The company said the machines may not correctly analyze a patient's heart rhythm, possibly preventing the machine from defibrillating the heart when it is needed.

The recall covers 1,924 Lifepak 500 AEDs made in 1997, the company said. Automatic defibrillators analyze heart rhythm in people that may be having a heart attack and, if necessary, shock the heart back into rhythm.

Medtronic said it has received 54 reports of instances involving this group of defibrillators, including eight cases where it may have prevented patient resuscitation.

The company said the devices being recalled are part of the first generation of automatic defibrillators and represent about 1 percent of the machines in use.

Medtronic said it has sent notices by mail to purchasers of the machines offering to upgrade the machines at no charge.

Defibrillator customers with questions can call the company at 1-877-873-7630 or visit its Web site: http://www.medtronic-ers.com/500 to determine if their device is included in the action.

Medtronic Inc. is based in Minneapolis; its business division that manufactures the defibrillators, Medtronic's Emergency Response Systems, is based in Redmond, Washington.