Athlete's heart: when a bigger heart is normal, and when it is not

A 34-year-old marathon runner has an echo done for a medical check. His left ventricle turns out to be roomier than average. At home he types "athlete's heart" into Google and finds two kinds of pages: one says it is completely normal, the other says it might be dangerous.

What neither of them does is explain how you tell the difference.

I find that the most annoying kind of health writing there is. There is in fact a measured boundary, established in 947 elite athletes. That is where I want to start.

What is an athlete's heart exactly?

An athlete's heart is your heart muscle adapting to years of training. Your left ventricle gets roomier, the wall slightly thicker, and your heart pumps more blood per beat. That is why your resting heart rate can drop a long way without anything being wrong. It follows training load, it is not a disease.

The term covers two things that often get mixed up. Endurance athletes mostly get a roomier chamber, strength athletes mostly a slightly thicker wall. Train both and you usually see both.

Your resting heart rate is the cheapest gauge you own. What it does and does not say sits in what your resting heart rate says about recovery.

How do you know if you have an athlete's heart?

With an echo of your heart, and no other way. An athlete's heart is a structural change, so somebody has to see it to establish it. A low resting heart rate or an unusual ECG make it more likely, but they prove nothing. A blood test can neither confirm it nor rule it out.

I want to say that last part plainly, because it is exactly what athletes hope for. A tube of blood tells you something about iron, inflammation and recovery. It tells you nothing about the dimensions of your left ventricle.

For that you need an echo, and your GP or a sports physician arranges one.

What are the features of an athlete's heart?

A low resting heart rate, a roomier left ventricle, a slightly thicker wall and a heart muscle that squeezes normally. On an ECG you often see slow conduction and tall deflections. The most telling feature may be what is absent: symptoms. Somebody with an athlete's heart simply performs.

Pelliccia and colleagues examined 947 Italian elite athletes by echo and described the upper limit. Wall thickness ranged from 7 to 16 mm, and only 16 athletes (1.7 percent) sat at 13 mm or more (PMID 1824720).

In all sixteen, the left ventricle was enlarged as well.

That is the sentence that matters. A thicker wall travels with a roomier chamber. If the thickness arrives on its own, the pattern fits training less well.

The great majority of trained athletes therefore sat comfortably inside the physiological range. Only a small minority came near the boundary, and those were rowers, canoeists and cyclists. Below, the features sit side by side, with the distinguishing tell on every row.

Is an athlete's heart good or dangerous?

For most trained athletes it is a normal adaptation and no cause for worry. The distinction doctors draw is not about size but about pattern. The ESC describes a cardiomyopathy as a heart muscle that is structurally and functionally abnormal, with no other explanation for it (PMID 17916581).

In an athlete's heart the function is normal. That is the hinge: a roomier chamber that pumps well is a different thing from a roomier chamber that does not.

The Hartstichting, the Dutch heart foundation, holds the same line: an athlete's heart is not in itself a disease.

There is one point where I would ask further, and that is your family. The ESC scheme splits heart muscle disease explicitly into familial and non-familial forms (PMID 17916581). If something like that runs in your family, or an unexpected death at a young age, it is worth mentioning to your GP.

Does an athlete's heart go away if you stop training?

Partly, and that is the most useful distinction available. An adaptation built by training can recede once the training stops. After a longer rest you often see the chamber diameter and wall thickness drift back towards normal. Heart muscle disease does not respond to detraining.

Cardiologists genuinely use that difference, for instance after a few months out with an injury. It takes time, but it is one of the few ways to answer the question directly.

Which is exactly why nobody is in a hurry here.

Does a resting heart rate of 38 fit an athlete's heart?

It can fit perfectly well, certainly in a well-trained endurance athlete. A low resting heart rate arises because your heart moves more blood per beat and your nervous system slows the rate. On its own the number says little about the dimensions of your heart. It mostly says something about your fitness.

What does stand out: a low heart rate paired with dizziness or blackouts is a different story. Then it is no longer about the number, it is about the symptom.

Why does your troponin rise after a long effort?

Because exercise itself can put troponin into your blood. Troponin is the protein doctors use to detect heart damage, and healthy athletes often show a rise after a long hard effort. Research suggests this usually does not reflect injury. So the timing of your blood draw matters.

Shave and colleagues summarised this in JACC. Post-exercise rises are common in healthy participants, they peak shortly after the effort and typically settle back within a day or two (PMID 20620736).

No Dutch page written for athletes mentions this. I think that is a real gap, because it is precisely the situation that causes alarm.

Imagine two cyclists, both with a resting heart rate of 42 beats per minute. One draws on Monday, two hours after a 160 kilometre ride. The other draws on Thursday, after three quiet days.

Only in the second are you measuring a resting value.

In practice that means one thing: do not book blood work right after your hardest session of the week. The same holds for inflammation markers such as hs-CRP, which can sit temporarily higher after hard training.

What a blood test does and does not do here

A blood test can neither confirm nor rule out an athlete's heart. What it does do is map the environment your heart works in: iron, inflammation, thyroid, kidney values and lipids.

That is context, not a cardiac assessment.

For endurance athletes that context is often more useful than expected, because a low iron store can push your heart rate up at the same effort. How that ties into recovery sits in blood values for endurance athletes and in faster recovery after training.

If your heart rate creeps up at a fixed pace, that is a recovery signal rather than a cardiac one. Why your zones are often set wrong sits in heart rate during training.

The wider picture of your risk as a fit thirty-something sits in cardiovascular risk in athletes.

What do you do next?

Do not go hunting for reassurance on forums, just make sure your draw is honest. Book your blood work at least two days after your hardest session. If you have symptoms during exertion, or heart muscle disease in the family, calmly make an appointment with your GP or a sports physician.

My own line: I treat an echo and a blood test as two separate tools. The echo answers the question about your heart. The blood answers the question about your recovery.

You draw those wider background values with 360 Health.

Every blood test result includes a professional assessment from a BIG-registered doctor. For treatment decisions, discuss your results with your GP.

References

• Pelliccia A, Maron BJ, Spataro A, Proschan MA, Spirito P. The upper limit of physiologic cardiac hypertrophy in highly trained elite athletes. New England Journal of Medicine, 1991;324(5):295-301. PMID 1824720.
• Shave R, Baggish A, George K, et al. Exercise-induced cardiac troponin elevation: evidence, mechanisms, and implications. Journal of the American College of Cardiology, 2010;56(3):169-76. PMID 20620736.
• Elliott P, Andersson B, Arbustini E, et al. Classification of the cardiomyopathies: a position statement from the ESC Working Group on Myocardial and Pericardial Diseases. European Heart Journal, 2008;29(2):270-6. PMID 17916581.
• Hartstichting. Information on the athlete's heart and cardiac symptoms in athletes.