INDIANAPOLIS – 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.

Indianapolis-based Guidant announced its second worldwide safety advisory in a week on Friday, after voluntarily recalling seven defibrillator models last week. The pager-sized 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.

Doctors said the new problems can be corrected without surgery, but some patients have begun requesting devices from different manufacturers.

“Patients are nervous,” said Dr. William J. Groh, director of the pacemaker and implanted defibrillator clinic at Indiana University’s medical school. “This has really hit the public airways in a big way.”

At Emory University in Atlanta, Dr. Jonathan Langberg was preparing to replace a defibrillator of a patient whose model was recalled by Guidant last week. But the model set to be implanted during a Friday morning surgery received a safety warning, too.

Langberg, director of cardiac electrophysiology at the university, said his patient received a defibrillator from Medtronic Inc., Guidant’s leading competitor.

“The patients I’ve seen in the clinic are understandably worried about whether their safety net has got holes in it,” he said. “The probability that the device will fail is very low. But we’re talking about a device that is necessary to keep you alive, so even a relatively small chance of a problem is something that needs to be thought about very carefully.”

A Medtronic spokesman said the company would be able to absorb any increased demand for defibrillators.

The latest Guidant safety announcement affects the Contak Renewal 3 model, the company’s largest seller, and Contak Renewal 4, as well as the Renewal RF. It also covers 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 limit a defibrillator’s battery life.

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 five defibrillator models 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.

“Guidant is in the very early stages of diligent evaluation of the component failure,” company spokesman Steve Tragash said Friday. “We are implementing a conservative step of halting implants until we are satisfied with device performance.”

Tragash said the faulty devices were found through the company’s “quality system.”

FDA spokeswoman Julie Zawisza said the agency was evaluating Guidant’s announcement and that it had not yet been classified as a recall.

“We support the company notifying the physician community and patients about this latest device issue,” she said. “We’ll be looking at the information they provided in the coming hours and days to evaluate the situation.”

Last week, Guidant said two deaths had occurred in patients using the models included in its initial warning, but didn’t specify which models were responsible. It also said the seven devices in question had 45 failures among them, with 43 among the Ventak Prizm 2 DR, the Contak Renewal and the Contak Renewal 2. Guidant said about half of 50,000 defibrillators recalled last week could be reprogrammed without removal.

A lawyer who filed suit against Guidant for failing to release information about faulty defibrillators said the most recent announcement is a sign of a troubled company.

“The truth is if this is evidence of anything, it’s that things are all upside down at Guidant,” said New York attorney Chris Seeger.

“Why does there have to be a major failure in one of the devices to learn there are potential failures in almost all of their devices? This is getting frightening,” he said.

Guidant reported $1.8 billion in defibrillator sales for 2004, an 18 percent increase over the previous year and 47 percent of the company’s total sales.