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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.
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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
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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.
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Imaging of the Atrioventricular Valve Function
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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.
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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.
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Electrophysiology of the AVSD
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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.
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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
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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
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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).
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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.
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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.
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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.
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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.