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Last Sunday, the Rock ’n’ Roll half marathon in Raleigh, N.C., experienced a very rare – and very tragic – event.

Two male runners, both in their 30s, collapsed before reaching the finish line and were pronounced dead at nearby hospitals. The causes of their deaths have not been confirmed, but health experts at the race speculated that they experienced fatal cardiac events.

Cardiac arrests during long-distance races are very rare. According to a study published in The New England Journal of Medicine, nearly 11 million runners participated in marathons and half-marathons in the United States between 2000 and 2010, and only 59 had cardiac events.

But when cardiac arrests do occur, they are almost always deadly. Of the 59 cases reported in the study, 42 were fatal.

So what can be done to prevent these tragedies? Should race organizers be stricter with their registration requirements?

Most endurance events, such as marathons or obstacle races like the Tough Mudder, require participants only to sign a waiver before running, which absolves the organizers of liability.

But some experts believe physicals or stress tests should be required, as well.

Why runners collapse

The sudden death of a marathon runner can be a shocking experience, since most  of them are relatively young and in peak physical condition. What’s even more frightening is that many of the conditions that lead to cardiac arrest in runners have few signs or symptoms – if any at all.

For older athletes, a major concern is coronary artery disease (CAD), which occurs when cholesterol and plaque build up in the inner walls of the arteries – causing the vessels to become hardened and narrow. CAD is often seen in patients who are overweight and unhealthy, but plaque buildup can also be genetic or a natural byproduct of aging.

"You can be totally asymptomatic, and your first symptom is sudden death.”

— Dr. Robert Tozzi, a cardiologist at Hackensack Medical Center

Normally, people with CAD will not experience heart problems from day to day, but when they strain their bodies through intense exercise, it can be hard for their hearts to keep up.

“In order to have symptoms, it’s a supply/demand situation,” Dr. Gordon Tomaselli, a cardiologist at Johns Hopkins Outpatient Center and volunteer for the American Heart Association, told FoxNews.com. “You only get chest pains when the demand on the heart outstrips its ability to supply blood and nutrients to other organs. A partial blockage of an artery that supplies blood to the heart may not manifest any symptoms at rest. So you can have lots of non-critical blockages that could, in fact, cause problems during intense exercise.”

While CAD typically affects older people, younger athletes may face a condition called hypertrophic cardiomyopathy (HCM), which causes the muscles of the heart to become thick, forcing it to work harder to pump blood.

Believed to be the result of a genetic defect, HCM affects one out of 500 people. But it largely runs in families, and if an individual develops the condition, approximately half of his or her family members will develop it as well. But, as is the case with CAD, many with HCM do not experience symptoms until it’s too late.

“You’re exercising and your heart doesn’t have the resilience of a normal heart,” said Dr. Robert Tozzi, director of the Gregory M. Hirsch Hypertrophic Cardiomyopathy Center and chief of pediatric cardiology at Hackensack Medical Center in New Jersey. “One of the things that happens is about 70 percent of these people with hypertrophic cardiomyopathy actually develop an obstruction; the thickening gets in the way of the blood flow, so they cannot generate enough blood flow during exercise.

“So you can be totally asymptomatic, and your first symptom is sudden death.”

Should EKGs be required?

Though HCM symptoms are hard to identify, there is a way to determine risk. Doctors advise individuals with a family history of HCM to undergo an electrocardiogram (EKG) – a painless test that records the heart’s electrical activity and often shows a higher wave or higher voltage of production in those who have thickened hearts.

So if testing often can detect these heart conditions, should every runner be required to undergo a physical exam or an EKG before running a marathon?

On this issue, cardiologists are split. Some say yes, testing should be mandatory.

“[HCM] is one of the things we can pick up, and one of the reasons I’m such a big advocate of screening,” Tozzi said. “…If they’re going to go out and do something like this, they need to be aware there’s a risk. Just like you’d suggest to anyone who’s older who wants to exercise – we always suggest seeing a doctor first.”

But others say the argument for mandatory testing is not persuasive. Although EKGs are the best method for picking up underlying heart conditions, they are often plagued by false results – sometimes indicating conditions that aren’t there or overlooking heart murmurs that could be deadly.

What’s more, hundreds of thousands of runners participate in long-distance races every year, making mandatory testing a costly – and time-consuming – endeavor.

“Logistically for a race that has 40,000 people, it’s impossible to check,” said Dr. Lewis Maharam, chairman of the board of governors for the International Marathon Runners Association. “The amount of time a doctor needs to spend to do a full physical and understand what specific tests they’re going to do is about an hour’s worth of time. That’s 40,000 hours, plus additional testing. It’s really on the person to decide, ‘I want to run this race, but I don’t want to be one of the statistics.’”

A better informed public

All cardiologists agree on one central thing: Runners and athletes must be better informed about their health and well-being before entering a race.

Individuals at highest risk of cardiac arrest are those with a long family history of underlying heart conditions and those who experience intense shortness of breath or chest pains during exercise. Health experts say these high-risk runners should be checked out by a doctor first and foremost.

Additionally, many cardiologists call for a fundamental change in the exercise mindset. Many runners and athletes will ignore a bad EKG or words of caution from their doctors, since they really want to participate in the events. Also, many runners will try to push through the pain during the race, not wanting to be perceived as weak or unable to finish.

“People who exercise know what their breathing pattern is with exercise. When you get cardiac arrest symptoms, your shortness of breath is way different; it’s much more severe,” said Dr. Norman Abramson, chairman of the board of directors at the Sudden Cardiac Arrest Foundation. “…So if you run a couple of miles and get chest pains, you should be going to your doctor…. We need that kind of education – making it psychologically acceptable to stop running.”

But the best way to save lives, Tozzi said, may be to have a better educated public. Race organizers could do a better job of informing their runners about the risks associated with running, as well as informing bystanders about what constitutes a cardiac episode.

Tozzi said race organizers also need to station CPR-certified staff members with automated external defibrillators (AEDs) throughout the course.

“The best predictor of who survives [a cardiac event] were those who got immediate bystander CPR,” Tozzi said. “….It’s common to collapse at the end of a marathon, but we need to be able to identify those who are having a cardiac arrest. If the actual person who is having a cardiac arrest does not immediately get CPR or have an AED placed on them, they will be much worse off.

“Increasing our ability to identify the individuals who need immediate attention – that’s going to save lives.”