Anatomy of mitral valve
The mitral valve, located between the left atrium and left ventricle, is a functional complex that relies on normal morphology, geometrical relations and function of all its constituents: the left atrium, the mitral annulus, the mitral leaflets, the subvalvular apparatus (tendinous chords and papillary muscles) and the left ventricle. Abnormalities in any of these constituents may lead to valvular dysfunction (stenosis or regurgitation). The mitral valve has a triple function:
- it regulates blood flow towards the left ventricle during diastole at low pressure gradient while preventing systolic backflow towards the left atrium
- it contributes to the formation of the left ventricular outflow tract during systole and
- its integrity is essential to maintain normal size, geometry and function of the left ventricle.
Knowledge of mitral valve anatomy is mandatory to gain insights in pathophysiologic mechanisms of mitral valve disease, to augment diagnostic performance and for subsequent clinical or therapeutic decision-making, procedural guiding and follow-up of patients with mitral valve disease. This web-page aims to provide a practical overview of mitral valve anatomy evaluation using transthoracic and transoesophageal echocardiography, including 2-dimensional and 3-dimensional techniques.
Schematic mitral valve The aortic valve is identified anteromedially to the valve; the left atrial appendage is located superiorly and laterally. The mitral valve is implanted between the left atrium and ventricle with a slightly oblique orientation.
Mitral annulus The posterior part of the mitral annulus is not enforced and rather discontinuous (making it prone to dilatation). The antero-posterior diameter forms the minor axis and the inter-commissural distance refers to the major axis. The mitral annulus is a 3-dimensional saddle-shaped structure (hyperbolic paraboloid) with highest points formed by the anterior and posterior annulus and nadirs at both commissures. Assessment of mitral annular diameters helps identifying the mechanism of valvular insufficiency, is useful to determine the type of mitral valve intervention in case of dysfunction and is of interest to size mitral valve prosthesis or annuloplasty ring.
Correct anatomic imaging planes are mandatory for accurate measurements of the mitral annulus. Conventionally, a parasternal long axis transthoracic view has been advocated for measuring minor annular diameter, however, this may slightly overestimate the true diameter. More appropriate measurement of the minor axis (antero-posterior diameter) of the mitral annulus can be performed at end-systole during transthoracic echocardiography in the apical long axis view (3-chamber view) or its transoesophageal equivalent, found at mid-oesophageal level with 135° tilt of the probe, as shown in figure 3. A minor axis diameter ≤35 mm or an annular diameter to mid-diastolic anterior mitral valve leaflet length ratio of 1.3 is considered the upper limit of normal. The major axis (inter-commissural diameter) of the mitral annulus is found at a bicommissural 2-chamber transthoracic echocardiographic view (when P1-A2-P3 mitral leaflet scallops are visualized, cfr infra) or a mid-oesophageal bicommissural view at 45-60° during transoesophageal echocardiography, as depicted in figure 3.
As indicated in figure 1 and 2, the mitral valve consists of an anterior and posterior leaflet that converge at the posteromedial and anterolateral commissures. The semi-circular anterior leaflet is anchored anteriorly to one third of the mitral annulus and the quadrangular posterior leaflet is attached posteriorly to the remaining two thirds of the posterior annulus. The largest part of the atrial floor is formed by the anterior mitral valve leaflet. Normal leaflets are thin and pliable structures with a thickness <5 mm. Normal mitral valve area is 4 to 5 cm2. Based on the presence of (usually) two indentations, the posterior leaflet is divided into a lateral P1 scallop (close to the left atrial appendage and the anterolateral commissure), a larger central P2 scallop and a medial P3 scallop (close to the posteromedial commissure). Arbitrarily opposing segments of the anterior leaflet are called A1,A2 and A3 scallops respectively. Of note, indentations never reach the annulus in a normal mitral valve. Correct anatomical identification of diseased mitral valve leaflet segments is of paramount importance for diagnostic, therapeutic and prognostic purposes in patients undergoing mitral valve surgery or percutaneous mitral intervention.
Figure 4 and 5 show segmental mitral valve anatomy on transthoracic echocardiography. The A2-P2 scallops of the mitral valve are seen on conventional parasternal long axis view and apical long axis (3-chamber) view. Slightly inferior tilting or superior translation of the probe in parasternal long axis view reveals the A1-P1 level (typically the tricuspid valve is seen as well). Slightly superior tilting or inferior translation of the probe reveals the A3-P3 scallops of the mitral valve (the left ventricle typically presents foreshortened). Subsequently, and shown in figure 2, a basal short axis view of the mitral valve reveals all different mitral valve scallops (A1-A2-A3 and P1-P2-P3) and both commissures with A1-P1 scallops typically closest to the probe. The apical 4-chamber view usually reveals medially the A3-A2 scallops and laterally the P2-P1 scallops. Finally, a bicommissural 2-chamber view reveals the P3-A2-P1 scallops. Of note, accurate evaluation of the anterolateral or posteromedial commissures using transthoracic echocardiography is extremely challenging.
Figure 6 and 7 depict systematic transoesophageal echocardiographic evaluation of mitral valve leaflet segments. Different approaches for systematic mitral valve leaflet evaluation during transoesophageal echocardiography have been reported, depending on whether one starts in a transgastric or a mid-oesophageal position. A mid-oesophageal 4-chamber view usually reveals A2-P2 scallops at 0° probe tilt. Slight withdrawal and/or anteflexion of the tip of the probe will show A1-P1 scallops (when the aortic valve is seen as well). The A3-P3 scallops are identified by slight advancement and/or retroflexion of the tip of the probe. Tilting the probe to 45° usually visualizes A3-A2 and (P2)-P1 scallops. Further tilting to 45-60° will show the bicommissural view with from lateral to medial P1-A2-P3 scallops. Commissural leaflet evaluation can be performed starting from the bicommissural view; clockwise rotation and anticlockwise probe rotation will point out the posteromedial and the anterolateral commissure respectively. Subsequent 90° tilting depicts scallops A1-A2 and P3 and tilting the probe to 135° will reveal the A2-P2 scallops where mitral leaflet length can be measured at mid-diastole. Finally, advancing the probe to a transgastric 0° position with slight retroflexion of the probe tip will lead to a short axis view of the mitral leaflet, as depicted in figure 2, showing all different anterior and posterior leaflet scallops as well as both commissures. Of note, in this view A3-P3 scallops will be closest to the probe as the mitral valve is visualized from its inferior portions.
In addition to correct identification of mitral leaflet segments, evaluating functional mitral anatomy is mandatory. The normal anterior and posterior mitral leaflet close (coapt) from their lateral to medial portions (coaptation line) during systole, preventing backflow of blood towards the left atrium. The tips of both leaflets actually overlap at the ventricular side, maintaining a sort of coaptation reserve to prevent development of mitral regurgitation; a coaptation length of 5 mm is considered a minimum to ensure adequate leaflet function. As indicated in figure 8, normal leaflet coaptation is usually situated at the ventricular side and occasionally on the level of the mitral annulus. Systolic bulging of leaflet bodies into the atrium above the annular level but with preserved coaptation point is referred to as `billowing`. `Prolapse` is used for a similar phenomenon, but with abnormal coaptation point of the affected leaflet segment(s) above the annular level. `Flail` is an extreme form of the former, indicating systolic leaflet tip reversal towards the left atrium above the annulus, due to chordal rupture for instance. Leaflet bodies that do not show systolic bulging and maintain a rather fixed position during systole or the complete cardiac cycle are referred to as `restricted`. Of note, due to the 3-dimensional shape of the mitral annulus, leaflet prolapse should not be diagnosed on a transthoracic 4-chamber view.
The subvalvular apparatus consists of the papillary muscles and tendinous chords. The anterolateral papillary muscle originates from the apicolateral third of the myocardium. The posteromedial papillary muscle is implanted on the middle third of the inferior wall. Several dozens of tendinous chords originate form these papillary muscles and attach to the ipsilateral half of the anterior and posterior mitral leaflets. Commissural leaflets only relate to tendinous chords from the respective papillary muscle located beneath. Primary and secondary chords attach to the leaflet tips and ventricular side of the mitral leaflet bodies respectively. Few tertiary chords attach directly from the posterior left ventricular wall to the posterior mitral leaflet. Strut chords, finally, are thicker secondary chords attached to the anterior mitral leaflet only and are part of a ventriculo-valvular fibrous loop, essential to maintain normal left ventricular shape and function. Synchronous papillary muscle activity and preserved geometric relations (cfr. infra) of the subvalvular apparatus are key to maintain normal valve function. Evaluation of the subvalvular apparatus helps to determine the pathophysiologic mechanism of valvular dysfunction and to plan subsequent surgical or interventional management strategies that aim to restore normal mitral valve geometry.
Echocardiographic evaluation of the subvalvular apparatus can be performed on mid-ventricular short axis views at parasternal or transgastric position during transthoracic and transoesophageal echocardiography respectively, as indicated in figure 9. The anterolateral and posteromedial papillary muscles are easily identified at the lateral and inferior ventricular wall segments respectively. A bicommissural view is used for evaluation of both papillary muscles and tendinous chords. During transoesophageal echocardiography a transgastric 2-chamber view at 90° is ideally suited for evaluation and assessment of the length of the tendinous chords, as indicated in figure 9.
Mitral geometric indices
Normal mitral leaflet function relies intimately on geometric relations of the mitral valve complex. Local or global left ventricular remodeling leads to papillary muscle displacement with subsequent tethering of mitral leaflets resulting in coaptation failure, a situation referred to as functional mitral regurgitation. Identification of these geometric indices on echocardiography in heart failure patients is of interest as it determines mitral regurgitation severity and has been related to success prediction of mitral valve repair. Several parameters reflecting tethering have been described. The end-diastolic distance between the center of the mitral annular plane and the coaptation point is known as the coaptation distance or height and can be evaluated in parasternal long axis transthoracic view or a long axis transoesophageal view at 135° tilt on echocardiography, as indicated in figure 9. Normal coaptation height or distance is ≤ 10 mm. The triangular zone comprised between the mitral annulus, both leaflets and the coaptation point is referred to as tenting area. The anterior and posterior angles of the anterior and posterior mitral leaflet and the mitral annulus can be quantified in addition. Papillary muscle displacement is represented by additional parameters. The systolic interpapillary muscle distance at the tips or base of the papillary muscles in a short axis view has been suggested, as shown in figure 9. The length of the anterolateral or posteromedial papillary muscle tip and the mitral annulus or its respective angle formed in between can be measured in addition. A parasternal long axis transthoracic view or a true bicommissural 2-chamber view are suited to assess these parameters.
Three-dimensional echocardiography of mitral valve
Real-time 3-dimensional echocardiography of the mitral valve allows easy identification of different anatomical segments of the mitral valve, including both commissures. As shown in figure 2, the `en face` view of the mitral valve can be constructed and refers to exposure of the mitral valve from the atrial perspective, similar to the surgeons view during mitral valve surgery. Therefore this technique is very helpful for segmental morphological or functional analysis, in particular for novice readers and for the dialogue between the echocardiographer and the surgeon. As the mitral valve is comprised within a 3-dimensional pyramidal dataset, the valve can additionally be evaluated off-line in infinite 2-dimensional reformation planes. Standardized acquisition protocols have been reported; a 3-dimensional zoom mode is ideal to study mitral leaflet morphology. A full volume acquisition including the left ventricle is more appropriate when the entire mitral complex needs evaluation. Of note, (3-dimensional) transoesophageal compared to transthoracic echocardiography provides more anatomical detail due to higher spatial resolution. Therefore transoesophageal echocardiography is indicated when transthoracic imaging is inadequate. In addition, dedicated software applications have been developed to build a 3-dimensional model of the mitral valve complex. These applications allow unique quantitative anatomical and geometrical evaluation of the mitral valve, as indicated in figure 10. These techniques are time-consuming and subject to a significant learning curve, restricting its use currently mainly to mitral valve scientific research.
- ↑ 1.00 1.01 1.02 1.03 1.04 1.05 1.06 1.07 1.08 1.09 1.10 1.11 1.12 1.13 Silbiger JJ, Bazaz R. Contemporary insights into the functional anatomy of the mitral valve. American Heart Journal. 2009;158:887-895
- ↑ 2.00 2.01 2.02 2.03 2.04 2.05 2.06 2.07 2.08 2.09 2.10 Ho SY. Anatomy of the mitral valve. Heart. 2002;88 Suppl 4:iv5-10
- ↑ 3.0 3.1 3.2 3.3 Foster GP, Isselbacher EM, Rose GA, Torchiana DF, Akins CW, Picard MH. Accurate localization of mitral regurgitant defects using multiplane transesophageal echocardiography. Ann Thorac Surg. 1998;65:1025-1031
- ↑ Salcedo EE, Quaife RA, Seres T, Carroll JD. A framework for systematic characterization of the mitral valve by real-time three-dimensional transesophageal echocardiography. J Am Soc Echocardiogr. 2009;22:1087-1099
- ↑ 5.0 5.1 Debonnaire PJMR, Palmen M, Ajmone Marsan N, Delgado V. Contemporary imaging of normal mitral valve anatomy and function. Curr Opin Cardiol. 2012;27 (in press) DOI:10.1097/HCO.0b013e328354d7b5
- ↑ 6.0 6.1 6.2 6.3 Foster GP, Dunn AK, Abraham S, Ahmadi N, Sarraf G. Accurate measurement of mitral annular dimensions by echocardiography: Importance of correctly aligned imaging planes and anatomic landmarks. Journal of the American Society of Echocardiography. 2009;22:458-463
- ↑ 7.00 7.01 7.02 7.03 7.04 7.05 7.06 7.07 7.08 7.09 7.10 7.11 7.12 7.13 7.14 7.15 7.16 7.17 7.18 Lancellotti P, Moura L, Pierard LA, Agricola E, Popescu BA, Tribouilloy C, Hagendorff A, Monin JL, Badano L, Zamorano JL, European Association of E. European association of echocardiography recommendations for the assessment of valvular regurgitation. Part 2: Mitral and tricuspid regurgitation (native valve disease). Eur J Echocardiogr. 2010;11:307-332
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