The study showed that by programming implantable cardioverter defibrillators, or ICDs, to only kick in when a heart rate rises above 200 beats per minute reduced the number of so-called inappropriate therapies by as much as 80 to 90 percent, the lead researcher said.

"But even more importantly it was associated with a 55 percent reduction in total mortality, and that's on top of the 40 percent reduction in mortality that you ordinarily see with the defibrillator," Dr. Arthur Moss, who led the study, said in an interview.

"This makes the defibrillator not only safer, but it also saves more lives, so it's a pretty significant finding," said Moss, a professor of cardiology at the University of Rochester Medical Center who presented the data at the American Heart Association scientific meeting in Los Angeles.

The ICD market, led by Medtronic Inc, has been shrinking and is now estimated to be worth about $6.5 billion a year. If the study findings are implemented in practice, it could help bolster the market going forward.

"I do hope it is going to help get more devices implanted in patients who are indicated for them. This helps shift the balance between risks and benefits greatly in favor of the device," said Ken Stein, chief medical officer of Boston Scientific's cardiac rhythm management group. Boston Scientific sponsored the study and supplied the ICDs used.

"My jaw dropped when I saw these numbers," Stein said of the added survival benefit. "This was a completely unexpected finding and a wonderful finding."

Currently, most ICDs are set to shock the heart back to normal rhythm when the rate exceeds 170 beats per minute. But a heart rate of 180 or 190 is not always dangerous and may be associated with normal exercise.

An earlier study found that some 20 percent to 25 percent of ICD therapy is inappropriate, meaning shocks are delivered when a patient is not in immediate danger. The shocks are often described as being like a mule kick to the chest and are not only painful but can cause serious anxiety.

"For 20 years the programming has been pretty similar in most of these devices and despite very sophisticated algorithms the ability of the devices to separate what we call benign rhythm disorders from malignant rhythm disorders has not been very good," explained Moss, who also led earlier ICD studies that helped to set guidelines for use of the devices and cardiac resynchronization therapy.

Setting the device to fire at a higher rate of 200 beats per minute reduced the risk of experiencing a first inappropriate therapy by 79 percent compared to standard programming, researchers found.

"We were pretty confident we could reduce inappropriate therapy, but we were concerned there might be a slight increase in mortality," Moss said. "The fact that we had a significant reduction in mortality indicates that a lot of this inappropriate therapy was in fact risky and that hadn't been appreciated before."

He called the life saving benefit "a surprise to us".

Moss said that not only should ICDs be programmed to kick in at 200 beats per minute going forward, but that reprogramming of already implanted devices can be done easily without the need for any invasive procedures.

"It's the simplest thing compared with all the other programming that goes into the device," he said. "It's like opening your garage door with radio frequency."

The three-armed study followed 1,500 patients from September 2009 through October 2011.

One group had ICDs with standard out of the box programming and a second group got ICDs programmed to deliver shock only when the heart rate exceeded 200 beats per minute.

A third group got devices with somewhat more sophisticated programming set to go off with a 60-second delay so that it could monitor the high heart rate to see if it continued before delivering a shock.

Those patients fared better than the ones who had traditionally programmed ICDs, but not as well as those whose device simply fired at above 200 bpm, researchers said.

"We've made true progress in making ICD therapy safer, more effective, and more acceptable to patients," Moss said.

(Reporting by Bill Berkrot; editing by Andrew Hay and Carol Bishopric)

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