Boston Scientific Corp. confirmed Friday that two recently launched implantable defibrillators may deliver unneeded shocks to patients, or fail to shock the heart when needed.

The malfunction in the Cognis and Teligen defibrillator models has occurred 15 times and may prove a problem in 8,000 patients who use a respiratory sensor on the device. No deaths have been reported as a result of the potential malfunction, the Natick, Mass.-based company said.

Implantable cardioverter defibrillators are stopwatch-sized devices that shock an errantly beating heart back into rhythm.

The March 23 "Dear Doctor" letter said insulated wires, or leads, used to connect the defibrillator to the heart could fracture. The risk can be eliminated by switching off the device's respiratory sensors, which track a patient's breathing. They are not essential to the delivery of the device's therapy.

Boston Scientific's letter has not been classified as a recall by the Food and Drug Administration.

Boston Scientific spokesman Paul Donovan said the company had contacted regulatory authorities, but has no plans to recall the devices or stop selling them. The respiratory sensor can be switched off wirelessly, he said.

Some of the lead wires that fractured were made by other manufacturers, Donovan said. He declined to say how many or who they were made by. Boston Scientific has sold 34,000 Cognis and Teligen devices, he said.

Industry analysts were divided on the significance of the letter.

"More than anything, I think doctors and patients want to be assured that Boston Scientific is delivering top-notch stuff," said Christopher Warren, a Caris & Co. analyst in New York. "Any time something like this comes out it shakes confidence in not just one company but the industry as a whole."

But Tim Nelson, an analyst at Minneapolis-based FAF Advisors, said the letter to doctors shouldn't be a problem for Boston Scientific. "You can program the device to avoid the consequences,'' he said. "It doesn't mean the features that everyone likes so much about them are no longer valid."

Boston Scientific's stock closed Friday at $8.20, down 37 cents.

The company says the Cognis and Teligen models, released by its Arden Hills division in May 2008, are thinner and last longer than the competition's -- features desired by doctors and patients alike.

The release of the two models has been a high point for the former Guidant Corp. business, which Boston Scientific bought in 2006 for $27 billion. After 11 safety advisories concerning its cardiac rhythm products in 2005, the company worked to improve its quality systems, resulting in just one advisory in 2008.

'Core of what we're doing'

The defibrillators are among the major products Boston Scientific has introduced over the past year that could help it wrest sales from rivals Medtronic Inc. and St. Jude Medical Inc. of Little Canada, CEO Jim Tobin said at a March 17 health care conference with analysts.

"Cognis and Teligen are at the core of what we are doing," he said. "They are nicer than what the other guys have been doing for years."

But growth in the $9 billion-a-year global market for defibrillators and pacemakers has slowed due to safety warnings and patient deaths, including 13 linked to a potentially faulty lead made by Fridley-based Medtronic.

Bloomberg News contributed to this report. Janet Moore • 612-673-7752