lesions involving the atrial, ventricular, and valvular portions of the AV canal defects, preoperative definition of the anatomic features in each case is helpful. Information regarding the mechanism of valve regurgitation and LVOTO are especially valuable in planning operative intervention. Intraoperative confirmation of the anatomic features is imperative to guide decisions regarding patch size, position of valve leaflets, and extent of cleft closure.

Physical examination correlates with the type of AV canal defect. In patients with partial AV canal defects, a systolic pulmonary flow murmur is heard, along with fixed wide splitting of the second heart sound, as in a secundum ASD. An apical blowing systolic murmur of MR may be present. If a VSD is present, as in transitional or complete AV canal defects, then a harsh holosystolic murmur is present, best heard at the left lower sternal border or the ventricular apex. A hyperactive precordium may be present. P2 is often loud, with fixed splitting of the second heart sound. Often a mid-diastolic low-pitched rumble is present. A pulmonary flow murmur may also be present.

The electrocardiogram reveals marked RV hypertrophy and sometimes may reveal LV hypertrophy and biatrial enlargement. The PR interval is often prolonged. The chest radiograph shows evidence of increased pulmonary blood flow, along with cardiomegaly. The presence of left AV valve regurgitation accentuates these findings; the left atrium may be severely dilated, with resultant elevation of the left main-stem bronchus.

In nearly all cases, the anatomic features, associated defects, and mechanism of valve dysfunction can be obtained from two-dimensional (2D) echocardiography, along with Doppler evaluation of blood flow direction and velocity.³⁵ The four-chamber view shows the left and right AV valves at the same horizontal level, which is in contradistinction to the normal situation, in which the tricuspid valve is attached to the AV septum more toward the apex of the heart than the mitral valve. Other features on echocardiography include the elongated LV outflow tract and the unwedged aortic valve. It is important to assess the presence of chordal attachments to the LV outflow tract, because this is a substrate for the development of LVOTO. The AV valve leaflets should be examined, and the mechanism of AV valve regurgitation should be assessed carefully. The availability of real-time three-dimensional echocardiography (RT-3DE) has been very helpful in the assessment of the defect and to determine the mechanism of regurgitation. We use either epicardial or transesophageal RT-3DE in the assessment and management of AV valve repair, especially in reoperations for AV valve regurgitation. Studies have shown that RT-3DE can be successfully used in the repair of AV canal defects with a very short learning curve.^35b

For the majority of patients, cardiac catheterization and angiography rarely add information not obtained by echocardiography. One notable exception is the unoperated patient older than 1 year in whom measurement of pulmonary vascular resistance is needed to determine operative risk. Another exception is the postoperative patient in whom quantification of residual postoperative lesions such as residual VSDs and LVOTO are required to assess the need for reoperation.

SURGICAL TECHNIQUE

Median sternotomy, either partial or complete, has been the standard technique for exposing the heart and great vessels. Some have advocated a right anterior thoracotomy for partial AV canal defects in older female children to improve cosmesis. More recently, however, techniques to minimize surgical trauma by limiting the extent of the sternotomy have been used, such as a lower mini-sternotomy (Fig. 116-4).³⁶ This approach can be applied to infants and children of all ages and may be helpful in decreasing postoperative pain and development of chest wall deformities such as pectus carinatum. We are now using this mini-sternotomy approach for nearly all AV canal defects, except when there are major associated cardiac lesions such as aortic coarctation, TOF, or TGA, where a full sternotomy is required for optimal exposure.

Figure 116–4 Mini-sternotomy setup for repair of atrioventricular (AV) canal defects. A, Representative parasagittal cross-section with bicaval cannulation and aortic cross-clamping. B, Surgeon’s view of the operative field. The inside of the atrium is well visualized for AV canal repair. A, Bookwalter retractor arm connected to an army-navy retractor; B, pediatric sternal retractor; C, aortic cannula; D, superior vena cava cannula via the right atrium, although direct cannulation is preferred for complete AV canal repair; E, inferior vena cava cannula; F, cardioplegia cannula; G, aortic crossclamp.

(Modified from del Nido PJ, Bichell DP. Semin Thorac Cardiovasc Surg Pediatr Card Surg Annu 1998;1:75-80.)