You're sweating, your heart feels like it’s beating out of your chest, and you feel dizzy, faintly nauseous, and fatigued beyond measure. This may sound like just another day of endurance training, but if symptoms like these fail to subside, there may be something more sinister going on.

“When the heart beats normally, its natural pacemaker - the sinoatrial node - in the top right chamber (atrium) of the heart sends out electrical impulses at regular intervals signaling the ventricles to pump,” says Frederick Ehlert, MD, an associate clinical professor of medicine and the director of the Cardiac Electrophysiology Fellowship Program at Columbia University’s College of Physicians and Surgeons in New York. “Atrial fibrillation is the result of electrical chaos in the atria that causes the heart to beat rapidly and erratically.”

In contrast to AF, atrial flutter refers to a heartbeat that is extremely fast but still regular. It can, however, progress to AF.

Running Wild

“The concern about either condition is that when the atrium beats too fast, it bombards the ventricle with over 300 impulses per minute and can raise the ventricular rate to uncomfortable and potentially dangerous levels,” says Herbert A. Insel, MD, FACC, senior cardiologist at EHE International, clinical instructor in medicine at New York University School of Medicine and associate physician at Lenox Hill Hospital in New York. “At the very least, when left untreated it can have a significantly deleterious effect on athletic performance.”

Compared with ventricular fibrillation, the instigator of cardiac arrest, AF is usually not life threatening on its own, but coupled with other conditions, it can lead down a risky road.

“A continually elevated heart rate has the propensity to weaken the heart sufficiently to allow congestive heart failure to develop,” Ehlert says. “In addition, AF is associated with stroke, so it is important that other stroke risk factors be considered in patients presenting with AF symptoms so that they can be prescribed anticoagulants if indicated.”

Cardiologists concur that the most significant risk factor in AF is age: 0.1% in the 30-year-old population, increasing to 2.4% by age 80. Aging is accompanied by muscle loss, fibrosis, and a greater likelihood of other factors predisposing to AF, such as leaky heart valves and hypertension. Although additional risk factors abound, “lone atrial fibrillation” can occur in the absence of any.

Moderate Rewards

One factor that remains unlisted is exercise. In moderation, it s heart health’s biggest helper, but taken to the extreme it may be exactly the opposite.

“Statistics show that more and more people are getting involved in endurance sports,” says Stephen Black, DSc, PT, ATC/L, NSCA-CPT, an Ironman triathlete as well as an athletic trainer and strength and conditioning coach at Rocky Mountain Human Performance Center in Boulder, Colo. “However, the typical endurance athlete exercises too long, too often, and at too high a heart rate, which significantly elevates their odds of sudden-onset exercise-induced AF.”

The most prevalent explanation for this correlation is that sustained physical training enlarges the diameter and volume of the atria. Such substrate alterations force changes in the generation and dispersion of electrical impulses, thus predisposing athletes to an uneven and/or rapid heartbeat. Sometimes, that’s all it takes to trip the wire. “Exercise is proven to be good for the heart because it enhances its structure, increases vagal tone, and lowers its resting rate,” Ehlert says. “However, all those things also can cause extra beats, and the more of those you have the higher the likelihood of triggering AF.”

Heartbeat Treatments

Although the road to AF varies from person to person, it’s not necessarily a dead-end street as long as prompt and proper treatment is sought.

“Anyone with AF should seek medical attention, [because] it can be persistent or self-resolve but recur without notice,” Insel says. “Athletes with new-onset AF should not train until it has been addressed and they are cleared to resume.” Once AF is diagnosed, the first line of defense is to re-establish a regular rhythm, either with anti-arrhythmic medication or cardioversion (shocking the heart into a normal rhythm), after which patients should rest a few days before being re-evaluated. Often this process is sufficient, but some patients require long-term treatment, including medication, cardiac ablation (electronic destruction of short-circuiting tissue), modified training programs, and routine follow-ups.

“While our approach is to find a workaround that returns athletes to their former training levels, it’s not always possible,” Black says. “I’ve treated a few whose extenuating circumstances — valve issues, previous lifestyle and/or genetic predisposition — required us to reduce their activity to just a daily walk or a downhill ski at a conservative level,” he says. “It’s often such a dramatic lifestyle change that I strongly encourage psychological counseling through the transition.”

Forewarned Is Forearmed

When it comes to endurance training, it’s vital to obtain professional guidance before embarking on a training program. “Athletes should consult a cardiologist for a baseline stress EKG to determine their appropriate heart rate, and work with a coach to devise a training program according to their own heart rate and athletic goal,” Black says. “I always advise my clients to train with a heart rate monitor using their prescribed heart rates, rather than the preprogrammed and less accurate formulae.”

AF is a serious but not necessarily permanent condition. Experts agree there are actions that can be taken to alleviate or at least accommodate the condition and continue training. It also offers an ideal opportunity for PT involvement. “PTs have a lot to offer in the realm of physical and cardiac health, and it’s a two-way street,” Black says. “From a revenue perspective, the increasing popularity of endurance sports together with an aging population makes a cardiology specialty well worth considering.”