Although cardiologists have been performing therapeutic intervention on the stenosed mitral valve for three decades, percutaneous intervention on the regurgitant mitral valve can be considered to be a therapy in evolution. Transcatheter technology for intervention on the aortic valve has become more commonplace over the last decade, since the first report of a percutaneous aortic valve replacement in 2002 by Alain Cribier.⁽¹⁾ The reason for this difference in progress can be explained by the more complex nature of the anatomy of the mitral valve, together with the heterogeneous aetiology of the pathological process that affects it.

Despite the infancy of the subspecialty, progress has been rapid with many different technologies being advanced within the field of percutaneous mitral intervention. Mirroring the surgical experience, aortic valves are replaced percutaneously whereas the preference for intervention on the mitral valve is towards repair.

There are currently numerous percutaneous mitral valve interventions being trialled clinically worldwide with many more devices being evaluated in the preclinical phase. Reflecting the diverse underlying pathophysiological processes of mitral regurgitation (MR), the techniques being developed can be broadly separated into those that permit leaflet repair, those that alter the annular dimensions, either directly or indirectly, and those that remodel the mitral valve complex as a whole. 

Percutaneous leaflet repair is primarily of use in a mitral valve that exhibits regurgitation through the central segments of the anterior or posterior valve leaflet without significant damage to the annular ring or subvalvular apparatus. Alfieri⁽²⁾ can be considered the pioneer of perhaps the simplest surgical form of leaflet repair in his technique that requires suturing of the free edge of the mid-section of the line of coaption between the anterior and posterior leaflets. This repair results in a double-orifice mitral valve reducing MR. Owing to the relatively simple nature of the repair, it does lend itself to replication using the percutaneous route. Two devices have been used clinically; the Mitra-Clip™ (Abbot Vascular, Abbot Park, IL, USA) and the Mobius II™ (Edwards Lifesciences, Irvine, CA, USA). 

Annular dilation is a significant contributing factor behind mitral regurgitation. The surgical results following annuloplasty are excellent, with the vast majority of patients being free from MR at long-term follow-up. Cardiologists have, therefore, been keen to emulate this surgical success using a percutaneous catheter delivered device. Two different technologies have emerged. The first group uses direct annuloplasty techniques, whereas the second group use the anatomical proximity of the coronary sinus as a route to modify the mitral annulus.

The Mitralign (Boston, MA, USA) was one of the first direct annuloplasty systems available percutaneously. Using the retrograde aortic approach, a steerable catheter is manipulated across both the aortic and mitral valves into the left ventricle. Radiofrequency is used to align several sutures attached to anchor pledgets around the annulus. The pledgets are then pulled towards each other using a guide wire, thus reducing the annular dimensions. The QuantumCor system (QuantumCor Inc, CA, USA) again uses radiofrequency, but applies it directly to the mitral valve annulus. The aim is to induce localised thermal damage, causing shrinkage of the annular dimensions. The GDS AccuCinch System (Guided Delivery Systems, CA, USA) has a similar mode of action to the Mitralign, again using endomyocardial anchors. A cinching wire is then attached allowing fine adjustment of the annular dimension.

Indirect annuloplasty relies on the direct anatomical relationship of the coronary sinus and great cardiac vein to the postero-lateral circumference of the mitral valve annulus. This approach is particularly attractive as it may prove to be less technically demanding on the beating heart. A change in the conformational dimensions of the coronary sinus will have an impact on the septo-lateral dimensions of the mitral annulus.

The Monarc device (Edwards Lifesciences, Irvine CA, USA) is one such indirect annuloplasty device. Venous access is gained through the right internal jugular vein to allow cannulation of the coronary sinus. The device has both proximal and distal self-expanding stent anchors, which are connected by a bridging element. This bridge is composed of a coiled spring that gradually contracts 3 to 6 weeks after implantation. However, bridge fracture led to a need for a redesign following the first-in-man trials. Although the device was feasible to implant, efficacy data was confounded by instances of coronary compression and anchor separation. 

The common theme in patients with functional MR is the distortion of the left ventricular geometry surrounding the mitral valve. Several novel devices are in development in an attempt to ameliorate this distortion. 

The Percutaneous Septal Sinus Shortening System (Ample Medical, CA, USA), has three elements; an atrial septal occluder, an interconnecting cinching wire, and a permanent small coronary sinus T bar element positioned behind P2. These magnetic catheters are used to create a connection from the great cardiac vein and the intra-atrial septum. Tension is then applied to the bridge element shortening the septo-lateral distances. First-in-man studies have shown initial promise.  

Clinical studies in the field of percutaneous mitral intervention currently lack the robust data seen in the equivalent percutaneous aortic valve trials. First in man and animal data abounds, and is likely to be converted into clinical trial evidence in the near future with certain technologies. The gold standard at present is surgical repair or replacement of the mitral valve, although this is often not a viable option in patients at high surgical risk due to other multiple comorbidities. The next decade is likely to see a refinement of the technology, and the indication for percutaneous mitral valve intervention is likely to widen.

Crucial to the successful implementation of transcatheter valve technology in the aortic setting has been the close collaboration between cardiac surgeons and interventional cardiologists. The birth of the Heart Multidisciplinary Team has been important in ensuring that the gold-standard of open surgical intervention has not been replaced by a substandard transcatheter approach. Indeed, in the transcatheter aortic field we have witnessed arguably one of the most rigorous and evidence based adoptions of a new medical technology thus far. At present, transcatheter mitral valve intervention remains limited in application, although the future will undoubtedly be bright for those devices that show efficacy and endurance. 

 

1.  Cribier A, Eltchaninoff H, Bash A et al. Percutaneous Transcatheter Implantation of an Aortic Valve Prosthesis for Calcific Aortic Stenosis. First Human Case Description. Circulation 2002;106(24):3006-8

 

2.  Alfieri O, Maisano F, De Bonis M et al. The double-orifice technique in mitral valve repair: a simple solution for complex problems. J Thorac Cardiovasc Surg. 2001;122:674-681.