Guidant's defibrillators sense an irregular heartbeat and shock the heart back into proper rhythm. But some of the recalled models can develop an internal short circuit that prevents them from working.

The Food and Drug Administration is not urging that the recalled defibrillators be removed. But it used Friday's action to urge patients to contact their doctors to decide appropriate next steps.

Guidant recalled a series of defibrillators last month. The new FDA action lists a portion of them as Class I recalls, the most serious type -- a move that may help doctors in deciding how aggressively to notify and treat their patients.

Replacement surgery is risky, too, cautioned FDA medical device chief Dr. Daniel Schultz in a teleconference.

"It's important to understand that in most cases these defibrillators work well and save many lives," Schultz said. "We believe very, very strongly that those decisions need to be made between the patient and the doctor."

Internal Short Circuit

The FDA is investigating how Guidant notified the agency and the public of the problem, said Timothy Ulatowski, FDA's medical device compliance chief.

"We are continuing to evaluate the circumstances surrounding the notification by Guidant," he said. The agency will announce its findings in "a matter of weeks, not months."

Now listed as a Class 1 recall are three models: Ventak Prizm 2 DR (model 1861), Contak Renewal (model H135) and Contak Renewal 2 (model H155.) The FDA said about 20,600 of these devices are still believed to be implanted; at least two patients have died.

The devices, about the size of a mini-cassette tape, sense an irregular heartbeat and shock the heart back into proper rhythm. But these models can develop an internal short circuit that prevent it from working.

The problem is caused by deterioration of electrical insulation in the device, and there's no way to predict which ones will fail, the FDA said.

Additional Class II Recalls

Listed as a lower-priority Class II recall Friday are the Guidant models Ventak Prizm AVT, Vitality AVT and Renewal AVT. A Class II recall means that any injury caused by a malfunction should be temporary or reversible.

About 21,000 of these devices are implanted worldwide, including 18,000 in the United States. There are two confirmed malfunctions with these models; neither resulted in injury, the FDA said. Guidant has said this defect can be detected and fixed non-invasively.

Also given a Class II priority are Guidant's newest models, the Contak Renewal 3 and 4, Renewal 3 and 4 AVT and Renewal RF brands. A magnetic switch may become stuck, inhibiting the ability to treat certain irregular heartbeats unless doctors fix it, also possible non-invasively. Four such malfunctions have been confirmed among about 46,000 implanted devices; a fifth is suspected, FDA said.

Tones sounded by the device signal the malfunction; patients who hear those tones should go to the emergency room, the FDA said.

Guidant, based in Indianapolis, Ind., has a $25.4 billion acquisition offer pending from Johnson & Johnson.

Shares of Guidant stock were down in afternoon trading.

Associated Press writer Ashley M. Heher in Indianapolis contributed to this report.