In the last decade, there has been a revolution in transcatheter therapies for structural heart disease. The most widely embraced, transcatheter aortic valve replacement (TAVR), was originally intended for patients with aortic stenosis, in whom surgery was considered prohibitive, but it has now been utilized as an excellent alternative to surgical aortic valve replacement in patients at intermediate or high risk and is being trialed for the low risk patients as well. As TAVR has become established with well-designed devices and acceptable safety and efficacy, it has inspired several device manufacturers to push the envelope of innovation to meet the large unmet need for the percutaneous treatment of patients with mitral valve disease, primarily for mitral regurgitation and also for mitral stenosis.

The mitral valve is a one-way valve between the upper (atrium) and lower (ventricle) chambers on the left side of the heart. When open, the mitral valve permits oxygenated blood from the lungs to fill the left ventricle. Once the left ventricle pumps to deliver blood to the body, the mitral valve closes to prevent blood from flowing back toward the lungs.

Mitral valve regurgitation (MR) occurs when imperfect closure of the mitral valve permits blood from the left ventricle to leak back towards the lungs. It is most often caused by myxomatous disease (a kind of degenerative disease). It can also be caused by senile calcific degenerative disease, conditions that cause enlargement of the left ventricle, infections, and trauma. Severe MR affects approximately four million patients in the U.S. alone.

Mitral stenosis, a narrowing of the valve, is mostly caused by rheumatic fever (now rare in the United States) or senile calcific degeneration creates an obstruction to the forward flow of blood and thereby increases the fluid pressure inside the lungs.
Both conditions present with shortness of breath and fatigue, which initially can be managed by medical therapy but will eventually need surgical correction with a repair or a replacement. If left uncorrected on failing medical therapy, it can lead to irregular heart rhythms, increased risk of stroke, heart failure, and death. Mitral valve surgery is a conventional open heart procedure performed through a standard midline sternotomy incision or through minimally invasive approaches through the front or side of the chest. These procedures necessitate the use of the heart-lung bypass machine and needs the heart to be stopped for the duration of the repair/replacement. As already seen with TAVR, percutaneous valve procedures are least traumatic. Recovery in the hospital and return to normal lifestyle are quicker than any other operation.

A variety of repair techniques have been and are now being tested for transcatheter mitral valve correction, including replacement valves and repair devices used on the mitral leaflets, the implantation of neochords, and the remodeling of the mitral annulus. While the comparison of TAVR to surgical aortic valve replacement was fairly straightforward, comparing surgical and catheter based mitral valve repairs is going to be a challenge, given the complexity and artistic nature of mitral valve repairs.

The ability to percutaneously repair a mitral valve with the same degree of accuracy and reproducibility will be a challenge. Concurrent development of advanced imaging technologies will play an important role in the success of these procedures. Anatomically, the mitral valve poses a greater test to percutaneous treatment due to its complex structure and integrated relationship with the left ventricle. Some specific difficulties facing the development of transcatheter mitral technologies are: the mitral is a larger valve; it is difficult to access; it is asymmetrical; it lacks an anatomically well-defined annulus to which to anchor the replacement valve; its geometry changes throughout the cardiac cycle; and placing a replacement valve in it entails the risk of left ventricular outflow tract obstruction.

Despite these challenges, a number of experimental devices are being developed. Initially, the good candidates for transcatheter mitral valve replacements will be patients with a failed mitral valve bioprosthesis, failed mitral valve repairs, and senile calcific degeneration. With ongoing technological advancements in the field of percutaneous valve therapies, within the next decade it is expected that transcatheter mitral valve therapies will become a valuable, minimally invasive alternative to mitral valve surgery for patients with severe mitral valve disease, particularly MR and variable surgical risk.

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