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.

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.

In a letter to heart doctors sent earlier this month, Medtronic said the affected devices were manufactured primarily in 2006 and can be traced to a specific subset of low voltage capacitors. The company said it had received no reports of death or injury related to the issue.

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 1.6 percent at $38.39. in morning New York Stock Exchange trade.