SAN FRANCISCO (KCBS) – They may look like the fittest of the fit, but there are increasing signs that some endurance athletes are destroying their hearts by pushing them too hard.

We’re talking about the kind of jocks whose training routines put their hearts in the red zone and keep them there, sometimes for hours every week. They’re the hard-core types who’ve been training like this for years. Many of them are cyclists–people who think nothing of a six-hour training ride and another one tomorrow.

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But an increasing number of them are reporting cases of atrial fibrillation–an out-of-control heartbeat that some say feels like having a fish flop around in your chest. A-fib, as it’s known, can leave someone’s heart racing at over 200 beats per minute.

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Treatment can include drugs that hold the heart rate down, implantable defibrillators, or ablation surgery. Doctors who’ve treated these cases say the first line of defense is for more people to recognize that something’s wrong when their heart starts behaving strangely.

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KCBS Anchors Susan Leigh Taylor and Stan Bunger spoke with cardiologist Dr. John Mandrola about these cases: