Asian Cardiovasc Thorac Ann 2008;16:497-502
© 2008 Asia Publishing EXchange Ltd
Surgical Anatomy of Atrioventricular Septal Defect
Iki Adachi, MD,
Hideki Uemura, MD1,
Karen P McCarthy, BS,
Siew Yen Ho, PhD
Cardiac Morphology Unit, National Heart & Lung Institute, Imperial College London
1 Department of Cardiothoracic Surgery, Royal Brompton and Harefield NHS Trust, London, United Kingdom
For reprint information contact: Siew Yen Ho, PhD, Tel: 44 20 7351 8751, Fax: 44 20 7351 8230, Email: yen.ho{at}imperial.ac.uk, Guy Scadding Building, Dovehouse Street, London SW3 6LY, United Kingdom.
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ABSTRACT
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This review aims to describe the anatomic spectrum of hearts classified with the collective term atrioventricular septal defect. Despite their anatomical variety, hearts with the stigmata of atrioventricular septal defect share the characteristic feature of a common atrioventricular junction guarded by a 5-leaflet valve. The lack of normal atrioventricular septation makes the aorta un-wedged, resulting in an elongated outlet length on the left ventricular surface (known as inlet-outlet disproportion). The major determinant of anatomic variations is the relationship of the bridging leaflets to the septal structures. This important relationship determines not only the level of intracardiac shunting (interatrial only, interventricular only, or both) but also the propensity for left ventricular outflow tract obstruction. Furthermore, the location of the atrioventricular node, which is posteroinferiorly displaced from the tip of the triangle of Koch, is also affected by this relationship. Understanding the cardiac anatomy in this malformation is an absolute prerequisite for successful surgery, and should be facilitated by recognizing the fundamental nature of the morphology.
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INTRODUCTION
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Atrioventricular septal defect (AVSD) is a collective term for certain types of congenitally malformed heart, characterized by a deficiency of myocardial tissue at the aspect of the atrioventricular junction.1,2 There have been many discussions on this malformation. Herein, we review the surgical anatomy of hearts with AVSD, focusing on what unites all these hearts under the designation of AVSD and what defines the varieties among them.
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TERMINOLOGY
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Amongst the many terms used to describe this group of hearts are endocardial cushion defect, atrioventricular canal defect, ostium primum atrial septal defect, common atrioventricular orifice, and atrioventricular septal defect. It was in the late 1960s that the concept of absence or deficiency of an atrioventricular septum as a common feature uniting these hearts began to emerge.1,2 This important concept received further support when variations of these hearts were recognized as a spectrum of the same defect.3,4 We prefer the term atrioventricular septal defect because, unlike the other terms, it does not have embryologic connotations. However, this term should not be taken too literally because in the normal heart the region previously described as the atrioventricular septum is a sandwich comprising atrial myocardium, epicardial fat, and ventricular myocardium. According to the strict definition of a septum as a partition between chambers, which can be crossed without exiting the heart, this region is not a true septum. Nevertheless, the term refers to the septal aspect of the atrioventricular junction, which is deficient. Although some clinicians still persist in using the other classic terms, the general trend appears to have shifted toward AVSD. It is interesting to see changes in how this malformation has been described in "Cardiac Surgery" edited by Kirklin and Barratt-Boyes, which is a representative textbook in the field of cardiovascular surgery. Whereas atrioventricular canal defect was used in the 1st and 2nd editions published in 1986 and 1993, authors have started to use atrioventricular septal defect in the latest edition published in 2003.5–7 This term is applied only to hearts with biventricular atrioventricular connections. The term is not used in the setting of hearts with univentricular atrioventricular connections because one of the ventricular chambers in these hearts is usually too small to be incorporated into biventricular repairs.
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ANATOMY
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ATRIOVENTRICULAR JUNCTION AND VENTRICULAR MASS
Whatever diagnostic name is applied, these hearts share a basic characteristic at the atrioventricular junction. All hearts with the stigmata of AVSD have a common atrioventricular junction. This is in marked contrast to the figure-of-eight configuration of the atrioventricular junction in normally structured hearts (Figure 1
). However, we should not confuse the valvar junction with the valvar orifice(s). The valvar orifices can be double in the setting of a common atrioventricular junction, as seen in hearts with separate valvar orifices (so-called partial form; Figure 1). Lack of the usual figure-of-eight configuration leaves the aorta in a more anterior position than normal due to the lack of a recess to accommodate the aortic outflow tract (Figure 1). This anterior position of the aorta is part of the deformity of the ventricular mass; outlet length is longer than inlet length (Figure 2). This inlet-outlet disproportion in AVSD contrasts with the nearly equal lengths in normally structured hearts. Furthermore, the atrial septum fails to meet the ventricular septum, usually leaving a large gap between which is the atrioventricular septal defect. The crest of the ventricular septum usually has a scooped appearance.
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Figure 1. Diagram showing the normal atrioventricular junction (left) and the common atrioventricular junction with a common valvar orifice (middle) and 2 separate orifices (right). AS = anterosuperior, IB = inferior bridging, LM = left mural, SB = superior bridging, RM = right mural.
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Figure 2. Diagram showing the inlet-outlet disproportion in hearts with atrioventricular septal defect.
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LEAFLETS AND SUBVALVAR APPARATUS
Another key component for better understanding of AVSD anatomy is the atrioventricular valve that guards the common atrioventricular junction. The valve usually consists of 5 leaflets (Figure 3). Two of these leaflets are across the interventricular septum, inserted into both the right and left ventricles through their supporting apparatus. Because of this, they are called bridging leaflets. In contrast, one leaflet is completely related to the left ventricular chamber, guarding the lateral aspect of the valvar junction (left mural leaflet). The other 2 leaflets are associated with the right ventricular cavity, one being located anterosuperiorly (anterosuperior leaflet) and the other inferiorly (right mural leaflet). If both bridging leaflets were connected through a strap of leaflet tissue, there would be separated valvar orifices as seen in the partial form (Figure 1
). Nevertheless, the basic 5-leaflet configuration of the atrioventricular valve would remain, irrespective of the number of orifices.
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Figure 3. Diagrams showing, in surgical orientation, the anatomical variations of the atrioventricular valvar leaflets according to the Rastelli classification. The classification can be interpreted in terms of the degree of extension of the superior bridging leaflet into the right ventricle. AS = anterosuperior, IB = inferior bridging, LM = left mural, SB = superior bridging, RM = right mural.
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As described above, the left-sided valve comprises 3 different leaflets: the mural and both bridging leaflets. Although this valve is occasionally and mistakenly called a mitral valve, there is no resemblance between the two. The mural leaflet in AVSD guards approximately 20% of the entire circumference of the left orifice, whereas that in the normal mitral valve supports approximately 67% of the circumference (Figure 4).8 The papillary muscles supporting the mitral leaflet in normal hearts are obliquely oriented, whereas those in AVSD are in a superior-inferior relationship. This is a useful sign in cross-sectional echocardiography. When the fact that the left valve in AVSD is quite different from the normal mitral valve is fully appreciated, we should note that the term cleft is inappropriate to describe the gap between the bridging leaflets (Figure 1). This is because cleft is a term used to describe a slit-like space within a single leaflet rather than a gap between 2 separate leaflets. If the gap in AVSD was a real cleft, it would be possible to close it without loss of effective valvar orifice area; however, this is not the case. Because the gap is a space between 2 separate leaflets, its closure will inevitably result in loss of effective valvar area. Inappropriate designation with the use of "cleft" may cause surgeons to misunderstand that the gap could be closed at no cost. This is why we have abandoned the use of "cleft" to describe the gap.
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Figure 4. Diagrams showing differences between the normal mitral valve and "left-sided atrioventricular valve" in atrioventricular septal defect. The papillary muscles insert close to the sites marked by stars. There is no resemblance between the two.
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In contrast, the right-sided valve shares some features with the normal tricuspid valve, despite having 4 leaflets. The anterosuperior and mural leaflets in AVSD are broadly comparable with those of the normal tricuspid valve. Interestingly, the circumferential proportion of the mural leaflet in the right valve is almost identical with that of the counterpart in the left valve.9 Furthermore, this ratio is almost consistent, irrespective of the degree of extension of the superior bridging leaflet into the right ventricle, ranging from minimal extension (so-called Rastelli type A) to the most significant extension (so-called Rastelli type C). This consistency might support the assumption that as the superior bridging leaflet extends further into the right ventricle, there is concomitant diminution in the size of the anterosuperior leaflet (Figure 3).6
LEVEL OF INTRACARDIAC SHUNTING
AVSDs are usually grouped according to the anatomic status of the atrioventricular orifice, either being essentially common to both ventricles or divided by connecting leaflet tissue between the facing bridging leaflets (Figure 1).4,10 From a surgical point of view however a crucial distinction is the level of intracardiac shunting through the defect.11 In this regard, 3 distinct patterns of shunting can exist: at the atrial level only, at both atrial and ventricular levels, and at the ventricular level only. This additional variation might make these hearts look more complex, but this is not the case. According to Becker and Anderson,4 "if the valve leaflets are removed, there is no way of distinguishing the variants of this group from one another". This contention is based on the observation that all hearts with the stigmata of AVSD have comparable features in the ventricular mass and a common atrioventricular junction. To put it another way, the major anatomical variations should be determined only by how the bridging leaflets are related to the septal structures. The level of intracardiac shunting is also a product of this relationship (Figure 5). In the largest subset, the bridging leaflets adhere to neither the crest of the ventricular septum nor the underside of the atrial septum, making shunting possible at both atrial and ventricular levels (Figure 5). This is usually accompanied by a common atrioventricular valve, forming a so-called complete type. The second popular group has both bridging leaflets firmly attached to the crest of the ventricular septum, resulting in shunting being possible only at the atrial level (Figure 5). In this setting, both bridging leaflets are usually linked, dividing the common orifice into separate right and left valvular orifices (so-called partial form, or ostium primum defect). The final variant is hearts with only interventricular shunting owing to attachment of the leaflets to the underside of the atrial septum (Figure 5). This is far rarer than the previous 2 variants.9 In fact, this subset was not specified even in large series of morphologic investigations of AVSD.8,12 This apparent under-reporting of a well-recognized subset is probably due to previous diagnostic confusion, and its true prevalence could be higher.13–15 Although all these hearts possess the unifying features of AVSD, namely a common atrioventricular junction guarded by the characteristic 5-leaflet valve, there are some anatomical differences among these subtypes. First, the degree of cardiac deformity varies considerably. Measurements in large series showed the most severe deformity characterized by a deeper ventricular scoop and larger inlet-outlet disproportion in the "complete" form, whereas hearts with only ventricular shunting were at the mildest end of the spectrum, with the "partial" form having a moderate deformity.8,9,12 There is also a clear difference in the level of the bridging leaflets relative to the level of the atrioventricular junction. The leaflets in hearts with only a ventricular component are always upwardly displaced (prolapsed), while those of an ostium primum defect are always downwardly displaced (tethered).9 Because the leaflet arrangement at the septal aspect cannot be modified with surgery, such prolapsing and tethering would remain after repair of AVSD, explaining the development of late valvular dysfunction.
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Figure 5. Diagrams showing the variations of the relationship between the bridging leaflets and the septal structures. The level of intracardiac shunting can only be determined by this relationship.
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ATRIOVENTRICULAR CONDUCTION
Differences in the leaflets’ relationships to the septum affect not only the shunting level but also the disposition of the atrioventricular node. In hearts with AVSD, the atrioventricular node is displaced posteroinferiorly from the apex of the triangle of Koch, which is the anatomical landmark.16 When closing the defect with a patch, surgeons should be aware of the location of the displaced node and penetrating bundle of His, to avoid conduction injury. The degree of displacement however varies according to the shunting level. In hearts with only the ventricular component of the defect, owing to a shorter distance between the anterior and posterior margins of the defect, the node is likely to be located closer to the apex of the triangle of Koch compared to other forms (Figure 6). Because of this proximity, it might be better for surgeons to limit excision of the septum primum to within the muscular rim of the fossa when entering the left atrium through a right atriotomy, as in typical AVSD repair. This anomalous node gives rise to the conduction bundle running on the crest of the ventricular septum and then branching to the left and right bundle after the relatively long non-branching portion (Figure 7). As with the other 2 subtypes, the distance between the orifice of the coronary sinus and the edges of the atrial and ventricular septum will determine whether it is feasible to secure the patch here without causing heart block. Another important consideration for surgeons is the course of the non-branching and branching bundle. It runs on the septal crest, slightly to the left side of the septal crest. In hearts with partial AVSD, fusion between the superior and inferior bridging leaflets and their adherence to the septal crest allow the entire atrioventricular conduction bundle to be protected by leaflet tissue when sutures are placed on the ventricular margins of the patch. In contrast, the branching atrioventricular bundle is exposed on the septal crest in hearts with a common valvular orifice. Consequently, if stitches need to be placed in the septum to anchor the patch, the right ventricular aspect of the septum would be the best place to avoid branch injury.
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Figure 6. Diagrams illustrating, in surgical orientation, the "Koch" area of hearts with different levels of shunting. In hearts with only ventricular component, the atrioventricular node is likely to be located much closer to the apex of the triangle of Koch.
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Figure 7. Diagram showing, in surgical orientation, the displaced atrioventricular node and its continuation to the non-branching bundle. The non-branching bundle does not run exactly on the top of the ventricular crest, rather it deviates a little toward the left ventricular side.
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ASSOCIATED ANOMALIES
A variety of cardiac anomalies can coexist with AVSD, including tetralogy of Fallot, additional ventricular septal defects, double-outlet right ventricle with or without pulmonary stenosis, total anomalous pulmonary venous connection, transposition of the great arteries, and Ebstein malformation.17 Of these anomalies, it is left ventricular outflow obstruction that surgeons should pay particular attention to because it may become evident only after surgical modification. The intrinsically elongated outlet tract as well as adherence of the superior bridging leaflet to the scooped ventricular margin of the septal defect are thought to be important for the development of left ventricular outflow tract obstruction.18–20 Any additional tissues (tissue tags, accessory muscle bundles) in this area can exacerbate the narrowness of the outflow tract. In view of the degree of outlet elongation as well as the leaflets’ relationship to the septum, one can reasonably assume that the so-called partial or ostium primum type carries the highest risk, while hearts with only the ventricular component have minimal potential. Nonetheless, such possibility should be kept in mind when performing surgical treatment, regardless of the type that is being operated on.
CONCLUSION
In this review of AVSD, we have highlighted anatomical features relevant to cardiac surgeons. We have not included any discussion on associated malformations such as tetralogy of Fallot, isomeric arrangement of atrial appendages, or chamber disproportion. Nevertheless, we emphasize the feature that characterizes all hearts with AVSD, namely, a common atrioventricular junction guarded by a 5-leaflet valve. This is what differentiates this group of hearts from those with other superficially similar anomalies such as cleft mitral valve. The key components that make the variations of AVSD are bridging leaflets and their relationship to the septal structures. This important relationship not only determines the level of intracardiac shunting but also affects the location of the conduction tissues. Furthermore, leaflet arrangement in relation to the septum also plays an important role in the development of left ventricular outflow tract obstruction which is one of the most formidable complications after surgery. In-depth knowledge of the anatomy should help surgeons reduce the risk of complications and achieve successful outcomes.
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ACKNOWLEDGMENTS
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This work is supported by the Francis Fontan Prize of the European Association of Cardio-Thoracic Surgery awarded to Iki Adachi, and a grant from The Uehara Memorial Foundation. The Cardiac Morphology Unit receives funding from the Royal Brompton and Harefield Hospital Charitable Fund.
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REFERENCES
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- Baron MG, Wolf BS, Steinfeld L, Vanmierop LH. Endocardial cushion defect. Specific diagnosis by angiocardiography. Am J Cardiol 1964;13:162–75.[Medline]
- Van Mierop LH, Alley RD, Kausel HW, Stranahan A. The anatomy and embryology of endocardial cushion defects. J Thorac Cardiovasc Surg 1962;43:71–83.[Medline]
- Piccoli GP, Gerlis LM, Wilkinson JL, Lozsadi K, Macartney FJ, Anderson RH. Morphology and classification of atrioventricular defects. Br Heart J 1979;42:621–32.[Abstract/Free Full Text]
- Becker AE, Anderson RH. Atrioventricular septal defects: what’s in a name? J Thorac Cardiovasc Surg 1982;83:461–9.[Medline]
- Kirklin JW, Barratt-Boyes BG. Atrioventricular canal defect. In: Kirklin JW, Barratt-Boyes BG, editors. Cardiac surgery. New York; Churchill Livingstone, 1986:541–91.
- Kirklin JW, Barratt-Boyes BG. Atrioventricular canal defect. In: Kirklin JW, Barratt-Boyes BG, editors. Cardiac surgery. 2nd ed. New York; Churchill Livingstone, 1993: 693–747.
- Kouchoukos N, Blackstone EH, Doty DB, Hanley FL, Karp RB. Atrioventricular canal defect. In: Kirklin JW, Barratt-Boyes BG, editors. Cardiac surgery. Vol 1. 3rd ed. Philadelphia: Elsevier Science, 2003:800–49.
- Penkoske PA, Neches WH, Anderson RH, Zuberbuhler JR. Further observations on the morphology of atrioventricular septal defects. J Thorac Cardiovasc Surg 1985;90:611–22. Erratum in: J Thorac Cardiovasc Surg 1988;95:146.[Abstract]
- Adachi I, Uemura H, McCarthy KP, Ho SY. Morphologic features of atrioventricular septal defect with only ventricular component: further observations pertinent to surgical repair. J Thorac Cardiovasc Surg 2008 (in press).
- Ho SY, Rigby ML, Anderson RH. Atrioventricular septal defects. Echocardiography in congenital heart disease made simple. London: Imperial College Press, 2005:76–86.
- Ebels T, Anderson RH. The concept and definition of an "intermediate form" of atrioventricular septal defect. J Thorac Cardiovasc Surg 1991;102:799–800.[Medline]
- Ebels T, Anderson RH, Devine WA, Debich DE, Penkoske PA, Zuberbuhler JR. Anomalies of the left atrioventricular valve and related ventricular septal morphology in atrioventricular septal defects. J Thorac Cardiovasc Surg 1990;99:299–307.[Abstract]
- Smallhorn JF, de Leval M, Stark J, Somerville J, Taylor JF, Anderson RH, et al. Isolated anterior mitral cleft. Two dimensional echocardiographic assessment and differentiation from "clefts" associated with atrioventricular septal defect. Br Heart J 1982;48:109–16.[Abstract/Free Full Text]
- Sigfusson G, Ettedgui JA, Silverman NH, Anderson RH. Is a cleft in the anterior leaflet of an otherwise normal mitral valve an atrioventricular canal malformation? J Am Coll Cardiol 1995;26:508–15.[Abstract]
- Sulafa AK, Tamimi O, Najm HK, Godman MJ. Echocardiographic differentiation of atrioventricular septal defects from inlet ventricular septal defects and mitral valve clefts. Am J Cardiol 2005;95:607–10.[Medline]
- Thiene G, Wenink AC, Frescura C, Wilkinson JL, Gallucci V, Ho SY, et al. Surgical anatomy and pathology of the conduction tissues in atrioventricular defects. J Thorac Cardiovasc Surg 1981;82:928–37.[Medline]
- Studer M, Blackstone EH, Kirklin JW, Pacifico AD, Soto B, Chung GK, et al. Determinants of early and late results of repair of atrioventricular septal (canal) defects. J Thorac Cardiovasc Surg 1982;84:523–42.[Abstract]
- Piccoli GP, Ho SY, Wilkinson JL, Macartney FJ, Gerlis LM, Anderson RH. Left-sided obstructive lesions in atrioventricular septal defects: an anatomic study. J Thorac Cardiovasc Surg 1982;83:453–60.[Abstract]
- Ebels T, Ho SY, Anderson RH, Meijboom EJ, Eijgelaar A. The surgical anatomy of the left ventricular outflow tract in atrioventricular septal defect. Ann Thorac Surg 1986;41:483–8.[Abstract]
- Silverman NH, Gerlis LM, Ho SY, Anderson RH. Fibrous obstruction within the left ventricular outflow tract associated with ventricular septal defect: a pathologic study. J Am Coll Cardiol 1995;25:475–81.[Abstract]
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ABSTRACT
INTRODUCTION
