After an unusual third review, a panel of doctors has narrowly recommended that the Food and Drug Administration allow Boston Scientific to sell a new heart device to prevent strokes.

But the device, known as Watchman, is likely to be available to a smaller group of patients than the manufacturer wanted.

Invented by a trio of physicians at the Minneapolis Heart Institute in 1996, Watchman is among a handful of cutting-edge devices that are intended to block circulation in the left atrial appendage, which is a sort of cardiac cul-de-sac of the heart where blood can form deadly clots in patients with irregular heartbeats.

The potential market for the device is large, because atrial fibrillation is the most common heart-rhythm dysfunction in the United States. Blood clots that break free from the hearts of patients with atrial fibrillation can be deadly if they reach the brain and cut off blood flow, causing ischemic stroke.

Boston Scientific, which maintains a sizable workforce at its Maple Grove and Arden Hills campuses, is seen by analysts as leading the race for approval of a left atrial appendage plug. St. Jude Medical in Little Canada is assessing a similar device known as the Amplatzer Amulet, but it's still in early data-gathering stages in the United States. Testing by smaller companies is also ongoing.

If the Watchman were approved in 2015, Boston Scientific could sell about $27 million worth of the devices that year, climbing to $90 million in 2018, according to a note to investors from Wells Fargo Senior Analyst Larry Biegelsen.

Although Boston Scientific is leading the pack of medical device companies, the Watchman has hit an unusual level of scrutiny at the FDA.

"It was almost unprecedented to have three panels," Dr. Rob Schwartz, one of the co-inventors of Watchman, said in an interview. "I don't know of anyone who has heard of three panels before. But they wanted to be very clear, because the device was an unusual device and they wanted longer-term follow-up."

The device got a lukewarm recommendation for approval by a panel of experts in 2009, but the FDA disagreed and denied permission entirely. Last year, a second panel of experts recommended approval, but the FDA asked to see longer-term patient data after concerns were raised about the design of the clinical trial.

On Tuesday night, a third panel of cardiac experts voted 12-0 that there was evidence the Watchman was safe for use. However, the same panel split 7-6 in voting that the device was not effective for use in the population specified by Boston Scientific.

"It was clear that the panel was uncomfortable with the broad indication for the use proposed by BSX," wrote Dr. Joshua Jennings, an analyst for Cowen & Co., using Boston Scientific's stock-ticker abbreviation. "We now believe that the FDA will ultimately approve the device with a label that identifies the Watchman as a second-line therapy."

The first-line therapy would be blood-thinning warfarin drugs like Coumadin, which reduce the danger that pooled blood would form clots in the atrial appendage. However, blood thinners can be dangerous for some patients because they reduce the ability of blood to coagulate. This can lead to uncontrolled bleeding from cuts and a dangerous type of brain damage called hemorrhagic stroke, in which weakened vessels leak blood into the brain.

Patients with atrial fibrillation who can't take blood thinners would be good candidates for a plug to block the appendage, Schwartz said.

"The question came up, could we have a device that would substitute for all of these blood-thinner issues? The solution was to create a plug," he said. "It's like closing off a dangerous mine … nothing gets in and nothing gets out. The body forms a tissue that covers the surface of the device."

He noted that doctors may eventually use the device for off-label purposes after FDA approval, which is legal as long as the company doesn't promote those unapproved uses.

Joe Carlson • 612-673-4779

Twitter: @_JoeCarlson