Leads, or wires that carry electrical pulses from the defibrillator - which is implanted in the chest - to the heart have always been the weak link in these systems, used to treat irregular heart beats. All of the device makers have had problems at one time or another with leads.

Boston Scientific bought the leadless technology, known as a subcutaneous implantable cardioverter-defibrillator, or S-ICD, when it acquired Cameron Health last year. The Cameron device was approved by the U.S. Food and Drug Administration in the fourth quarter.

"The S-ICD has not yet been shown to be safe and effective in a diverse patient population," Robert Hauser wrote in the January issue of the Journal of the American College of Cardiology.

Hauser, of the Minneapolis Heart Institute, who has been a vocal critic of medical device companies, also noted that the leadless ICD has not even been shown to be as good as the traditional ICDs that use leads.

He said the technology is a promising one that could fill important gaps, especially by bringing it to countries where facilities to implant traditional devices is not available, but more data are needed first.

"Unless critical questions with regard to safety and efficacy in primary and secondary prevention are addressed, the S-ICD should be confined to certain subgroups," he said.

Dr. Ken Stein, Boston Scientific's Chief Medical Officer for Cardiac Rhythm Management, said there was more in Hauser's editorial that he agreed with than disagreed with.

"But it was approved by the FDA as being safe and effective," he noted in a telephone interview. "It's an alternative for physicians to consider. We agree there are absolutely some patient populations that shouldn't get it," he said.

For example, patients who have a slow heartbeat or uncoordinated pumping function should not get it because the S-ICD cannot pace a slow heart beat or resynchronize contractions of the heart's ventricles. On the other hand, the S-ICD may be a better choice for patients who are at risk for sepsis, a severe blood infection.

"For some patient populations, this is the best option," Stein said. "Somewhere in that middle ground is where the line should be drawn."

Stein said the company is currently running clinical trials and registers that will render more data, some of which will be presented in the spring at the Heart Rhythm Society meeting.

Wells Fargo analyst Larry Biegelsen called the S-ICD technology "transformational" but said this first generation device is likely to be a niche product.

"We estimate the eligible S-ICD opportunity is currently about 14 percent of the total worldwide ICD market, but estimate penetration will be low due to limitations of the first generation device," he wrote in a research note.

Biegelsen estimated worldwide sales of the S-ICD device to be $46 million in 2013. Boston Scientific had sales of $5.4 billion in the first nine months of 2012.

(Reporting By Debra Sherman; Editing by Bernard Orr and Nick Zieminski)

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