by Tara Parker-Pope in NY Times Health

About two million people participate in marathons or half marathons every year, and you might think that that kind of strenuous exercise involving so many people would lead to a lot of heart attacks. But you would be wrong.

A study published Thursday in The New England Journal of Medicine covered 10 years of running and almost 11 million runners, and found that only 59 people had a cardiac arrest during a race - 51 of them men.

Forty-two of the events were fatal - a 71 percent fatality rate. But the overall case of fatality rate for out-of-hospital heart attacks is 92 percent, so runners were actually safer, partly because marathoners may be healthier than the general population, but also because of the presence of bystanders and on-site medical services.

The average age of a cardiac arrest victim was 42, and most of the arrests occurred near the finish line - after the 20th mile of a marathon and the 10th of a half marathon. The rates of cardiac events were three to five times higher in marathons than in half marathons. People who survived were older on average than those who did not, and having a bystander perform CPR was one of the strongest predictors of survival.

The researchers were able to obtain clinical data on about half of the deaths. For most of these, the cause was a condition called hypertrophic cardiomyopathy, a genetically-caused thickening of the heart wall that makes it electrically unstable. Dehydration was the cause of one death, hyperthermia another, and two people with structurally normal hearts apparently died of arrhythmias. Nine of the 23 people who died had more than one cardiac problem.

"In my mind, the most important public health message that comes from this study is the importance of timely bystander CPR," said Dr. Aaron L. Baggish, the senior author. "That's really the most actionable item - we've found something here that really makes a difference when someone goes down."

The number of race-related cardiac events over the past 10 years has increased, but probably only because more people are participating. Compared with other athletic populations, the death rate in distance running, one per 259,000, is low. The rate for collegiate athletes is one death per 43,770 per year, and for triathletes it is one per 52,630.

"For the runner and the doctor reading this, there are two important messages," said Dr. Baggish, an associate director of the Cardiovascular Performance Program at the Massachusetts General Hospital in Boston. "The first is that exercise and running are healthy. But the second is that exercise is not completely protective against heart problems."

He advises that every runner should have a discussion with their doctor about personal factors that may increase heart risks - and not just distance runners. Among previously healthy middle-aged joggers, one person dies every year for every 7,620 runners.