Atrioventricular Septal Defects

International Pediatric and Congenital Cardiac Code Classification

The International Pediatric and Congenital Cardiac Code (IPCCC) has classified AVSDs into four main groups: (1) complete, (2) partial, (3) intermediate/transitional, and (4) AVSDs with ventricular imbalance ( Fig. 31.2 ).

Complete AVSDs include defects in which an ostium primum atrial septal defect coexists with a nonrestrictive inlet ventricular septal defect in the context of a common AV valve (ie, with a common AV valve annulus with a single AV valve orifice). The common AV valve has two bridging leaflets (superior and an inferior) that override the ventricular septum, a left lateral (mural) leaflet, a right anterosuperior mural, and a right inferior mural leaflet. Shunting occurs at the atrial and ventricular levels. Most AVSDs are complete (56% to 75%).

Approximately 80% of these complete defects are isolated defects, that is, without other associated cardiovascular anomalies. Of these isolated defects, up to 60% are associated with chromosomal abnormalities, the most frequent of which is Down syndrome. Down syndrome accounts for 84% of those with chromosomal abnormalities. The remaining 17% have other karyotype abnormalities including trisomy 18, trisomy 13, Turner syndrome, Klinefelter syndrome, and various deletions and unbalanced translocations.

Partial AVSDs can have an isolated atrial-level shunt or an isolated ventricular-level shunt. The former is more common and is also referred to as an ostium primum atrial septal defect (ASD). The anatomic features include bridging leaflets that attach to the ventricular septum, leaving only an interatrial and no interventricular connection. The attachment of the bridging leaflets to the ventricular septum also serves to create two valvular orifices despite there being a single valve annulus. Less commonly, isolated ventricular-level shunts can occur, which are effectively an inlet ventricular communication (ventricular septal defect [VSD]). In these cases, partially fused bridging leaflets attach to the atrial septum leaving only a ventricular-level but no atrial-level shunting.

Intermediate AVSDs include the spectrum of defects that comprise primum ASD in association with a restrictive VSD, commonly a result of chordal attachments to the septal crest. They are distinguished from complete AVSDs in that there are two valvular orifices rather than one, and the presence of a restrictive ventricular defect. The two separate AV valve orifices, as with partial AVSDs, are a result of fusion of the bridging leaflets by a “tongue” of tissue that divides the common valve. A single annulus remains. These defects have also been called transitional AVSDs .

The IPCCC classification groups intermediate and transitional AVSDs in the same category and names them intermediate (transitional) AVSDs.

AVSDs with ventricular imbalance arise when there is unequal commitment of the common AV valve to both ventricles resulting in hypoplasia of one of the ventricles. This may occur with any of the previously mentioned AVSDs where there exists a primum atrial defect.

Congenital Gerbode defects are left ventricular–to–right atrial shunts, and may occur through a defect in the atrioventricular septum that permits a direct left ventricle (LV) to right atrium (RA) shunt. Alternatively, the shunt may be through a perimembranous VSD and associated perforation in the septal leaflet of the tricuspid valve, that is, indirect. Although not classically considered part of the AVSD, Gerbode defects by definition represent a defect in the atrioventricular septum ( Fig. 31.3 ).

Rastelli Classification

Complete AVSDs may also be classified by the so-called Rastelli classification (three types). The classification is largely based on the anatomy of the superior bridging leaflet ( Fig. 31.4 ).

• Type A (most common of Types A to C): The superior bridging leaflet is almost entirely contained within the left ventricle. It has chordal attachments to the crest of the ventricular septum. The right ventricular medial papillary muscle arises in relatively normal fashion from the interventricular septum.

• Type B: The superior bridging leaflet extends farther into the right ventricle. The leaflet is attached to an anomalous right ventricle (RV) papillary muscle that arises from the trabecular septomarginalis.

• Type C (often seen in association with other cardiac defects): The free-floating superior bridging leaflet extends even farther out into the right ventricle and is attached to an anterior papillary muscle.

Ventricular shunting increases progressively from Types A to C. Although the Rastelli classification was originally designed to predict surgical outcomes, its use is becoming increasingly obsolete because no consistent correlation exists between the Rastelli classification and surgical outcomes.

Atrioventricular Valves, Left Ventricle Outflow Tract Anatomy, and Other Anatomical Characteristics

• Characteristic features of the AV valve: In contradistinction to normal AV valve anatomy, where the presence of an intact membranous septum facilitates relative apical displacement of the tricuspid valve with respect to the mitral valve, deficiency of the membranous septum in AVSD results in both AV valves being at the same septal insertion level. This is consistent with the notion of a common atrioventricular valve ( Fig. 31.5B ).

  

• Cleft, right and left AV valve characteristics : Among AVSDs that have separate left and right orifices, a cleft in the left AV valve is commonly observed. In truth this is a misnomer because it refers to a functional commissure, that is, a zone of apposition between the superior and inferior bridging leaflets ( Fig. 31.6 ). In contrast to isolated clefts (directed toward the aortic valve annulus), AVSD clefts are directed toward the midventricular septum.

  

The left atrioventricular valve (LAVV) in AVSD is a trileaflet valve, composed of the left halves of the superior and inferior bridging leaflets and the left mural leaflet. The right AV valve is a quadrileaflet valve, composed of the right halves of the superior and inferior bridging leaflet, and the right anterosuperior and right inferior leaflets ( Fig. 31.1C ). These valves should therefore not be referred to as mitral or tricuspid valves . A more appropriate terminology is left or right atrioventricular valve.

The characteristic “gooseneck deformity ” (elongation of the left ventricular outflow tract relative to the ventricular apex, best seen on angiography) is attributable to cranial displacement of the aortic valve and root. This occurs in a fashion sometimes described as “unsprung” from its normal location, which is wedged between the right and left atrioventricular annuli. As a consequence of this unwedging, the distance from the apex to aortic annulus is longer than that of the apex to the mitral annulus ( Figs. 31.7 and 31.8 ), providing the substrate for an elongated left ventricular outflow tract and creating the potential for subaortic stenosis. In contrast, normal hearts have approximately equal aortic annular-apical and mitral annular-apical distances.

Atrioventricular Conduction Tissue

In AVSDs, the AV node is displaced posteriorly and inferiorly compared with the normal position. AV conduction tissue penetrates only at the crux of the heart and the penetrating bundle is also displaced posteriorly. The His bundle is shorter than normal. The left bundle branch (also posteriorly displaced) gives rise to a longer than typical left anterior fascicle and shorter than typical left posterior fascicle. The right bundle branch is longer than that of normal patients. These patterns have been found to correlate with the electrocardiographic (ECG) patterns described in the following sections.