A tiny cardiac monitor implanted just beneath a patient's skin is giving doctors another tool in helping to prevent recurrent strokes.

According to results of a Medtronic-sponsored study presented Friday, an insertable device called the Reveal XT does a significantly better job of detecting atrial fibrillation in patients who have had a stroke than standard monitoring. That gives doctors a better chance of preventing another stroke.

Patients with atrial fibrillation, a fluttering of the upper chambers of the heart, can have clots form in the heart that later migrate to the brain. The condition is suspected in as many as 30 percent of strokes, and people with atrial fibrillation are five times more likely to have a stroke.

"Finding atrial fibrillation allows a more specific, tailored therapy to prevent a second stroke," said Dr. Richard Bernstein, author of the new study and a professor of neurology at Northwestern University's Feinberg School of Medicine in Chicago.

Bernstein, who presented study results at the American Stroke Association's International Stroke Conference 2014 in San Diego, said atrial fibrillation can be hard to detect. Many patients don't know they have it because they don't always have symptoms. In the study, 79 percent of patients who had the condition had no symptoms.

"For patients who have had a stroke but don't know its cause, this study shows that the Reveal monitor provides superior ability to diagnose atrial fibrillation," said Pat Mackin, president of the Cardiac Rhythm Disease Management business and senior vice president at Medtronic. "This diagnosis helps physicians to prescribe appropriate medications and potentially prevent future strokes in these patients."

The Medtronic-sponsored study, called Crystal-AF (CRYptogenic STroke And UnderLying Atrial Fibrillation), involved 441 patients who had an unexplained stroke. The study compared standard cardiac monitoring within 90 days of a stroke with Medtronic's insertable monitor. The Reveal XT monitor can give doctors continuous data for up to three years.

After six months of monitoring, the device found atrial fibrillation in 8.9 percent of patients. Standard monitoring detected the condition in 1.4 percent of patients. A year later, the Medtronic monitor detected atrial fibrillation in 12.4 percent of patients, compared with 2 percent of the others. And, after three years, the numbers were 30 percent with the insertable monitor vs. 3 percent for standard testing.

Medtronic conducted the study at 55 centers in the United States, Canada and Europe.

Most people who have a stroke caused by a blood clot are given aspirin or similar drugs, Bernstein said. Patients with atrial fibrillation are prescribed anticoagulants, which are more effective at preventing a stroke.

The Reveal XT resembles a USB flash drive and has two electrodes to monitor heart rhythm. The device is slipped beneath the skin through a small incision during an outpatient procedure. Less than 3 percent of the devices in the study had to be removed because of complications, but patients had no long-term problems, according to Medtronic.

Bernstein was an investigator during the trial and a member of the trial's steering committee, for which he was paid by Medtronic. He said he has no financial interest in the device itself. The study's results, he said, could make a significant difference in preventing stroke in some patients.

"I think it's a big deal. It's a big deal for doctors because we have an additional tool to prevent another stroke," he said Friday.