Mitral Valve Apparatus and Regurgitation
The mitral valve apparatus is a complex structure that requires the integrity of six anatomic components. These components include: the mitral annulus (MA) or left atrioventricular junction, the mitral leaflets, the chordae tendineae, the papillary muscles, the left ventricular myocardium and the posterior left atrial wall.1,2 Contraction of the left ventricle (LV) and papillary muscles during systole results in forces that drive the mitral leaflets into apposition. The elevation in LV pressure, compared with left atrial (LA) pressure, allows for coaptation of the free leaflet margins. The MA acts as the fulcrum for the leaflets and is reduced in size during each ventricular systole. Papillary muscle contraction applies the appropriate counterforces to the chordae tendineae, preventing eversion of the leaflets. During normal closure, both leaflets must align in the same plane during coaptation and require an optimal annular size, a geometrically correct orientation of the papillary muscles, appropriate tethering to the tendinous cords, and suitable closing forces generated by LV muscular contraction.3
For MR to occur, more than one of these components must be dysfunctional. Dysfunctional components to the mitral valve apparatus help to further categorise this disorder and offer many potential options for mechanical correction. This aids in tailoring TMVT according to functional anatomy and device action.2,4 Current active TMVT focus on the MA, mitral leaflets, chordae tendineae and papillary muscles. As these technologies improve, so do their applications and the possibilities for combining techniques. When percutaneous mitral valve repair approaches are unlikely to work, transcatheter mitral valve replacement (TMVR) may be a potential option.
Mitral Annulus
The mitral annulus is the D-shaped orifice formed by the convergence of the LA and LV.3,5 It is saddle-shaped with elevated septal and lateral segments, and a depressed medial segment along the central zone of apposition.6 The anterior mitral leaflet is in fibrous continuity with the aortic valve and the posterior mitral leaflet with the musculature of LV inflow. During systole, the MA contracts reducing the area that the opposing leaflets need to coapt by 20-50 %.7 Left ventricular dilatation distends the MA, reducing the ability for the annulus to contract.8 In the setting of significant mitral annular calcification, a loss of annular contraction can also lead to leaflet malapposition.9
The goal of treating the MA when it is dilated is to decrease the septal-lateral diameter by at least 8 millimetres (mm). Annuloplasty techniques are designed to restore annular size and shape, subsequently improving leaflet coaptation. Approaches for correction include indirect reshaping or restraining the valvular orifice via the coronary sinus (CS) versus direct via the LA or LV.
Coronary Sinus Indirect Annuloplasty
Indirect annuloplasty utilises the CS to reshape the mitral annulus. The close proximity of the CS to the MA allows for the placement of devices that will shorten the posterior annulus and bring it anteriorly, decreasing the septal-lateral dimension and improving leaflet coaptation. The main difficulty with this technique is that the CS only relates to the posterior annulus and that, in many cases, does not course in the atrioventricular groove along the MA. Furthermore as annular dilatation progresses with worsening MR, the distance from the CS to the mitral annulus increases. Other considerations that would limit the use of these devices include mitral annular calcification that makes it difficult to reshape the MA. In addition, the CS may directly transverse the coronary arteries, most commonly the left circumflex.10-12 Coronary angiography is necessary to confirm the relationship of the coronary arteries to the CS prior to indirect annuloplasty.
The only available CS device is the CARILLON Mitral Contour System™ (Cardiac Dimensions Inc., Kirkland, WA, US). It consists of two self-expanding nitinol anchors connected by a fixed-length itinol intervening cable. After the distal anchor is deployed in the great cardiac vein, the bridge is unsheathed and tension is applied to the cable. Tension can be adjusted to achieve optimal results; subsequently, the proximal anchor is deployed in the anterior interventricular vein. The device is retrievable if results are non-optimal. In the Carillon Mitral Annuloplasty Device European Union Study (AMADEUS) and Tighten the Annulus Now (TITAN) trials, successful device implantation was demonstrated in 62 % of patients with a mean grade reduction of MR .13,14 This device has CE mark and is awaiting pivotal study in the US.
Mechanical Approach to Direct Annuloplasty
Direct annuloplasty reshapes the MA without use of the CS, preserving native leaflet function and restoring leaflet coaptation. These devices are delivered to the LA or LV and implanted into the MA. The Mitralign device (Mitralign Inc., Tewksbury, MA, US) is delivered retrograde transaortic into the LV periannular space. Two pledgeted anchors are deployed on the posterior mitral annulus at P1P2 and P2P3 locations, connected by a suture. The pledgets are plicated and secured into place by a stainless steel lock, cinching the posterior MA.15 First-in-man (FIM) study is ongoing.
The Accucinch Annuloplasty system (Guided Delivery Systems [GDS], Santa Clara, CA, US) utilises a similar technique; however, it has placement of up to 12 retrievable anchors applied from P1 to P3, extending from the right to left trigones. A suture connects the anchors with direct tension applied to decrease posterior annular size. Enrolment into Cooling in intracerebral Hemorrhage (CINCH) 2 safety and feasibility trial is underway. Moreover, the Cardioband device (Valtech, Or-Yehuda, Israel) is a sutureless technology where supra-annular fixation is made through an antegrade transseptal approach.16 Anchors are implanted individually along the posterior annulus and can be repositioned or retrieved, and adjustments made to fine-tune annular dimensions. FIM is also ongoing.
The major limitation of these devices is that they are partial rings that only affect the posterior MA. Currently, placement of a complete mitral annuloplasty ring is being evaluated in preclinical trials using the Millipede (Millipede LLC, Ann Arbor, MI, US) and enCorTC (Micardia Corporation, Irvine, CA, US) systems. Both are delivered via a transseptal approach and fixed into the periannular space. Unlike its surgical counterpart (enCorSQ device [MiCardia Corporation, Irvine, CA, US]) that requires exposure of a subcutaneous atrial lead for radiofrequency activation, the activation of enCorTC is wireless and is adjustable for appropriate MR result by echocardiography.12