My monthly column at EnduranceCorner this month was about endurance athletes and atrial fibrillation (AF). This is a topic that we’ve visited here at the blog previously, but I’ve added a recent race story of a triathlete who has been bothered by AF for a couple years. Interesting reading.
The Normal Heartbeat
First, let’s take a moment to consider the normal situation with the heartbeat. The heart is truly an amazing pump. Its pumping chambers (the ventricles) contract upwards of 5,000 times per hour (even at rest)—and this continues hour after hour, day after day, for a good many years, we hope!
There is an equally amazing electrical system responsible for making that pump work efficiently. The electrical activity of the heart begins with a self-initiating impulse in the sinus node, located in the right atrium (the upper chamber on the right side of the heart). That impulse travels across to the left atrium, then downward to the left and right ventricles. When this electrical system is working properly, the muscle of the atria and ventricles contracts at just the right time to make the pump work as efficiently as possible.
In the ordinary situation that I’ve just described, we say that there is a sinus rhythm, a heartbeat that starts in the sinus node. If the heart rate is 60 to 100 beats per minute, we call the rhythm normal sinus rhythm. Any rhythm other than normal sinus is called an arrhythmia.
When there is sinus rhythm and the heart rate is slower than 60 beats per minute, we call the arrhythmia sinus bradycardia. When there is sinus rhythm and the heart rate is greater than 100 beats per minute, we call the arrhythmia sinus tachycardia. In the interest of full disclosure, it turns out that these are actually the most common arrhythmias in athletes. We know that for many (if not most) well-trained endurance athletes that the resting heart rate is less than 60 beats per minute. In that case, sinus bradycardia is expected and totally healthy. It’s also true that, with exercise, the athlete’s heart rate will often exceed 100 beats per minute. So, sinus tachycardia is also totally expected and healthy.
AF is the most common sustained arrhythmia in the general population. With AF, the electrical activity in the upper chambers of the heart becomes totally disorganized. Instead of the usual clock-work contraction pattern, the atria almost quiver instead. The electrical signals reach the ventricles in a disorderly pattern and often at a faster-than-usual rate. The heartbeat becomes irregular (a hallmark sign of this problem) and this can be appreciated when you feel your pulse (at your wrist or neck). The AF may be constant (termed persistent or chronic) or intermittent (termed paroxysmal).
From the physiologic standpoint, the major immediate consequence of AF is a reduction in cardiac output, the amount of blood that the heart pumps (per unit of time). There is undoubtedly variability in the degree to which individuals are affected, but the reduction in cardiac output due to AF is often estimated to be 15% or so.
If we think about the general population, the chances of having AF increase with age, from nearly 0% at age 20 to more than 8% at age 80. This is a reflection that AF often accompanies other forms of heart disease such as mitral valve prolapsed or rheumatic heart disease, or a variety of medical problems such as hyperthyroidism (caused by an overactive thyroid gland). When AF occurs without other heart or medical diseases, we call it “lone atrial fibrillation.”
Why is AF important? In the United States, more than 2 million individuals have AF—and many of these are athletes. The arrhythmia is associated with a variety of poor long-term outcomes, including stroke, heart failure, and even death. The mortality rate for individuals with AF is nearly twice that for individuals without AF.
The Athlete and Atrial Fibrillation
It turns out that AF is 2-3 times more common in athletes than in non-athletes. The reason is not entirely clear, but there are several possible explanations. First, it’s likely that the cardiovascular stresses placed on the athlete’s heart over the long term result in structural changes (scarring) in the muscle of the atria which predispose the athlete to irregular electrical activity in the atria. Second, during exercise there is increased firing of various autonomic nerves that supply the heart—and these may disrupt the heart’s normal electrical activity. Lastly, low-level chronic inflammation produced by training has been proposed as a potential mechanism.
In athletes, paroxysmal AF is much more common than persistent AF, but either form can impair the athlete’s performance. During episodes of AF, the athlete might report a variety of symptoms, including fatigue, unusual or unexpected shortness of breath, dizziness, or palpitations (the physical sensation of the irregular heartbeat).
From a performance standpoint, the athlete is affected because of the reduced cardiac output that accompanies this arrhythmia. You can imagine the detrimental effect, during exercise, of a sudden reduction of 15% in the cardiac output. It would seriously limit any top-end effort and would add physiologic stress to any moderate effort.
A Triathlete’s Story
Let me share with you the story of a ~40 year old triathlete who recently competed in the Ironman Louisville race. He had been bothered by AF but was generally able to train well. At his blog, Run on Energy, he describes how the AF affected his race. The account is very typical for how paroxysmal AF can disrupt either training or racing.
This picture shows the heart rate data during the bike leg of his race. It’s remarkable for the number of spikes to heart rates of nearly 200. The accompanying firsthand account is interesting because it describes this athlete’s reaction to the problem and the resulting decrease in performance for the bike leg of the race. Again, this is very typical for athletes with AF.
Caffeine can be related to the occurrence of PAC’s or AF and we often ask affected patients to discontinue caffeine as a first step at treatment.
Medical treatment. A variety of medications are available for patients with AF and are focused on 2 approaches: 1) rate control, to limit the heart rate during periods of AF or 2) rhythm control, to try to prevent AF or convert AF back to sinus rhythm. The typical rate control medications include beta-blockers (eg, propranolol, atenolol, metoprolol) and calcium channel blockers (eg, diltiazem, Verapamil). The typical rhythm control medications include Amiodarone, among others. Unfortunately, though, the success rates for these medications are only about 50%, and the medications may be even less effective in athletes. Moreover, each of these medications comes with potentially adverse side effects.
Another consideration for patients with AF is anticoagulation, or blood thinning, to prevent small blood clots from forming in the atria. These blood clots can break loose, travel through the bloodstream to the brain, and produce stroke. For athletes with no other form of heart disease besides AF, an aspirin each day is probably prudent. For those with other forms of heart disease, a stronger anticoagulant such as Coumadin may be recommended to reduce the long-term risk of stroke. Unfortunately for the athlete, blood thinning with Coumadin also carries a risk of serious bleeding in the event of bodily injury (such as a bike crash), so decisions about anticoagulation deserve much consideration.
Ablation treatment. Another option for individuals affected by AF is ablation, which can be performed either as a catheter-based procedure or as a conventional surgical procedure. With the catheter-based procedure, small catheters can be threaded up to the heart, starting from the arteries or veins of the arms or legs. Electrical energy is then used to ablate (destroy) muscular tissue in the heart walls that is responsible for starting or propagating the AF. In the surgical version, small incisions are made on each side of the chest and specialized instruments are introduced into the chest cavity to perform the ablation on the outside of the heart muscle. For patients with paroxysmal AF, the success rates for these procedures is quite good. For patients with persistent AF, the success rates are not quite as good.
What to Do?
Athletes with an irregular heartbeat—either intermittently or permanently—should seek medical attention. It’s important to sort out the cause of an irregular heartbeat, particularly if the problem disrupts training or racing. Luckily, most athletes with AF can continue to participate fully in their sports.