The ventricles are not mirror-image pumping chambers. They differ in position, geometry, wall structure, trabeculation, valvar support, and the arrangement of their inlet and outlet components. The morphologically right ventricle is recognised by coarse trabeculations, the septomarginal trabeculation, and the moderator band, whereas the morphologically left ventricle has finer apical trabeculations, two major papillary muscles, and fibrous continuity between its inlet and outlet valves. These features remain valid irrespective of the position of the heart and are therefore more reliable than chamber location alone. [1-3]

For electrophysiologists and structural interventionists, ventricular anatomy is best understood in its attitudinally appropriate orientation. The spatial relations of the ventricular septum, arterial roots, atrioventricular valves, papillary muscles, and intracavitary trabeculations determine how catheters, pacing leads, prostheses, and ablation lesions approach the myocardium.

1  Spatial relationships and septum

In the normally positioned heart, the right ventricle is the most anterior cardiac chamber. It extends from the right atrioventricular junction towards the left and superiorly, wrapping around the anterior and rightward aspects of the left ventricle. The left ventricle is positioned more posteriorly and to the left, forming most of the left cardiac border and much of the diaphragmatic surface. When viewed in short axis, the left ventricular cavity is approximately circular, while the right ventricular cavity is crescentic because it curves around the ventricular septum. In frontal views, the right ventricle has a more triangular configuration and the left ventricle a more conical form. [1-3]

The ventricular long axes are oblique rather than vertical. The right ventricular inlet is directed from right and posterior towards left and anterior, while its outlet turns superiorly and leftward towards the pulmonary trunk. The left ventricular inlet is posterior and leftward relative to the outflow tract. Blood entering through the mitral valve turns around the aortic leaflet of the valve before reaching the subaortic vestibule. Consequently, fluoroscopic or tomographic projections should not be interpreted as if both ventricles occupied parallel sagittal planes. [2,3]

The muscular ventricular septum is a curved three-dimensional structure shared by both ventricles. Under normal loading conditions it is convex towards the right ventricular cavity. Its smooth left ventricular surface contrasts with the prominent muscular trabeculations on its right ventricular aspect. The septal myocardium cannot be divided accurately into simple flat anterior and posterior segments; attitudinal terms such as superior, inferior, anterior, and inferoseptal more faithfully describe its true position. [1,2]

At the ventricular base, the muscular septum is continuous with the small membranous septum. The hinge of the septal tricuspid leaflet divides the membranous septum into atrioventricular and interventricular components. Because the tricuspid valvar hinge is displaced apically relative to the mitral hinge, part of the region traditionally called the atrioventricular septum is not a true muscular septum separating atrial from ventricular cavities. It includes the inferior pyramidal space and the atrioventricular component of the membranous septum. Recognition of these relations is essential when mapping paraseptal pathways or the atrioventricular conduction axis. [2,3]

The ventricular conduction axis emerges through the central fibrous body and continues along the crest of the muscular septum. The right bundle branch descends on the right ventricular septal surface and commonly reaches the moderator band, while the left bundle branch spreads as a broad subendocardial sheet on the left ventricular septal surface. The course is variable and should not be inferred from a single external landmark.

2  The tricuspid and pulmonary valves

The tricuspid valve guards the inlet of the morphologically right ventricle. In attitudinal orientation, its major leaflets are described as septal, anterosuperior, and inferior. The junction is not a planar circular ring. Its hinge line is irregular and changes level around the ventricular base, with the septal leaflet attached more apically than the aortic leaflet of the mitral valve. The septal leaflet has chordal attachments to the ventricular septum, a feature that helps distinguish the tricuspid from the mitral valve. [1,2]

The tricuspid subvalvar apparatus is supported by papillary muscles of variable size and position. The anterior papillary muscle is usually the most prominent and is closely related to the parietal attachment of the moderator band. Septal papillary muscles may be multiple or represented by direct chordal attachments. The inferior papillary muscle is also variable. Chords frequently cross between adjacent leaflet territories, so the leaflets cannot be understood as isolated flaps supported by a single papillary muscle.

The pulmonary valve guards the outlet of the right ventricle at the distal end of the muscular infundibulum. The tricuspid and pulmonary valves are separated by the supraventricular crest, so there is no fibrous continuity between them. This muscular separation is a defining feature of the morphologically right ventricle and contrasts with the fibrous continuity between the aortic and mitral valves in the left ventricle. [1-3]

The pulmonary root is a short three-dimensional structure extending from the virtual basal plane, defined by joining the nadirs of the leaflet attachments, to the sinutubular junction. Each leaflet has a semilunar hinge and the three attachments form a crown-like pattern rather than a discrete circular annulus. Triangular extensions of the outflow tract wall between adjacent leaflets reach towards the sinutubular junction. Proximally, crescents of right ventricular myocardium are incorporated within the bases of all three pulmonary sinuses. [1,4]

The distal right ventricular outflow tract and the pulmonary root are common sites of idiopathic ventricular arrhythmias. Targets may lie below the leaflet hinges, within myocardial crescents at the sinus bases, near a commissure, or occasionally above the anatomical ventriculoarterial junction when myocardial extensions persist into the arterial root. The pulmonary and aortic roots are immediately adjacent, and mapping one root may record activation arising from the other. Coronary arterial proximity must be established before energy is delivered in this region. [1,4]

3  The mitral and aortic valves

The mitral valve guards the inlet of the morphologically left ventricle. It has an aortic leaflet and a mural leaflet, traditionally termed anterior and posterior. These positional labels are convenient but incomplete because the valve is obliquely oriented within the heart. The mural leaflet is divided by indentations into scallops, whereas the larger aortic leaflet forms part of the boundary between the inlet and outlet portions of the ventricular cavity. [1-3]

The mitral hinge is saddle-shaped and supported by a variable mixture of fibrous tissue and myocardium rather than by a complete uniform fibrous annulus. The two major papillary muscles are located inferomedially and superolaterally when described in attitudinally appropriate fashion. Each papillary muscle provides chordal support to both leaflets. The spaces between papillary muscles, chordae, and ventricular wall provide pathways for inflow and are relevant when devices or catheters traverse the left ventricular cavity.

The aortic leaflet of the mitral valve is in fibrous continuity with the left coronary and non-coronary leaflets of the aortic valve. This aortic-mitral continuity forms the fibrous roof of the left ventricular outflow tract and is anchored at its ends by the right and left fibrous trigones. The outflow tract is completed by the muscular ventricular septum. There is therefore no muscular structure equivalent to the right ventricular supraventricular crest separating the mitral and aortic valves. [1-3]

Like the pulmonary root, the aortic root extends from a virtual basal plane to the sinutubular junction and supports three semilunar leaflets. Their crown-shaped hinge lines cross the anatomical ventriculoarterial junction. Ventricular myocardium is incorporated into the bases of the right and left coronary sinuses, whereas the base of the non-coronary sinus is fibrous. The interleaflet triangles are extensions of the left ventricular outflow tract towards the sinutubular junction and should not be mistaken for valvar commissures or for parts of a circular annulus. [3,4]

The region between the right and non-coronary aortic sinuses is closely related to the membranous septum and the penetrating atrioventricular bundle. The superior fascicle of the left bundle may approach the nadir of the right coronary leaflet, depending on rotation of the aortic root, the amount of supporting outflow myocardium, and the depth of the inferoseptal recess. These relations are important during transcatheter aortic valve implantation and when ablating arrhythmias from the aortic sinuses or adjacent left ventricular outflow tract.

4  The components of the ventricles

Both ventricles can be analysed as having inlet, apical trabecular, and outlet components. These components are descriptive regions within a continuous chamber, not independent anatomical compartments. Their boundaries are particularly evident in the right ventricle, where prominent muscular structures separate and connect them. In the left ventricle, inflow and outflow share a common space and are separated mainly by the aortic leaflet of the mitral valve. [1-3]

The right ventricular inlet extends from the tricuspid junction to the papillary muscles and their chordal attachments. The apical component contains coarse trabeculations. The outlet, or infundibulum, is a complete muscular sleeve supporting the pulmonary root. Its posterior and inferior boundary is the supraventricular crest, which occupies the roof of the right ventricle between the tricuspid and pulmonary valves. [1,2]

The septomarginal trabeculation is a prominent Y-shaped muscular structure reinforcing the right ventricular septal surface. Its anterior limb extends towards the infundibulum and gives rise to multiple septoparietal trabeculations that run towards the parietal wall of the outflow tract. Its posterior limb extends towards the tricuspid valve. The body descends towards the apex and may continue through the moderator band to the anterior papillary muscle and parietal ventricular wall. [1,5]

The moderator band varies markedly in thickness, length, number of muscular strands, and level of parietal insertion. It should not be equated with the entire septomarginal trabeculation. It is a bridge from the septal complex towards the anterior papillary muscle or free wall and commonly carries a distal ramification of the right bundle branch. Besides serving as a morphological marker of the right ventricle, it can be an arrhythmic substrate and may complicate catheter stability, imaging, and device positioning. [1,5]

The left ventricular inlet surrounds the mitral subvalvar apparatus. Its apical component has finer interlacing trabeculations than the right ventricle and merges with the compact myocardium of the ventricular wall. The outlet is relatively short and partly fibrous. It lies between the aortic leaflet of the mitral valve and the ventricular septum, continuing superiorly into the aortic root. The smooth septal surface provides no equivalents of the septomarginal trabeculation, moderator band, or septoparietal trabeculations. [1-3]

These morphological differences permit ventricular identification in congenital malformations and in complex imaging planes. They also explain why an arrhythmia apparently located at the same fluoroscopic level may arise from different substrates: the right ventricular infundibulum, aortic sinuses, left ventricular summit, papillary muscles, moderator band, or septal conduction system may be separated by only a few millimetres in three-dimensional space.