“The tricuspid valve is designed to be (come) incompetent”
King TW, 1837.
Van de Spuy JC, 1965.
Normal tricuspid valve function depends on interactions between fibrous annulus, leaflets, papillary muscles, chordae tendinae and adjacent right atrial and right ventricular myocardium. Any congenital or acquired disorder of these individual components of the tricuspid valve complex will result in valve incompetence.
The tricuspid valve anatomy shows greater variability than the anatomy of the mitral valve.
The valve position
In normal heart, the tricuspid valve is located slightly closer to the apex than the mitral valve. The space in between the septal insertion of the tricuspid valve and the septal insertion of the anterior leaflet of mitral valve belongs to the membranous septum that separates the left ventricle from the right atrium. The displacement index is calculated by dividing the linear distance between septal insertions of the tricuspid and mitral valves by the patient’s body surface area [Figure 1].
The excessive apical displacement of the valve, i.e. the displacement index > 8 mm/m2, is associated with the Ebstein’s anomaly. On cross section, the tricuspid valve is located obliquely behind the aortic valve which has the central position. The pulmonary valve is positioned anterior, superior and slightly to the left of the aortic valve [Video 1 and Video 2].
The tricuspid valve complex components
The valve consists of three leaflets, named after their positions: anterior, posterior and septal [Figure 2]. The normal valve area in adults is 4-6 cm2. Since the valve is nearly vertical (approximately 45 degrees to the sagittal plane), the anterior leaflet is also referred to as superior, while the posterior leaflet has also been called inferior.
The anterior leaflet (also called anterosuperior and infundibular) is the largest leaflet; the septal leaflet (also medial) is usually the second largest leaflet, while the posterior leaflet (also called inferior and marginal) is most frequently the smallest of the three. The posterior leaflet often has multiple scallops, while in some patients, it is not possible to discern clear divisions between the anterior and posterior leaflets. As a consequence, the tricuspid valve has been described as having only two, or more than three leaflets [Video 3 and Video 4].
The papillary muscles and tendinous chords
The variability of the papillary muscles is a normal characteristic of the tricuspid valve. They can number from 2-9, but usually 2 or 3 papillary muscles can be seen: the anterior, which is the most prominent, and posterior, which is often bifid or trifid. The septal papillary muscle is the least prominent and sometimes can be even absent, with the multiple chordal attachments arising directly from the ventricular wall [Figure 3A, Video 5].
The papillary muscles usually provide chords for two leaflets, except for the anterior papillary muscle which supports only the anterior leaflet in approximately half of the cases [Video 6 and Video 7].
The anterior and septal papillary muscles are connected by the moderator band, which can be best seen in the apical 4-chamber view [Video 8]. This is a normal anatomical feature, (the septomarginal trabecula carrying the part of the right bundle of the conduction system to the anterior papillary muscle) and should not be confused with the right ventricular mass.
A tethering of the tricuspid valve leaflets by tendionous chords usually occurs with the dilation of the tricuspid annulus. Nevertheless, the tethering caused by short, aberrant chords can be the sole mechanism of congenital tricuspid regurgitation. Congenital aberrant chords are not the frequent cause of tricuspid regurgitation, but the severity of regurgitation due to aberrant cords may require surgical correction during the childhood. Both congenital and acquired tethering of the tricuspid valve leaflets will result in impaired mobility, incomplete coaptation and apical displacement of the regurgitant jet origin [Figure 3B, Video 9]. The regurgitant jet of physiological tricuspid regurgitation originates at the level of the annulus.
Figure 3B - Video 9
A real-time three dimensional echocardiography demonstrated that the normal tricuspid annulus is a saddle-shaped structure with the highest points in an antero-posteriororientation and the lowest points in amedio-lateral orientation [Figure 4]. In patients with functional tricuspid regurgitation, the annulus dilates along the right ventricular free wall and becomes more circular and planar.
The tricuspid annulus is a very dynamic structure and can change markedly with loading conditions. Even during the cardiac cycle, there is approximately 19% of reduction in annular circumference with atrial systole. Normal tricuspid valve diameter in adults is 28 ± 5 mm, as measured in the apical 4-chamber view. Significant tricuspid annular dilatation is defined by a diastolic diameter >21 mm/mm2 (>35 mm). Of note, it has been demonstrated that the annular diameter, as measured from the apical 4-chamber view, underestimates both major and minor annular dimensions.
Echocardiographic assessment of the tricuspid valve anatomy
Echocardiographic assessment of the tricuspid valve is challenging due to unfavorable retrosternal position of the valve and the inability to simultaneously visualize all three leaflets in standard transthoracic views.
The tricuspid valve morphology can be evaluated by 2D echocardiography from the standard parasternal right ventricular inflow view, parasternal short-axis, apical 4-chamber and subcostal views. Nevertheless, the leaflet identification from these views has become a matter of great controversy, as data from most influential textbooks, current guidelines and 3D echo studies are partially conflicting [Figure 5]. Therefore, the leaflet identification should ideally be done from the en-face view of the valve.
An en-face view of the tricuspid valve can be achieved by three-dimensional echocardiography (both atrial and ventricular perspective) and by two-dimensional echocardiography, from the modified subcostal view (only ventricular perspective) [Figure 6]