Percutaneous Valve Innovations: Transcatheter Mitral Valve Intervention
Can you tell us about your center and history with the mitral valve?
What studies are you currently involved in?
Are there advantages to mitral valve replacement vs repair? Will one or the other dominate in the future?
The appeal of mitral replacement is the promise of complete elimination of any residual mitral regurgitation (MR), while the appeal of the repair technologies is established safety and relative simplicity. How this balance unfolds will take at least several years.
Are trials looking at both functional and degenerative MR?
How does the anatomy of the mitral valve impact device design?
The mitral valve orifice is much bigger than the aortic valve orifice. It is D-shaped, so creating a replacement prosthesis for the mitral valve requires a much bigger device and a much more complex geometry. That is one of the challenges of mitral valve replacement efforts. The calcific mitral stenosis population tends to have a much smaller orifice, because of the presence of calcification, which, in highly selected cases, makes it possible to put in an aortic valve prosthesis.
What are some of the challenges of transcatheter therapy in the mitral valve?
Why does the device protrusion occur?
The mitral inflow and the left ventricular outflow are normally adjacent, and there are two ways that the left ventricular outflow can be compromised by a mitral device. One is directly. The mitral devices have to protrude to some degree into the ventricle in order to anchor, and the more they protrude, the more likely they are to push into the left ventricular outflow tract. The second mechanism is that the native anterior mitral leaflet can be displaced into the left ventricular outflow tract. Thrombus formation on the devices is another problem, and in fact, it has been an issue since the first commercial mitral valve surgical replacement device was developed in the 1960s. The left atrium is a much lower flow chamber than the left ventricle and aorta, and the mitral devices have more prosthetic material, and thus thrombus formation on these devices is an important problem. In the mitral space, for example, the Edwards Fortis program was discontinued in part because of thrombus formation on the devices, which occurred despite anticoagulation therapy. The need for anticoagulation is another important challenge for treating patients with these mitral devices.
What are the next steps for developing mitral replacement devices?
How will imaging continue to develop?
Patient selection and procedure planning are driven by computed tomography (CT). The use of CT for assessing mitral geometry and virtual valve implantation has rapidly become the standard approach. Software analysis packages to make these assessments, which have become the standard for TAVR CT analysis, are under development. Most mitral procedures are done with transesophageal echo (TEE) and fluoro interprocedurally, and for many procedures, the essential steps of anchoring the valve and orienting the valve at the key part of the procedure are predominantly echocardiographic. There are some commercial systems available that allow the echo and fluoroscopy images to be fused, and they are going to be helpful, but aren’t yet uniformly available. Imaging has really moved the whole structural heart field at every step, so we will see more and more development of hybrid imaging. There are systems that allow previously acquired CT images to be fused with fluoro images, and these are all very helpful. Historically, the most rudimentary form of imaging has been roadmapping with fluoroscopy, where a static reference can be displayed on a screen and a live image watched in an overlay. Multimodality imaging, having two completely different imaging modalities for guidance, is very exciting in that regard.
How do you see the heart team organizing around the mitral patient?
The heart team has developed, but in an evolutionary manner. You don’t just put together a roster of interested parties, instantly sit down in a room, and have a functioning heart team. In our program, the heart team started many years ago with patient selection for the SYNTAX and EVEREST trials. We now have a Wednesday afternoon weekly heart team meeting, where we review with our research and clinical nursing team, and our interventional and surgical physicians, a list of aortic, mitral, and left atrial appendage patients that are all undergoing screening and procedure planning. In many respects, that is the easiest part. There is an ongoing interaction among the team members at many stages. It becomes part of the whole practice. Our interaction with the surgeons has evolved and in terms of decision-making, it’s not only that it is team-oriented, but that it is a process. Decisions are not usually made in one step, because the evaluation takes face-to-face visits with both the interventional and surgical physicians, and the imaging has to be reviewed. So we need to do whatever it takes to get those major components all to drive toward a decision about best therapy for an individual patient.
What evolution do you see for the team during the procedure itself?
For TAVR, the surgeon and interventional physicians are required to be physically present for Medicare reimbursement. From one program to another, the degree to which both specialties are truly involved in the procedure varies. In our own practice, we have made a serious effort as the procedure has become less surgical to get our surgeon colleagues involved in some of the interventional steps, so that everybody can truly participate in the procedures. On the mitral replacement side, the procedures now are predominantly apical, so there is an obvious clear involvement of the surgeon. This is a work in evolution, so we will see how this changes as the mitral procedures become more percutaneous and less surgical. That is going to be a long road.
Can you talk about the role of the echocardiographer and their interaction with the interventionalist?
For the interventionalist to perform the procedure, an additional imaging specialist is required for echocardiography. For MitraClip, in many institutions, the cardiovascular anesthesiologist is managing the TEE, but it is very institution-specific. In our own program, it is a cardiologist echocardiographer who does the TEE. The learning curve for both the echo and interventional physician is critical, because each has to learn a great deal of new information to be able to take advantage of the echo during an intervention. Historically, there is no single language that both the echo and fluoro imaging interventional physicians have used, so communication and the ability to manage the interaction are key. The interventionalist absolutely has to be able to understand the echo imaging in real time. The demand for the interventional physician to learn CT interpretation has also been a big part of becoming involved with these therapies.
Can you share more about the applications of CT in the planning phase?
Three-dimensional (3D) modeling and rendering by CT that shows the appearance and placement of virtual valves is now part of the evaluation. It has become a common part of the mitral procedure planning for mitral replacement devices. On the aortic side, commercial software has developed to the point where there are several companies with TAVR software packages that will show you a virtual valve. Some programs have an option for importing an STL file, which is a CAD file that describes the specifics of a particular device, so it’s not just that you put a generic cylinder into the valve orifice in the 3D rendering, but rather a specific type of aortic or mitral valve. On the mitral side, this is not as well developed. It is critical to recognize how early the mitral valve replacement effort is. We have hundreds of thousands of TAVR implants, so that the experience, resources, and impetus to develop software and analytic programs are huge on the aortic side. On the mitral side, the total number of patients treated with replacement devices, the dedicated mitral devices, is still barely 100 patients worldwide. Most of the software is homemade, and there are companies working on mitral software packages, but those are programs that are in development.
Any final thoughts?
Patients who either had no option or were facing an open sternotomy procedure are now being treated with increasing frequency with a completely percutaneous procedure, a short hospital stay, and a remarkably short recovery time. It is exciting to do these procedures to replace or repair a valve, and now, for TAVR, to do so in an awake patient. For mitral, we are at the beginning of that journey, but with a lot of that same promise, typically validated on the one-month follow-up visit where these patients come for follow-up and they are transformed, without having had open surgery.
Disclosure: Dr. Feldman reports research grants and honoraria from Abbott, Boston Scientific, Edwards Lifesciences, and W.L. Gore.
Dr. Ted Feldman can be contacted at tfeldman@tfeldman.org.