Federal health regulators say an experimental catheter from Medtronic Inc. that uses extreme heat to correct irregular heartbeats caused safety side effects in nearly one-fifth of patients treated, far exceeding the study's safety goal.

Medtronic has asked the Forereuod and Drug Administration to approve its device for atrial fibrillation, which causes the heart's upper chambers beat rapidly and ineffectively. About 2.2 million adults in the U.S. have the condition, which accounts for about 15 percent of strokes, according to the American Heart Association. The condition is usually treated with blood thinning drugs, and in severe cases, open heart surgery.

In a review posted online Tuesday, FDA reviewers said about 56 percent of patients treated with the catheter saw their heart problem corrected. But FDA regulators note that more than 21 percent of those patients experienced a serious side effect shortly after the procedure, including stroke, heart failure and pneumonia. Medtronic's own safety goal for the study was no more than 16 percent of patients experiencing complications, though the company used a smaller segment of events for its calculations than the FDA.

"FDA is concerned about the overall safety profile of the system," the agency states in its review.

n particular, regulators drew attention to strokes in five patients within a month of being treated with the catheter. There were no strokes among patients taking traditional drug therapy.

Medtronic's Phased RF Ablation System uses a form of electrical energy to destroy diseased heart tissue that causes the irregular pumping. Competitors like Johnson & Johnson and St. Jude Medical Inc. have received approval for catheters that treat the temporary forms of atrial ablation, but Medtronic's catheter would be the first approved for persistent atrial fibrillation, which lasts more than a week.

Medtronic studied 210 patients with atrial fibrillation who were randomly selected to receive either treatment with the catheter, or traditional drug therapy. Patients were followed for six months with regular monitoring for symptoms of atrial fibrillation. At the conclusion of the trial, about 56 percent of patients in the catheter group had greatly reduced heart irregularities, compared with only 26 percent in the drug group. However, the FDA said that using medical society criteria, only about 37 percent of patients would have been considered successfully treated.

On Thursday a panel of cardiology experts will discuss and vote on the safety and effectiveness of Medtronic's device. The FDA is not required to follow the advice of the panel, which is composed of doctors from various hospitals and universities from across the U.S.

Minneapolis-based Medtronic is the world's largest medical device manufacturer, specializing in devices for the heart, spine and nervous system. Last year the FDA approved the company's Arctic Front cardiac cryoablation catheter system for temporary atrial fibrillation. The device uses extreme cold to treat atrial fibrillation.