Summary
The acknowledgment of tricuspid regurgitation (TR) as a stand-alone and progressive entity, worsening the prognosis of patients whatever its aetiology, has led to renewed interest in the tricuspid-right ventricular complex. The tricuspid valve (TV) is a complex, dynamic and changing structure. As the TV is not easy to analyse, three-dimensional imaging, cardiac magnetic resonance imaging and computed tomography scans may add to two-dimensional transthoracic and transoesophageal echocardiographic data in the analysis of TR. Not only the severity of TR, but also its mechanisms, the mode of leaflet coaptation, the degree of tricuspid annulus enlargement and tenting, and the haemodynamic consequences for right atrial and right ventricular morphology and function have to be taken into account. TR is functional and is a satellite of left-sided heart disease and/or elevated pulmonary artery pressure most of the time; a particular form is characterized by TR worsening after left-sided valve surgery, which has been shown to impair patient prognosis. A better description of TV anatomy and function by multimodality imaging should help with the appropriate selection of patients who will benefit from either surgical TV repair/replacement or a percutaneous procedure for TR, especially among patients who are to undergo or have undergone primary left-sided valvular surgery.
Résumé
Il est maintenant bien admis que l’insuffisance tricuspide (IT) significative est une entité propre qui aggrave le pronostic des patients, quelle que soit son étiologie, et ceci a conduit à un regain d’intérêt pour l’ensemble valve tricuspide-ventricule droit. La valve tricuspide est une structure complexe, dynamique avec une grande variabilité interindividuelle. Les différentes techniques d’imagerie moderne telles que l’échographie tridimensionnelle, l’imagerie par résonance magnétique et le scanner peuvent être utilisés en complément de l’imagerie bidimensionnelle classique pour analyser l’IT. Il est important d’analyser non seulement le degré de sévérité de l’IT, mais aussi les mécanismes à son origine, le mode de coaptation des feuillets valvulaires, le degré d’élargissement de l’anneau tricuspide et l’importance de la traction sur les feuillets valvulaires, ainsi que le retentissement hémodynamique sur l’oreillette et le ventricule droit. L’IT est dans la majorité des cas fonctionnelle et satellite d’une pathologie du cœur gauche et/ou d’une élévation des pressions pulmonaires. L’IT qui persiste et se majore dans les suites d’une chirurgie valvulaire du cœur gauche est une forme particulière à ne pas méconnaître car elle pose des problèmes de prise en charge et aggrave le pronostic des patients. Une description détaillée de l’anatomie et de la fonction de la valve tricuspide et de l’ensemble du cœur droit par l’imagerie multi-modalités devrait permettre d’affiner les critères de sélection des patients chez qui une correction de l’IT doit être envisagée, particulièrement parmi les patients candidats à une chirurgie du cœur gauche. De plus, ces éléments doivent entrer en ligne de compte dans le choix de la modalité thérapeutique optimale, à savoir réparation, remplacement valvulaire ou traitement par voie percutanée.
Background
The differences in anatomy and function between the mitral valve and the tricuspid valve (TV) have been recognized since the anatomical descriptions of the heart by William Harvey in 1628. Tricuspid regurgitation (TR) was first described by T.W. King in 1837, who showed that distension of the right ventricle (RV) with water induced considerable TV reflux. The authors thought that the TV, being weak, could act as a safety valve for the RV, and concluded that “the TV is designed to be(come) incompetent” . This statement was accepted as true, and for a long time the TV and TR were neglected while surgical techniques for treating left heart valvular diseases evolved. Unfortunately, TR turned out not to be as benign and physiological as had been thought. Some patients with progressive TR developed intractable right ventricular (RV) failure, especially those who had been operated on earlier for left heart valve diseases without concomitant TV surgery. These findings led to a renewed interest in the TV and, more globally, in the right heart valvular-ventricular complex. Multimodality imaging helped to better describe the morphology and function of the TV, and to fully assess the cause and impact of TR. This issue of TR is clinically important because it may presage a poor prognosis, and because surgical management of TR is far less codified than for left heart valves, and has poorer results and frustrating failures. This review will focus on the multimodality imaging of the normal TV, and on the pathophysiology, mechanisms and analysis of TR.
Anatomy
The TV is a complex entity of thin fibrous tissue, with three leaflets, chordae tendineae, papillary muscles and a fibrous annulus located between the right atrium (RA) and the RV . The normal area of the TV is 7–9 cm 2 , making it the largest of the four cardiac valves.
Tricuspid valve leaflets
The TV is nearly vertical and is oriented at approximately 45° to the sagittal plane, so that the margins of the valve are anterosuperior, inferior and septal . The three leaflets are the anterior, septal and posterior leaflets, which are thin and membranous, with commissures that appear more like indentations than true commissures. The anterior leaflet is the largest, with a semi-circular shape, and stretches from the infundibulum anteriorly to the inferolateral wall posteriorly. The posterior leaflet differs because of the presence of multiple scallops; it attaches along the posterior margin of the tricuspid annulus (TA) from the septum to the inferolateral wall. The septal leaflet is the smallest, and arises medially, directly from the annulus above the interventricular septum. The septal leaflet is characteristically inserted ≤ 10 mm more apically than the septal insertion of the anterior mitral valve leaflet.
Tricuspid subvalvular apparatus
The TV apparatus is similar to the mitral valve, but has greater variability. The tricuspid subvalvular apparatus consists of anterior, posterior and septal papillary muscles, and their true chordae tendineae. Each leaflet has chordal attachments to one or more papillary muscles. The anterior papillary muscle, the most prominent, provides chordae to the anterior and posterior leaflets, and the medial papillary muscle provides chordae to the posterior and septal leaflets. The anterior papillary muscle may have attachments to the moderator band. The posterior papillary muscle is smaller, and is missing in 20% of healthy subjects . The septal wall gives chordae directly to the anterior and septal leaflets, without a specific septal papillary muscle. In addition, there may be accessory chordal attachments to the RV free wall and to the moderator band. These multiple chordal attachments are important mediators of TR, as they impair proper leaflet coaptation in the setting of RV dysfunction and adverse remodelling.
Tricuspid annulus
The TV leaflets are attached to a fibrous annulus that is not as easy to define as it is around the mitral valve, although it remains identifiable . The septal leaflet, the least mobile of the three leaflets, has more support from the fibrous trigone than other leaflets. The normal TA is ovoid, and appears approximately one third longer in the mediolateral than in the anteroposterior direction . Furthermore, the TA is non-planar, with an elliptical saddle shape. The posteroseptal portion (close to the coronary sinus) and the anterolateral segments are the closest to the apex and the anteroseptal (close to the RV outflow tract and the aortic valve) and posterolateral segments are the closest to the RA, with a high-low distance of around 7 mm . The mean maximal TA circumference and area in healthy subjects are 12 ± 1 cm and 11 ± 2 cm 2 , respectively. The TA diameter varies according to the site of measurement, with reference values varying between 25 and 39 mm . From a dynamic point of view, the TA shows variability during the cardiac cycle, with an approximately 20% reduction in annular circumference with atrial systole . In pathological situations, as the septal leaflet is fixed between the fibrous trigones, the TA can only lengthen and dilate along the attachment of the anterior and posterior leaflets, resulting in a more circular shape; furthermore, it then becomes more planar with decreased high-low distance (< 4 mm) .
Echocardiographic imaging of the tricuspid valve complex
Assessment of the TV using transthoracic echocardiography (TTE) is challenging because of its unfavourable retrosternal position, the high variability of the TV anatomy and the difficulty in simultaneously visualizing all three leaflets in standard two-dimensional (2D) views; hence, all existing echocardiographic TV leaflet identification schemes are only partially correct. The use of an en-face view obtained by 2D TTE or, more easily, by transoesophageal echocardiography (TOE) and three-dimensional (3D) imaging is therefore recommended .
Tricuspid valve morphology
TV morphology can be evaluated by 2D TTE from the standard parasternal and apical RV views: RV inflow, parasternal short-axis, apical four-chamber and subcostal views ( Fig. 1 ). It is important to use all available views in 2D, colour and Doppler modes to obtain a complete evaluation of the valve by 2D TTE, and to rule out and not underestimate a flail leaflet or a localized abnormality. TOE helps to image the TV in multiple views and planes, although the incremental value of TOE is usually less for the TV than for the mitral valve. The deeper gastric view in a longitudinal plane (transgastric RV inflow view) often provides a nice long-axis visualization of the TV and the subvalvular apparatus. An en-face view is also quite easily obtained from the gastric approach ( Fig. 2 ).
Finally, 3D imaging is used mostly in TTE, from either the parasternal or the apical approach, with real-time zoom mode or after acquisition of a full-volume data set; this allows the display of the TV surgical view and visualization of all the components of the TV, enabling assessment of their dynamic spatial relationships and anatomical continuity ( Fig. 3 ). In patients with good echogenicity, assessment of TV anatomy and function with 3D TTE is often feasible, even if it is more difficult and requires more experience compared with mitral valve 3D evaluation . 3D TTE has the potential advantage of evaluating complex TV anatomy in organic TR, as may be encountered in Ebstein’s anomaly, carcinoid heart disease and TV prolapse . Recently, several publications have highlighted the usefulness of 3D TTE for detecting the location of the lead and its relationship to valvular leaflets and significant TR in patients with intracardiac devices . 3D TTE also enables us to locate the anatomical regurgitant orifice of TR and to measure the vena contracta, which was found to be more often ellipsoid than circular . Finally, the extent of TV tethering may be quantified with 3D echocardiography, in terms of the tenting volume and tenting angle of the three leaflets .
Tricuspid annulus
TA size and function play pivotal roles in the genesis of TR, and accurate analysis of the TA is required to determine the need for a combined procedure on the TV in patients undergoing cardiac surgery for left-sided valve diseases. However, normative data about TA diameter and function are limited, and are still a matter of debate.
As for the description of TV morphological details, 2D TTE has some limitations in the quantification of the TA diameter . In fact, the 2D view and the timing during the cardiac cycle of when the TA should be measured remain controversial . If 2D TTE is the only assessment tool available, the apical four-chamber view seems to be preferred, because of better interobserver agreement compared with other views , and it is the method recommended by current guidelines for making a decision about TV repair . In this view, the normal TA diameter in adults is 28 ± 5 mm, measured in diastole. The TA can also be measured from the mid-oesophageal four-chamber view in 2D TOE ( Fig. 4 ).
To assess cyclic changes in TA diameter during systole and diastole, TA fractional shortening may be calculated from the apical four-chamber view between the insertion sites of the septal and anterior TV leaflets at end-diastolic and end-systolic times; the normal value is around 25% .
The analysis of TA geometry and size with the use of 3D echocardiography shows interesting results and has good feasibility ( Fig. 4 ). Various measurements are reported, including major and minor TA diameters, TA fractional shortening, TA area and TA fractional area change. TA diameter is usually underestimated by 2D TTE compared with 3D TTE, and it seems necessary to re-establish normal TA values with 3D imaging. Furthermore, more complicated analysis of the non-planarity of TA can also be performed from 3D acquisitions, which have potentially important mechanistic and therapeutic implications for TV repair .
Value of other imaging modalities in the assessment of the tricuspid valve
Electrocardiograph-gated computed tomography (CT) scans and cardiac magnetic resonance (CMR) imaging are interesting adjuncts in the evaluation of the TV.
Computed tomography scans
The dynamic data set acquired in patients who undergo CT scans can be used to assess RV function. The thin slice thickness facilitates increased accuracy of RV delineation, and allows for precise recognition of the valvular borders. For the TV, only static anatomical information is relevant . Measurements of TA diameter and assessment of lack of coaptation of leaflets are generally simple on good-quality four-chamber cardiac CT images ( Fig. 5 ). Evaluation of TV prolapse is easier on coronal views. In functional TR (FTR), CT scans have been used to measure right atrial (RA) and RV volumes, TA diameters and areas, the distance between each commissure, the tethering angle of each leaflet and the tethering height, with these various indexes having interesting prognostic value . Some other potential benefits of CT scans are the detection of valvular calcifications, the evaluation of TV annuloplasty ring dislodgement and the assessment of the spatial relationship between RV pacemaker leads and related TR .
