Year : 2013  |  Volume : 16  |  Issue : 1  |  Page : 1--3

The saddle shape of the mitral valve: More than just a shape

Praveen Kumar Neema 
 Additional Professor Anaesthesiology, SCTIMST, Trivandrum, India

Correspondence Address:
Praveen Kumar Neema
Additional Professor Anaesthesiology, SCTIMST, Trivandrum

How to cite this article:
Neema PK. The saddle shape of the mitral valve: More than just a shape.Ann Card Anaesth 2013;16:1-3

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Neema PK. The saddle shape of the mitral valve: More than just a shape. Ann Card Anaesth [serial online] 2013 [cited 2020 Sep 28 ];16:1-3
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The knowledge about mitral valve (MV) has tremendously increased from the days when Andreas Vesalius suggested the picturesque term "mitral" to describe the left atrioventricular valve owing to its resemblance to a plan view of the bishop's mitre. In its open state, the MV leaflets extend like a funnel from its base at the atrioventricular junction to the free margins. Tendinous chords attach the MV leaflets to two closely arranged anterolateral and posteromedial papillary muscle. The MV leaflets, the annulus, the atrial and ventricular wall surrounding the annulus, the tendinous chords, the papillary muscles, and the myocardium surrounding the papillary muscle attachment constitute the MV complex/apparatus. For the prevention of the back flow during systole, precisely timed interaction of all the anatomical components of the MV complex is essential. Alterations in the structure and function of any of these elements can lead to MV dysfunction. [1]

The MV annulus is a concept and not an anatomically well-defined structure. The anterior one-third of the MV annulus that houses the anterior mitral leaflet (AML) is fibrous and well defined. Fibroelastic tissue extends posteriorly from the extremes of the anterior fibrous partial ring and forms the posterior two-thirds of the MV annulus; this part of the annulus tends to be "weak" and lacks a well-formed fibrous cord. The posterior mitral leaflet (PML) is attached to this part of the MV annulus. This is the area affected in "annular dilatation" and is often involved in calcification of the annulus.The MV is non-planar and obliquely located in the heart, and the annulus is saddle shaped. It is not known how the MV annulus acquires non-planar and saddle shape. Also, it is not known how this saddle shape is maintained. The finite element analyses consistently demonstrate that leaflet stress reductions occur because of the saddle shape. The saddle shape of the mitral annulus confers a mechanical advantage to the leaflets by adding curvature. The saddle shape of the mitral annulus is preserved across three mammalian species. [2] This fact provides an indirect evidence of the advantages it confers. It has been shown that the saddle shape of the mitral annulus and billowing of the MV leaflets reduces stress on the MV leaflets. [2] Perhaps, the saddle shape of the MV annulus evolves due to application of varying degree of forces all along the annulus and is designed by nature to increase the durability of the MV leaflets. The appreciation of this unique non-planar geometry of the MV annulus led to the evolution of the modern saddle-shaped annuloplasty rings to restore the physiologic shape of the mitral annulus, and thus optimize repair durability. [3] The preliminary data on saddle-shaped ring repair of MV regurgitation are promising and encouraging. [4] The life-span of the present generation tissue valves used in mitral position is limited due to degeneration and tear of valve leaflets. The natural corollary in the process of the development of tissue valves could be mounting of the valve leaflets on a saddle-shaped non-planar ring. However, it would be a challenge to house leaflets in a saddle-shaped ring.

It is interesting to know how the idea of non-planar shape of the mitral annulus came into being. The original 2D transthoracic echocardiographic (TTE) studies showed that normal mitral leaflets coapted below a line connecting the annular hinge points in the parasternal long-axis view of the left ventricle, which is oriented antero-posteriorly; [3],[5] leaflet displacement above this line correlated with angiographic prolapse. [3] Subsequently, the criteria for MV prolapse (MVP) were extended to include superior leaflet displacement in the apical four-chamber view also. [6],[7] Extending the criteria to include both views to diagnose MVP assumed that the mitral annulus was a Euclidean plane, so that leaflet-annular relationships would be comparable in the two views. Using these criteria, Sasaki et al. and Warth et al., while evaluating the MVP by 2D TTE, observed MVP in 11-13% or more of the general population, including the individuals preselected to be normal. [8],[9] Interestingly, the MVP was diagnosed in the apical four-chamber view, but it was frequently absent in the long-axis view, which was unexpected if the mitral annulus was truly a plane. [7],[9] The frequent presence of MVP in one view and its absence in an intersecting orthogonal view suggested that the appearance of prolapse in such instances reflects the non-planarity of the mitral annulus rather any localized leaflet distortion above a planar annulus. Levine et al. further described that the non-planar mitral annulus is saddle shaped, and the peaks of the saddle are located at the summits of AML and PML and the troughs at anterolateral and posteromedial commissures. [10] Later, sonomicrometry array localization, marker angiography, and 3D echocardiography confirmed both the saddle shape and dynamic changes in mitral annular geometry during the cardiac cycle. [11],[12],[13],[14],[15] Traditionally, the non-planarity or the saddle shape of the MV annulus is evaluated offline by transferring data acquired during 3D TEE to Matlab software (Math Works, Natick, MA, USA) for measuring the annular height commissural width ratio (AHCWR). [16] Recently, mitral annular non-planarity has been described as the echocardiographically measured non-planarity angle (NPA) with 3D datasets acquired using TEE. [17] The NPA is defined as an angle subtended by anterior and posterior landmarks (peaks of the horns of the AML and PML) at the commissural diameter.

In this issue of the Annals of Cardiac Anaesthesia, Govindan et al. evaluated and quantified the mitral valvular response, particularly, the MV non-planarity, by 3D TEE in patients with mild to moderate ischemic mitral regurgitation (IMR) undergoing myocardial revascularization. [18] The authors hypothesized that these patients after myocardial revascularization alone will show a decrease in NPA with or without changes in other geometric parameters. Apparently, the hypothesis that the mitral annulus NPA will recover after myocardial revascularization assumes that the loss of saddle shape or non-planarity of the MV annulus is because of myocardial dysfunction, and revascularization would improve myocardial functions and restore or improve non-planarity of the MV annulus. However, no change was found in the MV NPA with revascularization alone. Perhaps this finding, unchanged NPA, was not unexpected since the increase in NPA is related to asymmetric annular dilatation and remodeling of left ventricle, and the recovery from these changes is unlikely to be immediate. [19] Moreover, in the majority of these patients, the IMR is associated with myocardial infarction, scarring of the posterior myocardium, tethering of MV leaflets, and MV annulus dilatation. [19] The authors speculate that the same analysis if done at the time of discharge and after 6 months may throw more light into late remodeling. The reverse remodeling with or without resolution of MR is known to occur over time, but the scarring will remain in spite of revascularization.


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