Chapter 18. Congenital Heart Disease: Atrioventricular Septal Defect

Major advances in the field of pediatric cardiology and cardiac surgery over the last several decades have led to an improvement in survival rates of the patients with congenital heart disease. During this period, improvements in surgical and medical treatments have been accompanied by developments in a spectrum of diagnostic modalities. Integration of different modalities in clinical and research environments is being used for better understanding and better managing of complex cardiac conditions.

Ideally, few noninvasive imaging modalities should be able to delineate all aspects of the anatomy and evaluate physiologic consequences of the lesion(s) in patients with congenital heart disease. Moreover, the imaging modalities used should be cost effective and portable, not cause excessive discomfort and morbidity, and not expose patients to harmful effects of ionizing radiation. To limit the number of imaging modalities used in diagnosis and therapy, research is being performed to evaluate and validate techniques that could potentially serve as noninvasive or minimally invasive imaging modalities that would replace those which are invasive and less available.

The aim of this chapter is to present a model of how noninvasive multimodal imaging can improve the understanding of pathophysiology and increase the accuracy of the diagnosis of the specific congenital heart disease. The focus is on the heart with atrioventricular septal defect (AVSD).

Appropriate diagnosis and detailed assessment of the anatomic and functional features of AVSD presents challenges. Particularly, accurate preoperative assessment of the atrioventricular valve component is crucial since long-term outcomes for surgical repairs of AVSDs depend on the successful repair of the left atrioventricular valve. Moreover, surgical destruction of the conduction system may occur in cases of inappropriate diagnosis because AVSDs have different conduction pathways compared with defects that have separate atrioventricular valves.1

Understanding the electroanatomic and functional relationships in hearts with AVSDs is suggested to permit differentiation of atrioventricular defects from other similar abnormalities.

AVSD is anatomically characterized by abnormal development of the atrioventricular valve junction.2-6 In addition, the overall atrioventricular valve complex including valve leaflets and papillary muscles is abnormally developed. Figure 18–1 schematically depicts main differences in the normal (Fig. 18–1A) and primum AVSD heart (Fig. 18–1B) anatomy. Atrioventricular valve leaflets are different from normal mitral and tricuspidal leaflets because atrioventricular valve leaflets guard a common atrioventricular junction and are displaced. In addition, valve leaflets may be asymmetric, the mural leaflet may be abnormally small. Displacement and sometimes even dysfunction of the papillary muscles is present. Those changes are often associated with pre- and postoperative regurgitation of the valve.7-9 Left ventricular papillary muscles are positioned further from the septum compared with normal, with the posterior papillary muscle more so than the anterior papillary muscle.1 Thus, specifically, the posterior papillary muscle is a marker of distinction between AVSDs and other similar defects such as defects with mitral clefts.1

AVSD is further characterized by inflow tract shortening and by outflow tract lengthening. Left ventricular outflow tract is transformed into a longer channel, and therefore, the ratio between the inflow and outflow is smaller than that in healthy controls.10,11

Echocardiography

Two-dimensional (2D) echocardiography (ECHO) is the current standard noninvasive imaging tool used for the morphologic and functional assessment of the AVSD and provides global preoperative data in the majority of cases.12 Figure 18–2 and the related Moving Image show the 2D image of the primum AVSD. The morphology of the defect and atrioventricular valve is presented.

With advances in imaging technology, three-dimensional (3D) ECHO has been shown to primarily facilitate the accurate anatomic and functional assessment of complex spatial anatomic features and relationships of the anatomic and functional abnormalities involved in AVSD.12 3D ECHO provides more detailed information about anatomic and functional assessment of the atrioventricular valve, the relationships between the leaflets of the valves, with each other and with other heart structures.13 Figure 18–3 and the related Moving Image show an example of the assessment of the anatomy of the atrioventricular valve by using 3D ECHO imaging. Relationships between the leaflets and dynamics of the valve leaflet motion can be seen on the cine image. This anatomic information is of value for the clinician in planning surgical treatment because it provides a more detailed anatomic definition of interrelations between structures.14

The atrioventricular valve apparatus consists of the annulus, leaflets, chordae, and papillary muscles. Because the atrioventricular valve apparatus is complex, 2D ECHO is often not sufficient to provide full information of the atrioventricular valve relationships. Real-time 3D imaging permits display of the entire circumference of the atrioventricular annulus. This is important information in surgical repair of the valves (eg, annuloplasty). Whereas 2D imaging shows the edges of the leaflets in multiple different views, real-time 3D imaging shows the entire surface of the leaflet in a single display. Leaflets can be inspected from either the atrial side or the ventricular side, depicting, for example, buckling, cleft, or deficiencies. The advantage is that all of the information can be obtained from a single data set without the need to scan and sweep from different windows, as is required in 2D imaging.15 3D ECHO provides new insight into the dynamic morphology of the left-sided atrioventricular valve and left ventricular outflow tract anatomy.16,17 In the clinical scenario, it clarifies the pathology, particularly in complex lesions where the incremental information has an impact on therapeutic decision making.

Imaging of the Atrioventricular Valve Function

Morbidity of the left atrioventricular valve remains an important concern.18-20 Reoperation of valve failure is complex and involves annular reduction, commissurotomy, closure of the residual defect in the apposition of the bridging leaflets, and patch enlargement of the atrioventricular valve. Although the surgeon has the opportunity to inspect the atrioventricular valve and test its competency with saline, this latter technique is nonphysiologic and might provide an incomplete evaluation of the true nature of the valve failure. 2D ECHO with color Doppler scanning is the current standard for both preoperative and postoperative assessment of patients with AVSD. Figure 18–4 and the related Moving Image show the color Doppler ECHO used for assessment of the function of the atrioventricular valve. The limitation of the 2D perspective is its difficulty to provide details regarding the status of the commissures, the precise location of the regurgitant jets, sites of poor coaptation, and the presence of clefts. Color Doppler scanning is a helpful adjunct, but the true extent and location of the regurgitant jets might be difficult to appreciate in a 2D image, particularly in cases with multiple jets caused by the phenomena of jet entrainment.

3D color-flow ECHO is a complementary technique with potential to assess the more precise mechanism of the valve dynamics and valve insufficiency. Figure 18–5 and the related Moving Image show the 3D flow through the atrioventricular valve. 3D color Doppler images can identify regurgitation jets in three dimensions and in a more precise relationship to the leaflets of the atrioventricular valve. This facilitates the identification of anatomic details and mechanism of the valve failure. Redundancy of the valve leaflets and/or defective coaptation can be demonstrated during real-time 3D imaging of the motion of the leaflets. The contribution of poor coaptation and/or dysplastic leaflets to the degree of valve regurgitation can be documented with the 3D geometry of the regurgitant jet.

Electrophysiology of the AVSD

Left axis deviation is considered a "hallmark" of AVSD.21 Superior orientation of the QRS loop in the frontal and sagittal planes was even suggested as a sign that differentiates AVSD from other cardiac abnormalities.22 By gaining insight into the developmental relationships between the conduction system and heart structures, improvement in diagnosis can be achieved.

Electrophysiologic abnormalities were studied by using several different techniques including vectorcardiogram, body surface potential maps, and epicardial mapping, with the aim of describing the abnormality of electrical activation in detail and to use the activation patterns to differentiate AVSD from other similar lesions.21,22

Vectorcardiogram

As shown in Fig. 18–6, the major portion of the QRS loop in the frontal plane is superiorly oriented in AVSD patients compared with normals, in whom the QRS loop is rather inferiorly and horizontally placed.21 This displacement, also known as counterclockwise rotation, of the QRS loop in the frontal plane is a very common vectorcardiographic finding in patients with AVSD.21,22

Body Surface Maps

Spach et al23 used body surface maps to study electrophysiologic abnormalities in AVSD. The distribution of cardiac potentials on the body surface in patients with AVSD differed in a characteristic pattern from the normal group. Figure 18–6 shows an example of data from a child with AVSD (Fig. 18–7B) and a child with a normal heart (Fig. 18–7A).

Using Fig. 18–7 as reference, the initial phase of ventricular depolarization in the AVSD patient (a☆) started more inferiorly and more leftward, compared with the normal child (a). The depolarization wave then migrated more superiorly (b☆) and after that shifted to the right (c☆). During the second half of the heart cycle, depolarization was shifted superiorly and to the left (d☆, e☆) and finished by depolarization of the right ventricle (f☆). Depolarization followed then an inferosuperior direction in the AVSD patient (2f☆, 2g☆) and finished by depolarization of the right ventricle (2h☆, 2j☆). In summary, the main difference compared with the normal group was the late depolarization of the superior part of the left ventricle.

An interesting finding in f☆ is the emergence of nondipolar content on the surface of the child with AVSD. This means that within the heart, there was a presence of more than one overall wave front, which caused the simultaneous presence of more than one maximum or minimum. This finding is characteristic for patients with AVSD. It was suggested that the transient absence of a null zone suggests the marked predominance of active wave fronts within the heart progressing in an endocardial-to-epicardial direction. Durrer et al24 demonstrated that the diaphragmatic surface of the left ventricle activates abnormally early in AVSD patients. Wave fronts in the ventricular free walls are spreading in an endocardial-to-epicardial direction as the diaphragmatic surface of the left ventricle completes its activation with epicardial breakthrough.

Epicardial Mapping

Durrer et al24 examined the epicardial excitation in four patients with AVSD. An intramural electrode was used for recordings. The greatest differences in left ventricular excitation, as compared with normal, were present at the basal region of the posterior wall. In patients with normal hearts, the earliest epicardial activation time found was 70 ms after the beginning of the left ventricular cavity potential, whereas in patients with AVSD, values of 28 to 34 ms were found. Therefore, Durrer at al24 considered the finding to be suggestive of the existence of a relatively large posterobasal area activated at least 30 ms earlier than normal.

After epicardial breakthrough at the basal part of the posterior wall, however, the excitatory forces moved to the anterior and posterolateral parts of the left ventricle and progressed in a more superior direction.

Conduction System

It was recently proposed that abnormal development of the conduction system follows the development of the heart anatomy.25 The effort to understand and explain the reason for the previously described abnormalities of left ventricular depolarization with left QRS axis deviation as a result led to studies concerning electroanatomic aspects of the AVSD.

It was proposed that differences in atrioventricular valve relationships to the septum affect not only the shunting level but also the location of the atrioventricular node.7,26-28 The bundle of His was found to also be displaced.23 By performing iodine staining of the peripheral conduction system of the left ventricle, it was shown that the bundle of His entered the ventricular muscle area in a position that was located inferiorly compared with the normal entrance of the left bundle branch.29 From the bundle of His, multiple branches extended over the diaphragmatic surface of the left ventricle. No discrete organized pattern, as present in the normal heart with an anterior and posterior division, was visible in the AVSD hearts. There was general dispersion of multiple arborizations emanating over the inferior septal border out over the left ventricular free wall. A large septal area superior to the defect in the outflow tract of the left ventricle was completely void of conduction system. The arrangement of the conduction system in AVSD with inferior displacement of the bundle of His was suggested to result in different order of depolarization sequences in the heart and thus left axis deviation.30

Conduction System and Atrioventricular Valve Complex

From developmental studies, it is known that the conduction system develops from the trabecular system of the ventricle.31 The location of the papillary muscles was recently suggested to predict the location of the conduction system and thus the sequences of electrical activation in the heart.32

As stressed earlier, papillary muscles belong to the atrioventricular valve complex and are abnormally positioned in AVSD patients.1 The relationship between papillary muscles and conduction system may be key to understanding some electrophysiologic abnormalities.33 Recently, the association between QRS axis deviation and position of papillary muscles has been suggested in AVSD patients.34 The main finding was that in AVSD patients, the left deviation of the QRS axis correlated with the anatomic imbalance of the positions of the mitral papillary muscles.

Abnormalities associated with the AVSD are complex. Diagnostic confusions of AVSD with other heart defects have been reported.35,36 The ability to distinguish AVSD from other congenital heart defects early and precisely is important because AVSD patients may present with severe hemodynamic complications early in the course and require different surgical management with the awareness that the displacement of the atrioventricular conduction tissues varies markedly between AVSD and other lesions.1,36

Left atrioventricular valve regurgitation remains the most common residual defect.37 Deterioration of the atrioventricular valve can be progressive and rapid and can lead to congestive heart failure early in infancy.38 Prediction of the functional outcomes of left atrioventricular valves following the repair of AVSDs is of interest.38-40 It was shown previously that mitral valve regurgitation correlates with changes in the 3D geometry of the subvalvular apparatus and the annulus. Abnormal anatomic and electrical constellation leads to abnormal dynamics of the atrioventricular valve function. Important practical implications can be drawn from this information regarding approaches to restore a more favorable geometry and might explain why, in some cases, a simple annuloplasty ring might fail to improve mitral valve regurgitation.

Detailed understanding of the atrioventricular valve complex in association with electrophysiologic abnormalities may be beneficial in predicting the atrioventricular valve abnormalities and outcome of patients with AVSD according to the papillary muscle positions and abnormalities of the conduction system. Association between the papillary muscle positions, conduction system, and function of the left ventricle and atrioventricular valve may be of interest in designing surgical procedures dealing with anatomic and electrophysiologic relationships and may predict their effects on the restoration of the ventricular performance.

The major determinants of ventricular contraction and thus ventricular function are anatomic arrangements and the sequence of ventricular electrical activation. The ventricular activation sequence changes during heart development concomitantly with its anatomy.1 In patients with congenital heart disease, abnormal electrical activation reflects abnormal development of anatomic arrangements, and they both may influence heart performance and function.

This chapter highlights the main anatomic, functional, and electrophysiologic abnormalities associated with AVSD. The use of multimodal imaging modalities is suggested in the assessment of the electroanatomic and functional relationships.