Atrial Fibrillation in Endurance Athletes: Signs & Training

Anyone who runs, rides or swims regularly is doing their heart a favour — that is one of the best-established statements in medicine. Which is exactly why an observation discussed in sports cardiology for around thirty years is so irritating: endurance athletes who train very large volumes over decades develop atrial fibrillation more often than non-athletes of the same age. Both things are true at the same time, and there is no contradiction. Exercise lowers the risk of heart attack, stroke and heart failure — clearly, and dose-dependently. And a very high training volume sustained over many years can remodel the heart in a way that raises the risk of this one specific arrhythmia. Anyone using sport to build health should therefore know how atrial fibrillation feels, what role training dose plays, and what changes in training once the diagnosis is on the table.

What atrial fibrillation is

Atrial fibrillation is a loss of electrical order. Normally the sinus node in the right atrium fires a regular impulse that spreads in ordered fashion across both atria and makes the ventricles contract to an even beat. In atrial fibrillation, many small excitation waves circulate simultaneously and uncoordinated through the atrial tissue — the atria "fibrillate" at 300 to 600 impulses per minute instead of contracting. They then barely move blood into the ventricles; filling is taken over by passive pressure differences.

For performance, one consequence matters most: conduction to the ventricles becomes irregular. Instead of an even pulse you get intervals that are sometimes short, sometimes long. Two things follow. First, ventricular filling varies so much that each beat ejects less blood — the same heart rate delivers clearly less cardiac output in atrial fibrillation. Second, the rate often stays persistently high while the fibrillation continues, which adds further strain to the heart muscle. That is why performance drops abruptly during an episode, and why the typical symptoms are a suddenly "rummaging" pulse, breathlessness at loads you normally handle, and a pace you can no longer hold for no obvious reason.

Why endurance sport shifts the risk

The link is well documented in the literature but hard to reduce to a single number, because atrial fibrillation is common without sport too: roughly one in four adults in Europe and North America develops it at some point in life, mostly beyond the age of sixty. The review papers from sports cardiology describe an elevated relative risk for men with decades of high training volume in the order of two- to five-fold compared with an age-comparable general population. In women the association is clearly weaker or not consistently detectable.

Dose is what decides. A gradient appears across several studies: activity of up to a few hours per week carries the lowest risk — moderately active people sometimes show less atrial fibrillation than completely inactive ones. Above roughly ten training hours per week, and with more than ten years of disciplined training, rates in many cohorts rise above the level of the general population. Accumulated lifetime training also counts: in studies of former elite athletes it was mainly the lifelong sum of training hours that predicted the outcome, not the current week. Hard intensity and a high number of race starts act as additional factors, as does a dense block of hard sessions without genuine recovery days. Anyone already steering their weekly plan through a fitness curve (see CTL, ATL and TSB) has the instrument to spot sustained peaks instead of accumulating them.

What happens in the athlete's heart

The remodelling that accompanies long-term endurance training is not a disease picture in itself but an adaptation. The heart gets bigger, wall thickness rises only moderately, the left ventricle fills more slowly and more fully in diastole, resting heart rate falls, heart rate variability rises. These adaptations are why endurance athletes work more efficiently at rest and at moderate intensity than untrained people.

What becomes critical is not the size but the side effect: with decades of high-volume training, fibrotic deposits appear more frequently in atrial tissue, especially near the pulmonary vein openings, where the misdirected excitation waves originate. On top of that, increased resting vagal tone, inflammatory processes after hard sessions and an enlarged atrium all favour the onset and persistence of fibrillation. In endurance athletes the atrium is larger than in untrained people to begin with — physiologically unproblematic, but it increases the surface on which uncoordinated excitation can sustain itself.

There is also a factor that gets overlooked in daily life: behaviours that further irritate the atrium. These include alcohol (even moderate amounts directly before or after a race), dehydration with marked sodium loss, sleep deprivation, febrile infections, untreated hyperthyroidism, high blood pressure and obstructive sleep apnoea. In an endurance athlete with newly diagnosed atrial fibrillation it is therefore always worth checking these triggers — not only asking about training volume. If you sweat heavily during training, the right dosing of sodium and fluid is covered in the article on electrolyte intake.

Warning signs: recognising the pattern

Atrial fibrillation often arrives as a self-terminating episode — which is precisely why it goes undetected for so long. There is, however, a recognisable pattern:

• Irregular instead of even pulse. The key self-test: place two fingers on the neck or the inside of the wrist and count for a full minute. In atrial fibrillation the gaps between beats vary irregularly and without a pattern; there is no recognisable two-beat or four-beat rhythm. Rule-breaking single pauses or double beats are more likely extrasystoles, which a cardiologist needs to assess with a rhythm recording.
• A performance drop with no explanation. Pace collapses within minutes although load, weather and recovery state are familiar. Many athletes know similar drops from low blood sugar or overheating — but in atrial fibrillation the usual recovery after a short pause fails to appear, and the pulse stays irregular.
• Alternating between a very low and a suddenly high pulse. A resting rate of 42 while sitting and an abrupt jump above 150 during light work do not fit a normal heart rate profile.
• Unaccustomed breathlessness and dizziness. Breathlessness at a pace that used to be easy, a dull pressure in the chest, dizziness to the point of vision going grey, sometimes an unpleasant racing heart with obvious irregularity.
• Clustering after alcohol or hard blocks. If episodes occur preferentially 12 to 36 hours after a long run, an overly long ride or a convivial evening, that points to the two interacting.
• Odd values in the training log. A rising pulse at the same pace that cannot be explained by heat or overtraining, and a resting rate that stays permanently restless instead of consistently low. The resting heart rate monitor in the training log is a useful early signal here (see resting heart rate and recovery).

Differentiation matters: atrial fibrillation is not an extrasystole, and both must be distinguished from the harmless respiratory sinus arrhythmia (a pulse that varies with breathing — a normal sign of a healthy heart, especially in young endurance athletes). Without an ECG none of these can be separated reliably. So the rule is: this pattern is no reason to panic, but it is a reason for a timely rhythm assessment.

How the diagnosis is made

A single resting ECG only captures atrial fibrillation if it happens during the recording — which is rare. Cardiologists therefore work through a chain of methods when there is suspicion:

• Holter monitor over 24 to 72 hours. Standard for frequent episodes. For rare ones it is usually not enough.
• Event recorder or single-lead device. A small device the patient triggers when symptoms occur, worn over weeks — this documents sporadic episodes.
• Watch and chest strap heart rate monitoring as screening. Modern sports watches and chest straps detect irregular rhythms from the pulse curve and raise alerts; the measurement quality suffices as a signal, not as a diagnosis. The differences are explained in chest strap versus watch heart rate measurement.
• Implantable loop recorder. For rare but serious symptoms, a miniature device is placed under the skin and records for years.
• Echocardiography and laboratory tests. Assessment of atrial size, wall thickness and pumping function, exclusion of valve problems, plus thyroid values and electrolytes as possible triggers.
• Exercise testing. Ergometry or spiroergometry with rhythm recording shows whether the fibrillation starts under load and whether the rate response is excessively dense or overshooting.

For athletes, what else gets measured matters: an athlete's heart can show thickened walls on echocardiography, which without experienced interpretation could be mistaken for hypertrophic cardiomyopathy — one reason why sports cardiologists handle a referral note reading "irregular pulse during exercise" differently from general practitioners. Anyone documenting their own data should bring a rhythm trace. The principle that a pre-participation heart check is worthwhile before a first race applies here in particular — details are in pre-race health check.

Treatment at a glance

Treatment follows two goals: restore the rhythm or control the heart rate — and lower stroke risk, which in atrial fibrillation arises from clots forming in the fibrillating atrium. For athletes, both goals affect daily training:

• Rate control with beta blockers. Ventricular rate is capped so the heart muscle does not work permanently too fast. The side effect for athletes: maximum heart rate drops by 15 to 30 beats and heart rate responds more sluggishly. Training zones calculated as a percentage of maximum heart rate shift accordingly — power, pace or RPE become the more reliable anchor.
• Rhythm control by cardioversion. An electrical impulse resets the rhythm to sinus rhythm. It works reliably, but without treating the underlying causes the fibrillation often returns.
• Catheter ablation. Cauterising the tissue areas generating the misdirected excitation, mostly at the pulmonary vein openings. In athletes this is frequently the preferred option, because it does not force permanent medication and preserves endurance performance. Success rates are highest for paroxysmal, that is episodic, atrial fibrillation; with long-standing persistent fibrillation they fall clearly. After the procedure a training pause with staged rebuilding applies for several weeks.
• Anticoagulation. Blood thinners significantly lower stroke risk. For sport this mainly means avoiding contact sports, fall risk and impacts to the head. Technical mountain biking, obstacle races, combat sports and alpine skiing are candidates to discuss with the treating physician. Running, road cycling and swimming are generally unaffected.
• Trigger management. Avoiding alcohol, consistently treating high blood pressure and sleep apnoea, sufficient sodium and fluid, sleep and recovery. In intervention studies and in practice, a good number of athletes see episode frequency drop markedly from these measures alone.

Sport with the diagnosis: how to keep it healthy

The old idea that a heart with atrial fibrillation must be spared is considered outdated today. Exercise lowers blood pressure, improves circulatory recovery and works against the risk the arrhythmia carries long term. The decisive change is not abstinence but structure:

• Change the control variable. After diagnosis, heart rate is unreliable as a target, because beta blockers lower maximum heart rate and an episode distorts the pulse curve. Power on the bike, pace when running and perceived exertion on the RPE scale stay meaningful — more on this in pace versus heart rate.
• Reduce the sharp edges. Hard intervals above threshold remain possible in principle, but in smaller amounts and with clearly dosed recovery days. A common approach: reduce weekly load by roughly ten to twenty percent, limit hard work to one, at most two sessions per week, and avoid back-to-back load peaks.
• Make race decisions deliberately. Arguments against a start are an unclear treatment setting, an unassessed cause, fever, hard effort immediately after an episode, an unresolved trigger, or anticoagulation with high fall risk in the sport concerned.
• Thin out episode days consistently. After an episode, take a break, then re-enter with a staged build of easy sessions. It is not the one lost day that costs fitness but the attempt to make it up immediately.
• Document your load control. If your fibrillation is triggered by training patterns, you only see it with longitudinal data: resting heart rate, weekly load and recovery state read together. A training log built for that view replaces guesswork (see keeping a training log).

As always in training control: adaptation comes from the stimulus, but processing needs time and headroom. Anyone tracking weekly load through CTL and TSB spots sustained peaks early instead of explaining after the fact why form collapsed — and can build the training week around the arrhythmia rather than against it.

Common mistakes

• Ignoring symptoms because performance still holds. Atrial fibrillation is often paroxysmal at first and resolves by itself within hours. That self-resolution is exactly why those affected do nothing for years. The atrium takes damage during that time, and the prospects of rhythm therapy fall.
• Checking training as the only cause. Sleep apnoea, high blood pressure, thyroid function and alcohol are frequent triggers. Reducing training volume while leaving the other factors untouched often only partly lowers episode frequency.
• Stopping training after the diagnosis. This is usually the worse path: without exercise, blood pressure, weight and cardiovascular risk rise — the very factors that favour fibrillation. Adjusted training is the rule, not the exception.
• Solo endurance adventures while anticoagulated. Heading into technical terrain on blood thinners without regard for head injuries is not courage but an arithmetic error whose outcome you do not control.

Frequently asked questions