Top medical device makers are investing heavily in a new way to treat leaky mitral heart valves, a serious condition affecting millions of people, but it will be at least four years before the technology reaches the U.S. market.

The minimally invasive treatment, called transcatheter mitral valve replacement, lets a doctor implant an artificial valve using a catheter inserted through the arteries. The technique spares the patient from open-heart surgery, the traditional approach to valve replacement that is considered too risky for many people, especially the elderly.

Medtronic Plc, Abbott Laboratories, Edwards Lifesciences Corp and HeartWare International Inc in recent months have spent a total of $2 billion to buy young companies developing versions of the technology.

Boston Scientific Corp has an option to buy an early-stage company developing a mitral replacement system.

Nearly 6 million people in the United States may suffer from mitral disease, and Wall Street analysts say it could be the next multibillion-dollar market. But the rush by leading device makers into mitral replacement has raised questions for some investors and heart doctors.

Fewer than 100 people worldwide have undergone the procedure, and it is far from clear whether this technique will be significantly better than other treatments or how many patients would be candidates for the surgery.

"All these things are still several years away, especially from the U.S. market," said Jeff Jonas, a portfolio manager with Gabelli & Co, which holds shares of Medtronic. Mitral disease is "something that very gradually develops. A ton of people may have it, but a smaller number have it at a life-threatening stage."

The mitral valve helps regulate blood flow from the heart's left atrium into the left ventricle. Mitral valve regurgitation occurs when the valve does not close properly to prevent backflow. It can cause extreme fatigue and shortness of breath and increase the risk of stroke, heart failure and death.

"If we have a way where we can replace the valve without surgery, that is where all the excitement is," said Dr. Mathew Williams, chief of cardiac surgery at New York University's Langone Medical Center.

REPAIR, OR REPLACE

Abbott's MitraClip is the only minimally invasive product on the U.S. market for the condition. It repairs, rather than replaces, a patient's own mitral valve. It is approved for a limited group of severely ill patients and its performance improves when certain anatomical characteristics are present. Annual sales are about $250 million worldwide.

Abbott sees room for both repair and replacement in treating mitral disease and projects strong double-digit annual growth for the MitraClip over the next several years, said John Capek, a company executive vice president.

Rivals Edwards and Medtronic have found huge success with a minimally invasive technique for replacing malfunctioning aortic valves, and some see that as a benchmark for the mitral market.

Edwards projects the aortic market will grow to $3 billion in worldwide sales in five years, double its 2014 sales, driven by advancements in the technology and approvals to treat additional patient populations.

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Some Wall Street analysts, including Morgan Stanley's David Lewis, predict the mitral market could be two to three times larger because more patients have mitral valve disease than aortic.

"What you've got is this huge market opportunity. But the challenges are huge, and it's going to take some time," said Edwards Chief Executive Michael Mussallem. Edwards may be the first to launch a transcatheter replacement product in Europe, possibly by late 2017, with a U.S. approval possible by 2019.

Replacing a mitral valve is particularly tricky, partly due to the valve's asymmetrical structure, which varies by patient and is more difficult to address than the circular aortic valve, according to doctors. It can also be tougher to guide a mitral valve to the heart through the arteries because of its larger size, and harder to reach the original valve due to its location.

Results will not come until the end of 2017 from a key study enrolling up to 430 patients to determine whether repairing the valve using Abbott's MitraClip is better than medications alone.

Doctors say the research could bolster the case for minimally invasive approaches if the results support the procedure's effectiveness. Patients with mitral valve regurgitation often take medicines used to treat heart failure, but these do not correct the valve condition.

"It is a very interesting space, and there is a need for mitral valve replacement," said Dr. Saibal Kar, director of interventional cardiac research at Cedars-Sinai Medical Center in Los Angeles. But he cautioned it is much too early to predict it will become the industry's next multi-billion-dollar market.

"It is unbridled enthusiasm," he said.