Among 4,800 Norwegian heart patients, who racked up a combined total of over 170,000 hours of aerobic exercise, researchers found three cardiac arrests occurred during workouts and only one was fatal.

The number was too small to say for sure that high impact workouts are just as safe as moderate ones, but they show the overall risk of exercise bringing on cardiac arrest is fairly low, according to the authors.

"I think we're on the right track, but before we make it a standard recommendation, let's get our safety data," said Dr. Steven Keteyian, the director of preventive cardiology at Henry Ford Hospital in Michigan, who was not involved in the study.

There is plenty of evidence that the harder people work out, the more benefit they gain in cardiovascular function, said Oeivind Rognmo, a researcher at the Norwegian University of Science and Technology in Trondheim and lead author of the study.

It's therefore appealing to see whether heart patients could benefit from high-intensity exercise, he says.

The concern for heart patients, however, is that greater exertion comes with a small increased risk of a heart malfunction.

Rognmo and his colleagues wanted to see if people recovering from a heart problem who participate in harder workouts are at greater risk of a heart "event."

The researchers tracked 4,846 patients at three cardiac rehabilitation centers in Norway.

Cardiac rehab involves follow-up medical and lifestyle care to help people recover from a heart attack, heart failure or a heart surgery.

The researchers found that the people in rehab spent a combined total of 129,456 hours working out at moderate intensity and 46,364 hours at high intensity.

All people in the study participated in both types of exercise.

The moderately paced workouts included an hour of walking or other exercise that resulted in exertion at about 60-70 percent of people's maximum heart rate.

At high intensity, people trained with repeated four-minute intervals - four minutes of high impact exercise, such as cycling, jogging or cross country skiing, to get the heart rate up to 85-95 percent of maximum exertion, followed by four minutes of a more relaxed activity, such as walking.

Rognmo and his colleagues analyzed how many times people experienced a heart attack or cardiac arrest during exercise or within an hour afterward.

During the more than 129,000 hours the patients spent exercising moderately, one person died from cardiac arrest.

During over 46,000 hours of high-intensity workouts, two people had cardiac arrest but survived.

"We found that both types of intensities were associated with low event rates," said Rognmo. "I think (high intensity training) should be considered for patients with coronary heart disease."

The rates of heart events seen translate to one per 129,456 hours of moderate exercise by patients and one per 23,182 hours of high intensity exercise.

Rognmo and his colleagues write in their report in the medical journal Circulation that the differences in the number of cardiac arrests during moderate and high intensity exercise were too small to conclude whether high intensity exercise is more dangerous than less demanding workouts.

Because of this uncertainty, Keteyian said high intensity exercise is not yet ready to be broadly recommended.

"I don't think we're ready for prime time because there's a little bit of a difference between the two groups, but it's too small to even say from their study (whether it's meaningful), so we just need more data," Keteyian told Reuters Health.

Keteyian said he recommends that people stick to the current guidelines of keeping exercise to moderate intensity until more safety data show that high intensity workouts are just as safe.

SOURCE: bit.ly/OCEkC3 Circulation, online August 9, 2012.

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