CHICAGO - Medtronic Inc said it is warning doctors about problems with 6,300 implantable heart devices because the batteries in the devices drain sooner than normal.

Medtronic spokesman Christopher Garland said patient safety was not at risk because the affected devices, called Concerto CRT-D and Virtuoso ICD, will provide three months' warning before their batteries run down.

He said the issue related to a copper component.

CRT-D and ICD are implanted into the chest and help manage irregular heart beats.

There is no risk of sudden loss of output and these devices will continue to deliver therapy as needed until they reach end of service. Patients do not have to do anything differently. They should keep up with their regular device check-ups, Medtronic said in a statement.

Medtronic said it sent a letter to heart doctors this month advising them of the problem.

Although the affected population is relatively small and the safety risk appears contained, we believe this represents yet another black eye for Medtronic's Cardiac Rhythm Management franchise and could incrementally erode market share and their standing among physicians. said analyst Derrick Sung of broker-dealer Sanford Bernstein.

Rick Wise, an analyst with Leerink Swann, played down the issue, saying the number was small and that it should not be a surprise that from time to time there would be problems with sophisticated medical devices.

Shares of Medtronic were down 1.3 percent, or 51 cents, at $38.50. in early New York Stock Exchange trade. (Reporting by Debra Sherman; editing by John Wallace and Steve Orlofsky)