Image: Competitors dive into the water at the start of the men's elite race at the Hy-Vee Triathlon
Doug Wells  /  AP
Competitors dive into the water at the start of the Hy-Vee Triathlon on Sunday, June 13, 2010, in West Des Moines, Iowa. On Sunday during the Nautica New York City Triathlon, two competitors experienced problems during the swim phase of the race, and later died.
By
msnbc.com contributor
updated 8/9/2011 1:04:14 PM ET 2011-08-09T17:04:14

The heart-related deaths of two New York City triathlon competitors has once again opened the question of whether extreme exercise can be dangerous, or even deadly.

No one knows yet whether either competitor had, for example, an undiagnosed heart problem. But experts say that under certain conditions, even healthy, finely-tuned athletes can get into trouble, especially in the swimming phase of a triathlon. That's where both competitors experienced problems during Sunday's Nautica New York City Triathlon.

Part of the problem is the frigid water that the triathletes plunge into, said Dr. Ajay Kirtane, a cardiologist and an assistant professor of medicine at the New York-Presbyterian Hospital/Columbia Medical Center.

Two participants from NYC triathlon die

“Everyone is amped up, which really revs up their adrenaline levels,” Kirtane said. “And then, bang, they hit the cold water.”

That combination of skyrocketing adrenaline and the shock of the cold water can spark arrhythmias in some people, Kirtane said.

Even if the heart doesn’t develop an arrhythmia, it can be sorely taxed by the sudden cold on a hot day. When we’re hot, our bodies shoot blood to our extremities to help cool us off, Kirtane explained. When we hit the cold water, the blood vessels constrict, our blood pressure goes up and then our hearts have to work that much the harder.

Another element that adds to the risk is the water itself, Kirtane said. If a person develops a mild arrhythmia on land, she might pass out and, at worst, hit her head. But in the water, passing out can lead to death by drowning.

Kirtane points to a study published in 2010 in the Journal of the American Medical Association. The study found that athletes competing in triathlons died at a higher rate than those running in marathons: 1.5 deaths per 100,000 triathletes versus 0.8 deaths per 100,000 marathon runners.

The researchers documented 14 deaths in triathlons between 2006 and 2008. The vast majority of those deaths — 13 — were during the swimming phase of the races.

Initially, those deaths were put down to drowning. But when autopsies were done on nine of the athletes, coroners concluded that seven had evidence of heart problems.

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While there’s no way to make these competitions completely safe, the risk can be lowered if athletes get a thorough check-up, including an electrocardiogram, ahead of time, Kirtane said. The hope is that genetic abnormalities that could lead to heart-related fatalities might be caught before a race.

Dr. John DiFiori suggests that anyone over the age of 39 get a complete physical before starting even training for such an event.

"Far and away the most common cause for deaths in competitive events in people 40 and older is undiagnosed coronary artery disease,” said DiFiori, a sports medicine specialist and team physician for the University of California, Los Angeles. “If you are planning on participating you should consult with a physician and discuss what your plans are and have a thorough evaluation with that in mind.”

Ultimately, though, the risk is still pretty small.

“The rate of death is very low,” Kirtane said. “Frankly, there are a lot of other things we do that are more dangerous.”

Linda Carroll is a health and science writer living in New Jersey. Her work has appeared in The New York Times, Newsday, Health magazine and SmartMoney. She is co-author of the forthcoming book "The Concussion Crisis: Anatomy of a Silent Epidemic."

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