The tricuspid valve is part of a complex functional system that also includes the right atrium, the right ventricle, and the pulmonary circulation.
With the tricuspid valve exposed, the surgeon observes sequentially the right atrial cavity, the atrio-valvular junction, the tricuspid leaflets, and the subvalvular apparatus.
RIGHT ATRIUM
The right atrium consists of a curved posterior groove continuous with the superior and inferior vena cavae, a flat interatrial septum, a trabeculated dome, and the tricuspid valve ( Fig. 17-1 ).
The posterior groove is a smooth wall separated from the trabeculated dome by a ridge of muscle, the crista terminalis (a) , that extends from the superior vena cava to the inferior vena cava. The interatrial septum has a central shallow depression, the fossa ovalis (b) , and the orifice of the coronary sinus (c) guarded by a valvelike fold, the thebesian valve (d) . Another valvelike fold, the Eustachian valve (e) , guards the anterior extremity of the inferior vena cava. Both the thebesian valve and the Eustachian valve vary greatly in size and may be absent. The thebesian valve may be perforated by multiple holes, making it difficult to identify the true orifice of the coronary sinus.
The tendon of Todaro (f) is a fibrous structure formed by an extension of both the Eustachian and thebesian valves. This tendon in conjunction with the orifice of the coronary sinus and the base of the septal leaflet of the tricuspid valve delineate the triangle of Koch (inset) . This triangle helps the surgeon locate two important components of the conduction system: the atrioventricular node (g) , at the base of the triangle, and the bundle of His (h) , which extends towards the apex of the triangle. The bundle of His crosses the septal segment of the tricuspid annulus approximately 5 mm from the anteroseptal commissure. At this point the bundle pierces the membranous septum (i) or bypasses it posteriorly to reach the crest of the muscular septum, where it bifurcates into multiple branches.
THE ATRIO-VALVULAR JUNCTION
The junction between the right atrium and the tricuspid valve is usually well-delineated by the change in color and structure ( Fig. 17-2 ) at the junction. While the atrium is pale pink and trabeculated, the valve tissue is yellowish, smooth, and regular. In pathological valves, the atrio-valvular junction can be masked by endothelial thickening or jet lesions. In these circumstances, the atrio-valvular junction can be identified by mobilizing the leaflets upward and downward. This mobilization helps visualize the hinge of the leaflets, which in turn allows location of the annulus fibrosus, which attaches three leaflets—anterior, posterior, and septal—separated by three commissures—anteroposterior, posteroseptal, and anteroseptal. The annulus fibrosus is not visible from an atrial view because it is deeper and 2 mm external to the hinge (inset a) . This characteristic has important surgical implications: sutures passed only through the hinge would compromise the motion of the leaflets and would not reach the resistant fibrous body of the annulus; on the contrary, sutures placed 2 mm external to the hinge and oriented towards the ventricle reach the fibrous body of the annulus and preserve the free motion of the leaflets.
The annulus of the tricuspid valve is not visible from an atrial view. It can be located by knowing that it is deeper and 2 mm external to the leaflet hinge.
The annulus is a heterogeneous, almost virtual, structure composed of intermixed fibrotic and elastic fibers in continuity with the leaflet tissue, the atrium, and the ventricle. The posteromedial fibrous trigone reinforces the tricuspid annulus in the area of the anteroseptal commissure. From the anteroseptal commissure, four annular segments can be described in a clockwise fashion (inset b) . The “aortic segment” is defined as the portion of the annulus localized at the proximity of the neighboring aortic root. This identification is critically important because inadvertent suture placement in this area during annuloplasty may lead to aortic valve or sinus injury. The “anterior segment” corresponds to the right outflow tract. It attaches the remaining part of the anterior leaflet and the anteroposterior commissural leaflet. The “posterior segment” attaches the posterior leaflet and the posteroseptal commissural leaflet. The “septal segment” corresponds to the septum and attaches the septal leaflet and the adjacent anteroseptal commissural leaflet (inset c) . The septal attachment of the tricuspid valve is at a lower level than the septal attachment of the mitral valve (inset d) . As a result, a portion of the interventricular septum, the membranous septum, separates the left ventricle from the right atrium (inset d) , a configuration that explains the left ventricular to right atrial shunts seen in congenital malformations.
The annulus has a “wave form” instead of a planar configuration because of the bulging of the right outflow tract, the nonlinear anterior and septal segments, and the lower position (towards the apex) of the attachment of the septal leaflet ( Fig. 17-3 ). The two lowest positions (apically located) of the wave form are the anteroseptal commissure, where the anterior leaflet attachment overrides the attachment of the septal leaflet (a) , and the posteroseptal commissure (b ). The highest position is the portion of the aortic segment that corresponds to the right outflow tract (c) . The importance of the gap between the level of attachment of the anterior and septal leaflets at the anteroseptal commissure depends upon individual variations and valve pathology. In pathological conditions and long-lasting tricuspid regurgitation, the annulus tends to be more planar.
The shape of the “annulus” varies throughout the cardiac cycle ( Fig. 17-4 ). During diastole, it is grossly circular (a) with a transverse diameter barely larger than the anteroseptal diameter. During systole, the atrio-valvular junction has an ovoid shape with the transverse diameter significantly larger than the anteroseptal diameter (b) ( Table 17-1 ).
| Orifice area | Diastole: 7.1 ± 1.3 cm 2 |
| Systole: 5 ± 1 cm 2 | |
| Circumference | 9.3 ± 0.9 cm |
| % reduction of circumference | 13 ± 1% |
The concentric reduction of the tricuspid valve orifice during systole results from the contraction of the right and the left ventricles and the bulging of the aortic root. The contraction of the orifice is not uniform because of the following underlying structures: the septum, the right ventricle, and the aorta. In addition to concentric deformation, the tricuspid annulus displays three-dimensional (3D) deformation at the junction of its different segments as well as rotational movement with anterior displacement of its anterior segment in relation to its septal segment (c) . This results in a helicoidal shape with a 3- to 7-mm gap at the anteroseptal commissure. These spatiotemporal deformations vary, depending upon right and left ventricular contractility, pressures, and pathological processes.
Two anatomical structures close to the annulus are at risk during tricuspid valve surgery ( Fig. 17-5 ): (1) the non–coronary sinus of Valsalva, especially the commissure between the non–coronary and right coronary leaflets of the aortic valve (asterisk); (2) the bundle of His, which crosses the septal leaflet attachment 3 to 5 mm from the anteroseptal commissure and then either perforates or circumscribes the membranous septum before it bifurcates into two branches.
THE LEAFLETS
The tricuspid valve consists of three leaflets, which are the opening and closing structures of the tricuspid orifice. Optimal opening and closing implies the integrity and the free motion of the hinge of the leaflets. A perfect closure requires a precise fit between the shapes and surface areas of both the leaflets and the annulus.
The three leaflets of the tricuspid valve have different sizes and shapes ( Fig. 17-6 ). The anterior leaflet is larger than the posterior leaflet, which is larger than the septal leaflet ( Table 17-2 ). The anterior leaflet is primarily attached to the right ventricular outflow tract, the posterior leaflet to the muscular wall of the right ventricle, and the septal leaflet to the septum.
