Due to the extreme diversity of cardiac malformations, classification is necessary. It is useful to refer to the basic segmental analysis concept adopted by anatomical pathologists. This divides the heart into three segments [10,12]: 
 

These three segments are joined by two junctions (Figure 15.1):
 

Figure 15.1: Segmental analysis of the heart. The heart is divided into three segments (atrial, ventricular and arterial) connected to each other by 2 junctions (atrioventricular and ventriculoarterial junctions). The venous return is part of the atrial segment. The atrioventricular valves are part of the ventricular segment and the ventriculoarterial valves are part of the arterial segment [4].

Each segment is defined by its intrinsic morphology since it is not possible to reliably identify the cardiac chambers based on their position and size, because: each of them may be repositioned, atrophic, hypertrophied or even absent. Five criteria are used to determine the heart's anatomical structures [5].
 

The anatomical descriptions of structures and pathologies provided in this chapter are based mainly on echocardiographic imaging [2,3,7,9,11,13]. They are illustrated by diagrams and images obtained by transesophageal echocardiography (TEE).

Atrial segment

The cardiac situs is determined by the position of the atria. In the vast majority of cases, it is the same as the abdominal situs, with transdiaphragmatic discordances occurring only in severe and very rare pathologies that are not encountered in adults. The RA can be identified by the junction with the inferior vena cava (IVC) – if the IVC is absent, the hepatic veins flow into the RA. The RA and the liver, which are both easily identifiable, are therefore on the same side. The pulmonary veins may anastomose elsewhere than into the left atrium (LA) in cases of anomalous pulmonary venous return.

The atria may also be distinguished by their specific anatomy. The RA has a short, obtuse and pyramidal atrial appendage. It is the site of the fossa ovalis of the atrial septum and receives the insertion of the Eustachian valve. Its wall is partially trabeculated. The boundary between the smooth part and the trabeculated part is marked by a crest (crista terminalis), which runs from one vena cava to the other (Figure 27.129). The LA, whose wall is completely smooth, has a long, pectinate, finger-shaped atrial appendage. Sometimes both atria exhibit the same anatomical characteristics, which is described as isomerism or mirror-image atria. Right isomerism (two RAs) is normally associated with the absence of the spleen (asplenia), while left isomerism (two LAs) is characterised by multiple spleens (polysplenia) [6].


Figure 27.129: Anatomical criteria for the definition of right atrium (RA). A: Transesophageal bicaval view (90-100°). 1: IVC: inferior vena cava, SHV: supra-hepatic vein. 2: Eustachian valve. 3: fossa ovalis. 4: crista terminalis. 5: left atrial appendage (LAA). 6: trabeculated part of RA. B: right atrial appendage (bicaval view). C: left atrial appendage (2-chamber view).

Atrioventricular junction

The junction between the atrial septum, interventricular septum, septal leaflet of the tricuspid valve and anterior leaflet of the mitral valve forms the internal crux of the heart. These four structures may exhibit a coalescence defect, leaving an orifice in the centre of the heart – this is an atrioventricular canal (AV canal) or endocardial cushion defect. Normally, the septal insertion of the tricuspid valve is 5-10 mm distal to that of the mitral valve (0.7 cm/m²). Both valves are on the same plane in the event of atrioventricular canal defects (Figure 15.2) [8].




Figure 15.2: Atrioventricular junction. In normal circumstances, the septal insertion of the tricuspid valve is in a more apical position than that of the mitral valve. The difference is approximately 1 cm in adults (0.7 cm/m²). Both valves are on the same plane in cases of common atrioventricular canal defects (AV canal defects).

The atrioventricular junction is concordant if the atria and ventricles follow on from each other in the correct order. It is discordant if each atrium is connected to the contralateral ventricle. In the event of discordance, the tricuspid valve remains attached to the right ventricle (RV) and the mitral valve remains attached to the left ventricle (LV). If both atria are connected to a single ventricle, this is described as a double-inlet ventricle. If an atrioventricular valve opens into both ventricles due to a ventricular septal defect (VSD), this is described as “overriding”. This malformation may be supplemented by “straddling” if the valve’s chordae tendineae are inserted into a wall of the contralateral ventricle through the VSD (Figure 15.3).



Figure 15.3: Atrioventricular junction. A: Overriding – the mitral valve overrides the septum, with its orifice anastomosing to both ventricles, although the subvalvular apparatus is on the same side. It is connected to the ventricle that receives > 50% of its surface area. B: Straddling – the subvalvular apparatus of the overriding valve, in this case the mitral valve, is inserted into both sides of the septum, with part of the chordae crossing the VSD. 

Ventricular segment

A ventricle is composed of three parts: an inlet, a trabeculated body, and an outflow tract. A chamber that receives more than 50% of the opening of the corresponding atrioventricular valve is considered as a ventricle. Below this value, it is defined as a rudimentary chamber. Position, shape and size are unreliable indicators of which ventricle is right or left as these data may be profoundly disrupted. Other criteria are used to distinguish them (Figure 15.4) [3].



Figure 15.4: Differentiation of the RV and LV. The RV is connected to a tricuspid valve (1) and contains 3 (or more) papillary muscles, one of which is located on the septum (2); its outflow tract is entirely muscular (3); it exhibits significant trabeculation (4); and generally a clearly defined muscular moderator band (5). The LV is connected to a bicuspid valve (6) and only contains 2 papillary muscles (anterolateral and posteromedial, sometimes single) (7), neither of which is inserted on the septum; it is slightly trabeculated (8) and its outflow tract includes a fibrous continuity between the aortic valve and anterior leaflet of the mitral valve (9). The height difference between the septal insertions of the tricuspid and mitral valves (10) helps differentiate the RV (low insertion) from the LV (high insertion) (0-20° 4-chamber view). This indicator is lost if AV canal defects are present as both valves are inserted at the same level in patients with this pathology [1].

A right ventricle is distinguished by:
 

A left ventricle can be identified by:
 

Ventriculoarterial junction

The junction is concordant or discordant depending on whether or not the ventricle is connected to the appropriate vessel. Transposition of the great arteries (TGA) is the most common case of ventriculoarterial discordance: the RV is connected to the aorta and the LV to the PA (D-TGA). A double-outlet ventricle is characterised by a layout in which over 50% of a ventriculoarterial valve is connected to the same ventricle as the other vessel, whether this ventricle be right, left or single. While the valves may exhibit overriding, notably in the event of a membranous VSD as in tetralogy of Fallot, straddling is impossible as they have no chordae (Figure 15.5).



Figure 15.5: Pathologies of the ventriculoarterial junction. A: normal situation – the aorta (Ao) and pulmonary artery (PA) cross at 45°, whereby the PA is anterior. The PA can be identified by its division into two branches 4-5 cm after its origin. The ascending aorta is a long tube that has no branches before the vessels of the aortic arch. Since numerous coronary artery anomalies are possible, the positioning of the coronary arteries cannot be used as a criterion for identifying the aorta. B: double-outlet right ventricle (DORV) – due to the septum shifting, with or without VSD, both vessels originate from the RV. C: transposition of the great arteries (TGA) – the aorta emerges from the RV and the PA from the LV. On an echocardiogram, the two vessels are parallel in the long-axis view at 120° and the two valves are on the same plane in short-axis view (0-40°). Generally, the great arteries are parallel in the sagittal plane in cases of D-TGA (anterior aorta and posterior PA) and side-by-side in the frontal plane in cases of L-TGA.

Arterial segment

Two vessels emerge from the heart – the aorta, which is posterior, and the pulmonary artery (PA), which is anterior. In normal circumstances, they cross each other at their origin. While the aorta's first branches only emerge distally in the aortic arch, the PA splits into two branches after 4-5 cm. The coronary arteries are not a distinguishing feature as they may emerge from the aorta or the PA. If their embryonic septation has not occurred, the two vessels fuse to form a single vessel (truncus arteriosus) [6].

Shunts and obstructions

Shunting involves communication between the systemic and pulmonary circulation and may occur in different locations (see Shunts).
 

Obstructions mainly occur in the two ventricles’ ejection pathways and in the aorta. They may be dynamic (concentric muscular hypertrophy of the outflow tract) or static (subvalvular membrane, valvular stenosis, arterial stenosis).
 

Obstructive lesions cause excessive afterload in the affected ventricle, which undergoes concentric hypertrophy. If a shunt is present, these modify the pressure ratio between the right and left circulations.


 

Nomenclature

In segmental analysis, the heart is divided into 3 segments (atrial, ventricular and arterial) and 2 junctions (atrioventricular and ventriculoarterial). The anatomical examination of congenital heart diseases is a 5-step process:     - Defining the situs based on the RA     - Defining the various chambers by their structures (number, position, development)      - Examining the AV and ventriculoarterial junctions (concordance or discordance)     - Checking for shunts     - Checking for obstructive lesions

© BETTEX D, CHASSOT PG, January 2008, last update May 2018



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