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Bolton Wanderers' Nigel Reo-Coker helps a member of the club's medical staff attend to Fabrice Muamba after he collapsed on the pitch during their FA Cup quarter-final soccer match against Tottenham Hotspur at White Hart Lane in London March 17, 2012.Suzanne Plunkett / Reuters

Fabrice Muamba, a 23-year-old professional soccer player in Britain, collapsed during a game last month after suffering cardiac arrest. He's now up and walking. A 27-year-old Indian soccer player was not so lucky: He died a few days after having cardiac arrest during a match. At least six other professional soccer players around the world have died in the last three years from cardiac-related incidents. As well, not a year goes by now without news of a marathoner who died in a race because of a heart attack or cardiac arrest.

Stories such as Mr. Muamba's might shock us – a 23-year-old suffering cardiac arrest? Moreover, a professional athlete in peak physical condition? But it's not just pros who are felled by cardiac conditions; non-elite athletes who over-exert can be just as prone, and for just the same reasons. When a everyday person who looked to be in good health collapses during exercise, the reasons why are rarely a mystery.

"Most of these guys and women, mostly guys, are dying of very well-known causes," says Eric Larose, a cardiologist at the Quebec Heart and Lung Institute and a Heart and Stroke Foundation spokesperson. Many of these deaths could have be prevented if, beforehand, the victims knew their risk factors and the warning signs of an impending cardiac event.

Although deaths at marathons are certainly tragic, the incidence rate is far too low to make it practical for organizers to screen every runner who plans on participating, says Chris Woollam, medical director of the Mississauga Marathon and the Toronto Marathon. A study published in the New England Journal of Medicine in January that looked at running events between Jan. 1, 2000, and May 31, 2010, found that of the 10.9-million runners who participated in marathons and half-marathons in the U.S., 59 suffered cardiac arrest, an incidence rate of 0.54 per 100,000 runners. Most were men, and cardiovascular disease accounted for the majority of cardiac arrests.

And there is no higher incidence of sudden death in runners compared to the general population, Dr. Larose says, adding: "Because of this, the guidelines don't suggest that we specifically screen young populations that are participating in sporting events." Which means the onus is on us to do due diligence when it comes to matters of the heart.

What should we be looking for? First, it helps to know the difference between a heart attack and cardiac arrest.

"A heart attack is a plumbing problem," says Dr. Chris Simpson, chief of cardiology at Queen's University. An artery leading to the heart becomes blocked, usually from cholesterol build-up, and the part of the heart muscle that is fed by that artery becomes damaged.

"Cardiac arrest is an electrical problem," Dr. Simpson says. "The natural rhythm of the heart that generates every heart beat that you have suddenly either just completely stops and becomes electrically silent, or it becomes so fast and erratic that your heart muscle essentially can't keep up with all the electrical signals and so it just quivers – literally quivers – and it can't generate any pumping output."

Sudden cardiac arrest is usually due to a handful of genetically inherited heart rhythm disorders: Long QT Syndrome, Brugada Syndrome, Catecholaminergic Polymorphic Ventricular Tachycardia (CPVT), Arrhythmogenic Right Ventricular Cardiomyopathy (ARVC) and hypertrophic cardiomyopathy.

These conditions can be difficult to diagnose, Dr. Simpson says, "but one of the things we've found recently is that if you take the whole universe of people who just dropped dead at age 23, it turns out that over half of them, in retrospect, had some warning signs."

There are, in fact, three key warning signs. One is having dizzy spells or passing out during exercise. That can be a sign that a your heart's rhythm was out of whack long enough to make you collapse but went back to normal quickly enough to let you wake up, Dr. Simpson says.

Fainting as a result of a loud noise or being startled is a second red flag. "The reasons for it aren't entirely clear, but it's a well-described phenomenon in Long QT Syndrome," Dr. Simpson says.

Third is a family history of sudden death at a young age or of drowning. Many cardiac conditions can make you pass out during exercise, which may not be fatal in a marathon, when you can stand back up after you recover. "But if you pass out in the water, you drown," Dr. Simpson says.

Even if none of these red flags are raised in your case, there still may be reasons to see a physician. If you have a family history of heart disease, fainting, chest pain or cardiac irregularities, you should book a trip to the doctor's office. That trip could result in having your blood pressure checked or having an electrocardiogram done, which can pick up some electrical problems with the heart. More specialized tests include an echocardiogram, a heart ultrasound, which can identify whether or not a person has hypertrophic cardiomyopathy and can also often pick up ARVC, says Harry Rakowski, a senior staff cardiologist at the University Health Network in Toronto.

And if you're older and planning on running your first marathon, you may want to undergo a stress test, Dr. Rakowski says; people over 40 are more likely to have coronary artery disease. You'll run on a treadmill or ride a stationary bike to measure your heart rate and blood pressure while you perform at increasingly more difficult levels of exercise. Or you may have a nuclear stress test that looks at blood flow to the heart.

"These tests can predict whether you have significant coronary disease about 85 per cent of the time," Dr. Rakowski says. They can also pick out some electrical problems. "They'll basically tell you whether the heart's performance during exercise is reasonable or not," he adds.

But these tests are not perfect, which is why anyone exercising needs to know warning signs of a cardiac event. If you have chest pain, feel your heart beating too fast, shortness of breath, or feel yourself become faint during exercise, then "you just have to use common sense and stop," Dr. Rakowski says. "You can't be so macho that you push yourself to exhaustion and to having a heart attack or a fatal rhythm problem."

Few people with the drive to conquer a marathon this summer will enjoy being told to stop, especially those people who think pain is a wall you just push through.

"Marathons have that extra aura about them where people will push themselves just to finish it. Everybody knows you're doing it, your friends are out there, your family's at the finishing line. I've felt it myself," Dr. Larose says. "Don't push. You just want to finish. You don't want to finish at all costs."

Editor's note: Eric Larose is a spokesman from the Heart and Stroke Foundation. Incorrect information appeared in an earlier version. This version has been corrected.

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