(Reuters) - AtriCure Inc's Synergy Ablation System heart-rhythm device is safe and effective, but its trial size and design raise some concerns, U.S. medical device reviewers said.

The latest trial "appears to have succeeded" if all enrolled patients are considered, Food and Drug Administration researchers said in documents regarding AtriCure's device, which treats abnormal heartbeat in patients requiring open heart surgery.

Some patients in the trial had sudden-onset heart rhythm problems, meaning they may be healthier than those with more advanced forms of the heart condition. Excluding those "paroxysmal patients," the trials meet neither the safety, nor the effectiveness goals, researchers said.

Atrial fibrillation is the most prevalent heart rhythm disorder, affecting more than 2 million Americans, and a common cause of stroke.

AtriCure's Isolator Synergy ablation system was approved in 2001 to remove soft tissues during general surgery. In 2007 had its indication updated to allow use for ablation of specifically heart tissue during surgery.

The device helps restore normal heart rhythm in patients with permanent atrial fibrillation, or abnormal heart rate, who require open-heart surgery.

Without a specific indication for atrial fibrillation, the drug maker said it has been prohibited from training physicians to use the device for treatment of abnormal heart rhythm or to discuss the condition in any way.

Shares of AtriCure fell in afternoon trading on the Nasdaq, dipping 3 percent to $11.28, in contrast with the S&P Health Care Equipment Sub-Industry Index, which rose 1.9 percent.

While such devices are still a relatively small part of the multibillion atrial fibrillation market, analysts expect it to grow.

(Reporting by Alina Selyukh in Washington)