Unlike transcatheter aortic valve replacement (TAVR), the development of transcatheter mitral valve replacement faces unique challenges. The mitral valve is a complicated anatomic apparatus, including mitral leaflets, chordae, and papillary muscles. The non-circular saddle-shaped annulus is pliable and changes during the cardiac cycle. Several vital structures, such as the circumflex coronary artery, atrioventricular node, and left ventricular outflow tract, are adjacent to the mitral valve . Specifically, the anterior leaflet of the mitral valve is a part of the left ventricular output tract . All of these features create a series of technical difficulties.
Although several challenges exist in TMVR, the TMVR system is rapidly expanding, with over 10 devices currently under development . Several devices, such as Tendyne™ (Abbott Vascular, Santa Clara, United States), Intrepid (Medtronic, MN, United States), Tiara® (NeoVasc, Richmond, Canada), CardiaQ (Edwards Lifesciences, Irvine, United States), are presently under clinical evaluation [16,17,18,19,20]. The transapical approach is the chief approach for TMVR. It provides easy access to the mitral valve and a simple stent-release procedure. The use of large-bore catheters (34- to 36-F) is allowed in the transapical approach. The transseptal approach has emerged as a hopeful alternative for TMVR. The technical difficulty and smaller sheath size limit its development .
The atrial portion of our device is “D” shaped to match the native mitral annulus. It is conducive to stent fixation and reduces paravalvular leakage. The flange section of the two edges is wider than the flat aspect and the arc, specifically designed to fit the saddle-shaped mitral annulus. The asymmetric flange minimizes left ventricular outflow tract obstruction and prevents stent displacement. A circular ventricular stent has three anchoring structures. First, radially struts are arranged on the ventricular component of the valved stent to penetrate the mitral leaflet and the valvular apparatus and prevent the device retrograde dislodgement into the atrium during systole. Second, a clip on the ventricular portion corresponds to the position of the anterior leaflet of the mitral valve. It is released to hold the anterior leaflet of the mitral valve after the ventricular component opens completely. The design minimizes left ventricular outflow tract obstruction caused by the native anterior leaflet. It also prevents the movement of the device into the left atrium. During the operation, we did not deliberately capture the anterior leaflet. As long as the flat aspect of the D-shaped stent is aligned with the anterior annulus of the mitral valve, the clip can easily hold the anterior leaflet after the left ventricular body is fully opened. Third, three tethering strings are attached to the lower part of the stent and fixed to the apex of the heart to prevent stent displacement. The early experiments showed that the posterior side of the stent shifted to the left atrium during systole. This finding may be related to the insufficient anchoring force between the device and the posterior annulus. We did not include a clip on the posterior of the stent as Tiara to avoid the risk of penetrating the posterior wall of the left ventricle. Therefore, we added three strings attached to the stent symmetrically that could be pulled out with the delivery system and sutured to the apex. Current TMVR devices presenting anchoring systems with clips or tethering strings have proved to be reliable. Tiara® has three clips, two anterior and one posterior. Medtronic has two clips, called support arms. In our device, the single-clip design makes it easier to hold the anterior leaflet. Tendyne™ uses a ventricular fixation system connected to the LV apex through a polyethylene tether ; the complementary combination of the clip and tether string achieves good results. No displacement happened in the seven animals.
Our self-expanding stent valve achieved good hemodynamic performance in the acute porcine model with no stent displacement and left ventricular outflow tract obstruction. Although a significant difference exists in the effective functional area between the stent valve and the natural valve, it was comparable to the conventional bioprosthetic valve. Therefore, this study confirmed the feasibility of the new valved stent and the delivery system. We plan to conduct a long-term animal experiment to observe the reliability of valve stents.
A limitation of this study is the use of healthy animal models with normal cardiac anatomy and physiology. The healthy pig is significantly different from the patient presenting with functional mitral regurgitation. Another limitation is that the study is an acute experiment. It is necessary to carry out chronic experiments to verify the reliability of valve stents.