A second safety warning from Guidant Corp., one of the nation's largest medical device manufacturers, urged doctors to stop using five defibrillator models because they could malfunction and may have to be recalled.

Guidant announced its second worldwide safety advisory in a week on Friday, after voluntarily recalling seven defibrillator models last week. The devices sense an irregular heart rhythm and shock the heart back into correct beating. At least 74,900 defibrillators are now under a company warning.

The latest Guidant safety announcement affects the Contak Renewal 3, the company's largest seller; the Contak Renewal 4; the Renewal RF; the Renewal 3; and the Renewal 4 AVT models.

The five models have a magnetic switch that can become stuck in a closed position, preventing the device from treating irregular heart rhythms. The faulty switch also can drain the defibrillator's battery more quickly.

The devices have had at least four malfunctions among them, Guidant said. A fifth case is suspected but not confirmed. No patients using the devices have died or been injured, the company said.

Guidant said physicians should reprogram the defibrillators so their magnet use is off. The devices will continue to function. Patients who hear beeping tones from the defibrillators, however, should go to the emergency room immediately.