The initial review from the U.S. Food and Drug Administration comes after Boston Scientific Corp, a leading maker of heart devices, agreed in March to buy privately held Cameron Health.

But analysts said the FDA staff review, released online on Monday, may not dent Cameron's chances to win U.S. approval for its device to treat dangerously irregularly heartbeat.

Cameron's defibrillator has been sold in major European countries since 2009, and the FDA staff said on Monday that the company met safety and effectiveness goals during clinical trials - though they worried about its battery depletion issues.

A panel of outside advisers to the FDA is due to vote on the device on Thursday, and the FDA will make a final decision later.

Cameron Health said about 850,000 people in the United States are at risk of sudden cardiac arrest, when the heart suddenly stops beating, and are eligible for an implantable defibrillator. Defibrillators treat overly fast heart rhythms or quiver of heart muscles, which cause most cardiac arrests.

Like similar devices, Cameron Health's defibrillator delivers electric shocks to help a fluttering heart beat normally. But its electrodes are threaded under the skin along the breastbone, rather than in the heart. In-heart electrodes are known as transvenous leads.

Cameron Health has said its device is easier to implant than transvenous leads because it does not require an X-ray machine, and it may also be safer because it is easier to remove than devices inside the heart when there are problems.

Boston Scientific, Medtronic Inc and St. Jude Medical Inc all make implantable defibrillators. Shares of Boston Scientific were down 0.7 percent at $5.90 on Monday afternoon.

Michael Weinstein, analyst at J.P. Morgan, said the issues raised by the FDA staff should not prevent the device from being approved, but he questioned how many patients would want to use it.

For example, it is bigger than some other in-heart defibrillators and delivers stronger shocks, which can be more painful, he said in a research note.

"Our view remains that the Cameron (device), if approved, is likely to be niche product," David Roman, analyst at Goldman Sachs, wrote in a separate note.

FDA staff said in a clinical trials, 97.9 percent of people implanted with Cameron Health's device had no complications with their device after 180 days. The device also converted heart quivers to a normal rhythm in 98.9 percent of patients, meeting the company's goals.

But the device reviewers said they would not approve it until the company resolved issues with a battery that depletes earlier than it should.

Cameron Health's defibrillator should be able to give about 21 shocks over its five-year life, but the FDA staff said there have been three cases of premature battery depletion since June 2011, when the company announced the issue.

The FDA staff said they would not ask panelists to discuss the battery depletion issue on Thursday.

The FDA reviewers also focused on "inappropriate" shocks from the Cameron Health device, when 30.7 percent of patients received an electric shock to the heart without having an overly fast heart rhythm or quiver of the heart muscles. This could cause the device to run out of power even earlier.

(Reporting by Anna Yukhananov; editing by Sofina Mirza-Reid and Matthew Lewis)

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