Rastelli Classification of Complete AVSD #2: Rastelli Type B

Complete atrioventricular septal defect (AVSD) is characterized by a common atrioventricular junction, a common atrioventricular valve, and interatrial and interventricular communications of variable extent. The Rastelli classification describes complete AVSD according to the morphology and attachments of the superior bridging leaflet. In the original classification, Types A, B, and C represent progressively different relationships between this leaflet, the ventricular septum, and the right ventricular papillary muscles.[1]

Rastelli Type B is uncommon. Its defining feature is extension of the superior bridging leaflet across the ventricular septum into the right ventricle, where it is supported by an anomalously positioned right ventricular papillary muscle rather than attaching directly to the septal crest. This morphology places Type B between the septal attachment characteristic of Type A and the more extensively bridging configuration of Type C.

1. Fundamental Anatomy of Complete AVSD

The defining anatomical substrate of AVSD is not simply an atrial septal defect plus a ventricular septal defect. Rather, there is deficiency of the normal atrioventricular septal structures and a common atrioventricular junction guarded by a five-leaflet valve.[2,3] The principal valvar components are the:

• superior bridging leaflet,
• inferior bridging leaflet,
• left mural leaflet,
• right anterosuperior leaflet, and
• right inferior mural leaflet.

The superior and inferior bridging leaflets cross, or “bridge,” the plane of the ventricular septum to varying degrees. Their relationships to the septal crest, ventricular papillary muscles, and chordal apparatus determine much of the morphological variation among AVSDs.[2-4]

The absence of normal atrioventricular septation also alters ventricular geometry. The aortic root is relatively “unwedged,” the left ventricular inlet-to-outlet distance is elongated, and the left ventricular outflow tract assumes the characteristic elongated configuration associated with AVSD.[3] These features are common to the AVSD spectrum and are independent of Rastelli subtype.

The supplied surgical schematic emphasizes the key Type B relationship: the superior bridging leaflet crosses the septal plane, while its rightward division and supporting papillary apparatus are displaced into the right ventricle.

2. The Basis of the Rastelli Classification

Rastelli, Kirklin, and Titus originally classified complete AV canal according to the morphology of what was then termed the common anterior leaflet, now more commonly described as the superior bridging leaflet.[1] Although the classification remains useful for conceptualizing common-valve morphology, subsequent anatomical studies have demonstrated a continuum rather than three completely discrete configurations.[2,4]

In simplified terms:

Type A: The superior bridging leaflet has substantial attachment to the crest of the ventricular septum. Its rightward extension is limited, and the right anterosuperior leaflet is relatively well developed.

Type B: The superior bridging leaflet crosses farther into the right ventricle. Instead of attaching directly to the septal crest, its rightward aspect is supported by an anomalously positioned papillary muscle on the right side of the septum, frequently located toward the apical or septomarginal region.

Type C: The superior bridging leaflet has no direct septal attachment and extends extensively across the ventricular septum toward a more anterior right ventricular papillary muscle.[1,2,4]

Thus, the classification is fundamentally a description of how the superior bridging leaflet is supported as it crosses the ventricular septal defect, rather than simply how far the leaflet itself extends into the right ventricle.

3. Morphology of Rastelli Type B

3.1 Superior bridging leaflet

In Type B, the superior bridging leaflet crosses the ventricular septal plane more extensively than in Type A. Its rightward component continues across the ventricular septal defect before reaching its supporting subvalvar apparatus.[2,4]

The characteristic attachment is to an abnormally positioned right ventricular papillary muscle, rather than directly to the crest of the ventricular septum. Anatomical descriptions have located this papillary muscle close to the septum, often more apically than the usual medial papillary complex.[2,4,5]

This arrangement creates the visual impression that the leaflet is suspended over the VSD. However, “floating” should be used cautiously. A truly free or relatively unattached superior bridging leaflet is more characteristic of Rastelli Type C. In Type B, the defining feature is not absence of attachment, but transfer of its attachment from the septal crest to an anomalous right ventricular papillary muscle.[1,4]

3.2 Right anterosuperior leaflet

As the superior bridging leaflet extends farther into the right ventricle, the adjacent right anterosuperior leaflet becomes correspondingly smaller. Morphological examination of complete AVSD specimens has demonstrated a continuum in which increasing rightward extension of the bridging leaflet is accompanied by reduction in the size of the right anterior valvar component.[2]

This relationship is surgically important. The apparent division between the superior bridging leaflet and right anterosuperior leaflet lies farther to the right than in Type A, and the surgeon should identify the true commissural and chordal relationships before manipulating or dividing valvar tissue.

3.3 Papillary muscle abnormality

The abnormal papillary muscle is the hallmark of Type B. It may arise adjacent to the ventricular septum, from the trabecula septomarginalis/septomarginal region, or from a more apical portion of the right ventricle depending on the individual morphology.[2,4,5]

The supporting chordae therefore cross the ventricular septal defect and may occupy tissue planes that would otherwise appear available for patch placement. The exact chordal arrangement is more clinically important than assigning the categorical label itself.

3.4 Ventricular septal component

The inlet ventricular septal deficiency lies beneath the common atrioventricular valve. Because the superior bridging leaflet is not firmly tethered to the septal crest in the Type B configuration, the effective interventricular communication beneath the leaflet may be larger than in a typical Type A configuration.[4]

Nevertheless, ventricular communication size varies considerably. Rastelli classification alone should not be used to infer the exact dimensions of the VSD, ventricular balance, degree of shunting, or feasibility of a particular repair.

4. How Rare Is Type B?

Rastelli Type B is consistently described as the least common of the three classical patterns. Contemporary surgical series generally contain only a small proportion of Type B cases. For example, one series of 235 patients reported Type B morphology in 16 patients, approximately 7%.[6] Other surgical cohorts similarly demonstrate single-digit frequencies.

Accordingly, describing Type B as rare, approximately several percent of complete AVSDs, is reasonable. A precise universal prevalence such as 5% should be interpreted as an approximation because reported proportions depend on population, morphological definitions, associated lesions, and whether intermediate configurations are assigned to Type A or Type B.

Importantly, subsequent morphological investigations have challenged rigid application of the original A/B/C system. In a study of 70 hearts with complete AVSD, Piccoli and colleagues demonstrated a graded spectrum of superior bridging leaflet extension and papillary-muscle attachment rather than three sharply separated morphologies.[2] This explains why some hearts cannot be categorized unequivocally.

5. Echocardiographic Recognition

Echocardiography should define the anatomy directly rather than merely report a Rastelli letter. Standard subcostal, apical, and parasternal imaging can demonstrate the common atrioventricular junction, bridging leaflets, ventricular septal component, ventricular balance, and subvalvar apparatus.[7]

For suspected Type B morphology, the important questions are:

• How far does the superior bridging leaflet extend into the right ventricle?
• Does it attach directly to the ventricular septal crest?
• Where is the papillary muscle supporting its rightward component?
• Are chordae crossing the VSD?
• How large is the right anterosuperior leaflet?
• Is there additional straddling or overriding of the common AV valve?
• What is the relationship between the bridging leaflets and the ventricular septal crest?
• Is there left ventricular outflow tract narrowing or other associated intracardiac anatomy?

An anatomoechocardiographic study specifically demonstrated Type B morphology as a bridging leaflet crossing the ventricular septal defect and inserting into a papillary muscle adjacent to the septum, showing that the relevant subvalvar anatomy can be identified echocardiographically.[5]

Three-dimensional echocardiography may further clarify leaflet topology and chordal attachments when conventional two-dimensional planes do not adequately demonstrate the common valve.

6. Surgical Significance

The Rastelli classification was developed from morphology rather than as a prescriptive surgical algorithm. Consequently, Type B does not dictate a specific repair technique. Two-patch, modified single-patch, and other established approaches can be used according to ventricular septal depth, common-valve anatomy, ventricular balance, surgeon experience, and associated lesions.

The practical importance of recognizing Type B lies primarily in the subvalvar anatomy.

6.1 Preserve the anomalous papillary muscle and chordae

The anomalous right ventricular papillary muscle provides critical support to the superior bridging leaflet. Chordae crossing the VSD should therefore be identified before ventricular patch placement or leaflet manipulation.

Inadvertent division can alter leaflet geometry and produce common AV-valve regurgitation. Conversely, indiscriminate preservation without understanding their course may interfere with patch positioning or create distortion.

6.2 Define the leaflet division before reconstruction

Because the line of division between the superior bridging leaflet and right anterosuperior leaflet is displaced toward the right ventricle, the right anterosuperior component may appear unusually small. The surgeon should not assume the Type A configuration.

Saline testing and direct inspection help determine the functional line of coaptation and identify the portions of the bridging leaflets ultimately forming the left and right atrioventricular valve components.

6.3 Avoid distortion by the VSD patch

Whether a two-patch or modified single-patch repair is used, reconstruction should preserve the three-dimensional relationship between:

septal crest → patch → bridging leaflet → chordae → papillary muscle.

A patch positioned without accounting for an anomalous Type B papillary muscle may alter chordal tension, restrict leaflet excursion, or displace the coaptation zone. The objective is therefore not simply closure of the ventricular communication but reconstruction of the septal plane while preserving physiologic common-valve geometry.

7. Conduction-System Considerations

The conduction anatomy of AVSD is abnormal regardless of Rastelli subtype. Deficiency of the normal atrioventricular septal structures displaces the atrioventricular node posteroinferiorly, and the penetrating and nonbranching conduction axis follows an elongated course before reaching the ventricular conduction system.[3,8]

Traditional landmarks of the normal triangle of Koch are therefore unreliable. Morphological studies have shown that the relationship of the inferior bridging leaflet to the ventricular septum provides an important surgical landmark for the penetrating conduction axis.[8]

More recent high-resolution imaging of AVSD specimens has further confirmed the abnormal disposition of the conduction system and its close relationship to the posteroinferior region of the ventricular component.[9]

This conduction anatomy is a feature of the underlying AVSD rather than a unique consequence of Type B. Nevertheless, complex leaflet and chordal anatomy should not distract from meticulous protection of the conduction axis during VSD closure.

8. Clinical Meaning of the Classification

Rastelli Type B should ultimately be regarded as an anatomical descriptor rather than a complete surgical diagnosis. Modern operative planning requires substantially more information than the A/B/C designation alone.

The surgeon must independently evaluate:

• common AV-valve competence,
• location and mechanism of regurgitation,
• superior and inferior bridging leaflet morphology,
• chordal insertions,
• papillary-muscle configuration,
• ventricular balance,
• VSD depth and geometry,
• left ventricular outflow tract morphology,
• associated conotruncal or venous abnormalities, and
• conduction-system landmarks.

This reflects the broader contemporary understanding that AVSD represents a morphological spectrum. The relationships among the bridging leaflets, ventricular septum, chordae, and papillary muscles are more informative than a categorical label when planning repair.[2,3]

Key Surgical Principles

• Type B is defined by papillary-muscle attachment. The superior bridging leaflet crosses the septal plane and is supported by an anomalous right ventricular papillary muscle rather than attaching directly to the septal crest.
• The right anterosuperior leaflet is relatively small because the superior bridging leaflet extends farther into the right ventricle than in Type A.
• Do not equate Type B with a truly free-floating leaflet. Complete absence of septal tethering with extensive bridging is more characteristic of Type C.
• Inspect the subvalvar apparatus directly. Chordal and papillary-muscle anatomy is more important for reconstruction than the Rastelli label itself.
• Preserve leaflet geometry during VSD closure. Patch positioning should avoid abnormal chordal tension, leaflet restriction, and distortion of the reconstructed AV-valve components.
• Protect the posteroinferior conduction axis. The displaced AV node and elongated conduction pathway are fundamental features of AVSD.
• Treat Rastelli classification as descriptive, not prescriptive. Operative strategy should be individualized according to the complete valve, ventricular, septal, and subvalvar anatomy.

References

1. Rastelli G, Kirklin JW, Titus JL. Anatomic observations on complete form of persistent common atrioventricular canal with special reference to atrioventricular valves. Mayo Clin Proc. 1966;41(5):296-308. PMID: 5932615.
2. Piccoli GP, Wilkinson JL, Macartney FJ, Gerlis LM, Anderson RH. Morphology and classification of complete atrioventricular defects. Br Heart J. 1979;42(6):633-639. doi:10.1136/hrt.42.6.633. PMID: 534580.
3. Adachi I, Uemura H, McCarthy KP, Ho SY. Surgical anatomy of atrioventricular septal defect. Asian Cardiovasc Thorac Ann. 2008;16(6):497-502. doi:10.1177/021849230801600616. PMID: 18984764.
4. Anderson RH, Baker EJ, Ho SY, Rigby ML, Ebels T. The morphology and diagnosis of atrioventricular septal defects. Cardiol Young. 1991. doi:10.1017/S1047951100010362.
5. Espinola-Zavaleta N, Muñoz-Castellanos L, Kuri-Nivón M, Keirns C. Understanding atrioventricular septal defect: anatomoechocardiographic correlation. Cardiovasc Ultrasound. 2008;6:33. doi:10.1186/1476-7120-6-33. PMID: 18573220.
6. Cui H, Zhuang J, Chen J, et al. Surgical treatment and early-mid follow-up results of complete atrioventricular septal defect. Zhonghua Wai Ke Za Zhi. 2017. doi:10.3760/cma.j.issn.0529-5815.2017.12.011. PMID: 29224269.
7. Nayak S, Kanakriyeh M, Varadarajan P. Echocardiographic assessment of atrioventricular canal defects. Echocardiography. 2020. doi:10.1111/echo.14961. PMID: 33368544.
8. Seo JW, Zuberbuhler JR, Ho SY, Anderson RH. Surgical significance of morphological variations in the atrial septum in atrioventricular septal defect for determination of the site of penetration of the atrioventricular conduction axis. J Card Surg. 1992. doi:10.1111/j.1540-8191.1992.tb01022.x. PMID: 1482825.
9. Yoshitake S, Kaneko Y, Morita K, et al. Reassessment of the location of the conduction system in atrioventricular septal defect using phase-contrast computed tomography. Semin Thorac Cardiovasc Surg. 2020. doi:10.1053/j.semtcvs.2020.03.011. PMID: 32450213.