The Food and Drug Administration today approved a vest-like medical device that is worn under clothing to monitor and treat abnormal heart rhythms in people at risk of dying from sudden cardiac arrest.

The product, made by Lifecor, Inc., of Pittsburgh, Pa., is the first cardioverter defibrillator that can be worn outside the body, rather than implanted in the chest.

The device is a programmable system that senses heart function and automatically delivers an electrical shock when needed to restore normal heart rhythm.

Until now, people who needed a cardioverter defibrillator had to have it surgically implanted into their chest.

"Today's approval represents a significant advance in defibrillator technology. It means patients may now opt for a wearable defibrillator if they are not suitable candidates for an implanted defibrillator or do not want one," said FDA Acting Principal Deputy Commissioner Bernard A. Schwetz, D.V.M., Ph.D.

The new product consists of an electrode belt assembly that is worn around the chest, touching the skin. It is connected to a monitor, with an alarm, that is worn in a holster at the waist. This entire device assembly is worn continuously 24 hours a day, except when the wearer is bathing or showering.

The device continuously monitors the patient's heart to detect life-threatening abnormal heart rhythms. Typically, once a week the physician will want the patient to connect the monitor to an external modem and send the data over the phone to the physician's computer for his review.

The defibrillator detects abnormal heart rhythms by sensing the heart's electrical activity on the surface of the chest. If a life-threatening rhythm is detected and the person loses consciousness, the device delivers an electrical shock to restore normal rhythm.

FDA approved the wearable defibrillator based on a review of laboratory and animal tests and clinical studies of safety and effectiveness conducted by Lifecor.

Some 289 heart patients at 16 medical centers in the United States and Europe were fitted with the defibrillator vest. All were either awaiting heart transplants or had recently had a heart attack or a coronary bypass operation. The patients wore the defibrillator for an average of 20 hours a day for approximately three months.

The wearable defibrillator was 71 percent successful in treating sudden cardiac arrest episodes compared to 25 percent success for patients calling 911. It successfully detected and treated five episodes of sudden cardiac arrest, and detected but was unable to treat two other episodes. The two failures to treat resulted because the patients had incorrectly assembled the electrodes in the vest. Lifecor subsequently changed the design of the garment to make it more user-friendly.

Two percent of patients experienced an unnecessary shock from the wearable device, compared to 2.3 percent false shocks per patient-month received by patients with an implantable defibrillator. Lifecor subsequently modified the device to further reduce the rate of unnecessary shocks.

The most frequent side effect was a temporary skin rash, experienced by 5.9 percent of study patients.