Medtronic Inc. (MDT) has advised doctors of a potential battery issue in about 6,300 implanted defibrillators that could cause premature depletion, but not without ample advanced warning, a company spokesman confirmed Friday.

The issue involves "Concerto" cardiac resynchronization therapy defibrillators, which coordinate beating in erratic hearts and have defibrillators on board to shock hearts to stop life-threatening problems. It also involves "Virtuoso" defibrillators.

Medtronic spokesman Chris Garland said this is an issue that Medtronic identified, and that it doesn't alter the normal course of regular check-ups and device monitoring.

The company guarantees at least a 90-day warning from devices before the battery runs out, and though the battery may not last as long as normal in these devices, doctors and patients will still get that advanced warning.

"There's no safety concern at all, there's been no reports of injury," Garland said.

Medtronic issued a letter on the matter to doctors earlier this week, he said.

While small in scope, Bernstein Research analyst Derrick Sung still believes this issue could hurt Medtronic's $5 billion heart-rhythm business, which includes both implantable defibrillators and pacemakers. This advisory follows one in May regarding a potential problem with wires in about 37,000 pacemakers and a recall two years ago of faulty cables that connect defibrillators to the heart.

The latter issue cast a long shadow over Medtronic's heart-rhythm business, which has been fighting its way back from a stretch of market-share losses to rivals St. Jude Medical Inc. (STJ) and Boston Scientific Corp. (BSX). One reason for share losses "has been a desire for doctors to spread their implants more evenly among the three manufacturers in order to diversify their recall risk," analyst Sung said.

Shares of Medtronic opened lower Friday and were recently down 2.2% to $38.16.

-By Jon Kamp, Dow Jones Newswires; 617-654-6728; jon.kamp@dowjones.com

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