The Ventricles

The second of the three major cardiac segments is composed of the ventricles which also maybe in situs solitus, or D-loop, or in situs inversus, or L-loop. Only rarely is the situs ambiguous or anatomically indeterminate. At the ventricular level, there are no well defined syndromes of situs ambiguous as there are at the atrial level.

The Right Ventricle

The right ventricle can be identified externally by its pyramidal shape, and by its coronary distribution pattern, which is distinctive and typical. The left anterior descending coronary artery demarcates the right from left ventricles. The papillary muscles of the right ventricle arise from both the septal and from the free wall surfaces and they are numerous and relatively small, making a right ventriculotomy readily possible. Papillary muscles arising from the septal surface is unique to morphology of the right ventricle. The infundibulum is incorporated into the right ventricle, and forms the outflow track, whereas the right ventricle proper forms the inflow track. The junction between the infundibulum and the right ventricle is composed of the parietal band, the septal band and the moderator band. The pulmonary artery normally arises from the infundibulum of the right ventricle. The leaflets of the pulmonary valve and the leaflets of the tricuspid valve are widely separated by the infundibular musculature.

The internal appearance of the morphologically right ventricle is pathognomonic. The muscular trabeculations are relatively coarse, few, and straight, tending to parallel the right ventricular inflow and outflow tracts. The papillary muscles of the right ventricle are relatively small (making right ventriculotomy readily possible) and numerous, and they attach both to the septal and to the free wall surfaces. Because of its numerous attachments to the right ventricular septal surface (mostly to the posteroinferior margin of the septal band), the tricuspid valve may be described as 'septophilic'.

The three components of the definitive right ventricle are therefore the inlet (the tricuspid valve and atrioventricular septum), the apical trabecular portion, and the outlet portion (infundibulum).

The inlet component extends from the atrioventricular junction to the distal attachments of the chordae tendineae of the tricuspid valve.

The apical trabecular component extends from the two parts associated with the valves and out to the apex. This part is coarsely trabeculated and contains septomarginal trabeculation. It is the apical trabecular component which is the most constant and characteristic part of the right ventricle. In addition to the coarse trabeculations, the right ventricle possesses a series of septoparietal trabeculations which extend from the anterior surface of the septomarginal trabeculation onto the parietal wall of the ventricle. The moderator band is a prominent muscle bundle which crosses from the septomarginal trabeculation to the anterior papillary muscle and then to the parietal wall. The other ventricular components have specific features in the normal heart which aid in the recognition of the morphologically right ventricle.

The outlet component is the smooth-walled tube of muscle which supports the leaflets of the pulmonary valve. The normal right ventricle has a large infundibular component making up its outflow tract. The infundibulum, is incorporated mainly into the right ventricle, where it forms a conal ring consisting of three components: (1) the distal conal septum, which extends on to the parietal or free wall, forming the parietal band; (2) the septal band, or proximal conal septum; and (3) the moderator band. The infundibulum consists not only of the parietal band, but also of the septal and moderator bands, the latter two which are also known as the trabecula septomarginalis. An understanding of the four components of the interventricular septum aids in making ventricular septal defects understandable.

The right ventricular (tricuspid) valve

The right ventricular valve, or tricuspid valve, has three leaflets, these being the septal, inferior and anterosuperior leaflets. The muscular support of the tricuspid valve is made up of the anterior muscle which is the largest and usually arises from the septomarginal trabeculation. The complex of chords supporting the anteroseptal commissure is dominated by the medial papillary muscle (of Lancisi), a relatively small muscle which arises either as a single band or as a small sprig of chords from the posterior limb of the septomarginal trabeculation. The inferior muscle, smallest of the three, is usually single, and may be represented by several small muscles. The most characteristic and distinguishing feature of the tricuspid valve is the direct attachment of chords from the septal leaflet into the septum. Chordal attachments to the septal surface are never seen in the morphologically left ventricle except when the tricuspid valve straddles and inserts on to the left ventricular septal aspect. The major feature of the outlet component of the right ventricle is that it is a complete muscular structure. The muscular shelf which separates the tricuspid and pulmonary valves in the roof of the ventricle is called the crista supraventricularis. It is made up of the inner curvature of the heart wall, called the ventriculoinfundibular fold.

The left ventricle

The exterior of the left ventricle is shaped like a cone. Internally, the left ventricle is demarcated by its fine trabeculations, which are numerous, fine muscular projections. There are two papillary muscles; the anterior lateral and the posterior medial. Notably, the papillary muscles do not attach to the septum. Since the left ventricular papillary muscles are large and arise only from the free wall surface, this makes left ventriculotomy difficult, except at the apex or at the high paraseptal area. In addition to the anterior descending branch of the left coronary artery, which externally marks the location of the anterior portion of the interventricular septum, anterior and posterior obtuse marginal branches of the left coronary artery course across the left ventricular free wall. Also known as diagonals, these branches supply the large papillary muscles and the adjacent left ventricular free wall. Normally there is little or no conal musculature beneath the aortic valve, which results in aortic-mitral fibrous continuity. When the great arteries are normally related, the non-coronary-left coronary commissure of the aortic valve sits directly above the middle of the anterior mitral leaflet. The noncoronary-right coronary commissure sits directly above the membranous septum, which in turn is located directly above the left bundle branch of the conduction system. The conal septum runs beneath the right coronary leaflet of the aortic valve. The foregoing are highly important landmarks for transaortic operative procedures.

As with the right ventricle, the left ventricle is described in terms of inlet, apical trabecular and outlet components.

The inlet component contains the mitral valve (left ventricular valve) and extends from the atrioventricular junction to the attachments of the prominent papillary muscles.

The apical trabecular portion is the most characteristic feature of the morphological left ventricle which contains the fine characteristic trabeculations. The smooth septal surface also helps in identification, since the morphologically left ventricle never possesses a septomarginal trabeculation or a moderator band.

The outlet portion of the morphologically left ventricle is distinguished by its abbreviated nature. Part of two leaflets of the aortic valve have muscular attachments to the outlet component. The remainder of the leaflets take origin from the fibrous tissue of the aortic root, part of this being the extensive area of fibrous continuity with the aortic leaflet of the mitral valve. It is the posterior aspect of the roof of the outlet, therefore, which is particularly short. There is no muscular segment of the ventriculoinfundibular fold in the left ventricle such as separates the arterial and the right ventricular valves.