Medtronic's experimental new mitral valve has been implanted in the first of 1,200 patients in a clinical trial to test the device on people who could benefit from heart surgery for a condition called mitral regurgitation.

The new device, called the Intrepid, is implanted without open-heart surgery, making it possible to treat patients who might otherwise be considered inoperable with traditional tools. Mitral regurgitation happens when blood flows backward into the left atrium of the heart, and patients with severe cases of it may feel symptoms like fatigue, shortness of breath and leg swelling. It's much more common later in life.

Other treatments for serious mitral regurgitation are already available, like repair of the leaky mitral valve with surgery or with a special device like Abbott Laboratories' MitraClip. But interest among interventional cardiologists is growing for a new type of valve-replacement device like the Intrepid, which can be implanted through a small incision elsewhere on the body, allowing doctors to repair a heart valve without surgically opening the heart.

Although several companies are designing and testing transcatheter mitral valve replacement (TMVR) devices today, none are approved for U.S. use outside of clinical trials. The market for the devices is expected to hit $3 billion by 2022, though the costs of the devices and other factors may impact that, according to a recent market analysis by Infinium Global Research posted online.

The mitral-replacement devices are similar to transcatheter aortic valve replacement (TAVR) devices, which were first approved for U.S. sales in 2011 and today account for billions of dollars in sales, as the number of manufacturers and approved indications has grown over time.

On Monday, Medtronic announced its first minimally invasive Intrepid mitral valve implant, in a patient at St. Luke's Medical Center in Milwaukee. That trial, called Apollo, will enroll as many as 1,200 patients in two separate cohorts at up to 60 sites, the company said.

In the first cohort, up to 650 patients who are candidates for conventional open-heart mitral replacement surgery but not candidates for mitral repair will be randomized to get either an Intrepid device or traditional mitral valve surgery. Medtronic's stated goal with the trial is to show that the number of strokes, deaths, reoperations and hospitalizations after one year in the Intrepid group is statistically noninferior to patients treated conventionally.

In the second cohort in the Apollo trial, up to 550 mitral regurgitation patients who are considered too risky to undergo an open heart procedure will be selected to receive an Intrepid valve. The researchers' goal with this group is to show that the valve is a noninferior option to a "performance goal" after a year, according to Medtronic.

The Intrepid valve is composed of two wire-mesh stents plus a tissue valve, which are rolled into a capsule thin enough to be inserted between the ribs into a blood vessel, and then advanced into place between the left atrium and the left ventricle using live-motion X-rays. The first stent is expanded to make the device fit securely over the existing diseased valve, and the second holds the new tissue valve in place.

The device was already tested for safety in a 15-patient pilot study. The final results of that study are expected to be unveiled at an industry conference next week, but early results announced at a conference in Europe last year showed 14 of 15 patients had successful deployment of their Medtronic valves, and that one patient had measurable mitral regurgitation after the procedure, compared to 15 who had varying levels of the condition before it.

"This is the beginning of an important journey to establish a truly less invasive approach to treat severe mitral valve regurgitation in patients who are appropriate candidates for mitral valve replacement with a transcatheter technology that eliminates the need for open-heart surgery," Dr. David H. Adams, surgeon-in-chief of Mount Sinai Health System, and national co-principal investigator of the Apollo trial, said in a news release.