(Reuters) - U.S. device reviewers said a Medtronic Inc device was effective for treating a common heart rhythm disorder, but raised concerns about its safety.

In documents released on Tuesday, staffers from the Food and Drug Administration said they were especially worried about the high rate of stroke for patients who used Medtronic's Cardiac Ablation System to treat their persistent atrial fibrillation (AF), a major cause of stroke.

Medtronic shares fell 0.9 percent to $34.40 in morning trading on the New York Stock Exchange, in line with a 1 percent drop in the S&P Health Care Equipment index.

Atrial fibrillation, which affects more than 2 million Americans, is the most prevalent heart rhythm disorder, in which the heart's upper two chambers contract rapidly and irregularly, allowing blood to pool. This can form clots that travel to the brain and cause strokes.

The Medtronic device treats persistent AF, which consistently lasts for more than seven days, or recurs for as long as four years. The company said there are currently no FDA-approved devices to treat this condition.