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.