It is the valve you never want to use.

St. Jude Medical’s tiniest artificial heart valve is smaller around than a dime, intended to fit snugly inside the damaged hearts of the smallest cardiac patients. Some of them are ­toddlers, others are just weeks old. All of them are very ill, or will soon become so, from congenital diseases of the heart.

Not many kids need them — at the University of Minnesota Masonic Children’s Hospital, perhaps one or two kids a year will need them. Some years, none will. Pediatric heart surgeons would much rather fix a child’s heart tissue surgically than install a 15-millimeter metal valve.

Sometimes, however, you can’t fix it.

“You don’t like to replace valves in kids, because the heart is growing and the valve is just kind of stuck at this one size,” said Dr. Robroy MacIver, a pediatric cardiothoracic surgeon at Masonic in Minneapolis. “So if you are placing a valve in an infant, you are probably going to have to replace it twice before they are an adult size.”

The world’s smallest heart valve is a metal machine with pyrolytic-carbon leaflets that open and close with each beat, using only the force of the beating heart. Mainly intended to replace a mitral valve, it comes with a long-term prescription for blood-thinning medications and the stress of knowing open-heart surgeries are certain.

The Minnesota-manufactured device was originally available from Little Canada-based St. Jude only under a custom-device approval from the Food and Drug Administration, but in 2011 the company got permission to sell it on an emergency basis to patients with no other treatment options. Those approvals still left it difficult to obtain quickly because it had to be ordered well in advance and returned if not used. Now hospitals in an ongoing clinical trial can keep the device in stock.

Back in 2010, the FDA met with medical device companies and doctors to discuss the lack of artificial heart valves for children with full government approval on the market. The agency urged companies to find ways to move the lifesaving devices for the very young into clinical trials.

“It’s clear that too few device companies are stepping up to address the needs of pediatric patients, and we are not seeing the activity and applications we would like,” outgoing FDA ­Commissioner Dr. Margaret Hamburg said in a speech last October in Washington.

Companies have responded. Just last month, Fridley’s Medtronic gained FDA approval for commercial sales of its 18-millimeter valve, called Melody. But it’s only approved for the pulmonary valve on the right side of the heart, and it includes a label warning doctors not to use it on the mitral valve, on the left side.

Last month, St. Jude announced it was enrolling children in a 40-hospital national clinical trial of its 15-mm Masters Series valve, with the goal of proving its safety and attaining an investigational device exemption (IDE) from the FDA that will allow it to be stocked on hospital shelves with much less red tape.

“The FDA is interested in this being available to more patients who need it,” Dr. Mark Carlson, St. Jude’s chief medical officer, said in an interview. “We know that this is not a large market, if you will, but it is one that is extremely important.”

The study, known as the HALO U.S. IDE trial, is likely to move slowly because it will be tough to find enough patients who need the device, can take blood thinners and have damaged valves that can’t be repaired surgically. MacIver likened the situation to the so-called “orphan drug” problem, in which patients with rare diseases find it difficult to get affordable therapies because the cost of pharmaceutical development is so high and the potential market is small.

“St. Jude, they’re not going to sell a ton of these valves,” MacIver said. “But in these kids, it’s a life or death thing. If you don’t have a valve, you’re in trouble.”

At least one stock analyst sees a strong market. In a Zacks Equity Research blog post, the analyst wrote that an estimated 35,000 babies have ­congenital heart defects, and many require mechanical heart valves, making the device a potential “game-changer” for St. Jude if approved.

Carlson also noted that the 15-mm device is a therapy intended to allow younger patients to live long enough to get another valve implanted.

“It’s a bridge therapy,” Carlson said, “that will enable them to grow, and their hearts to be healthy and grow. And as they do grow, they’ll be candidates for the traditional valves that are more appropriate for larger patients.”