July Resource Post: The Athlete’s Heart

The heart and physical exercise

What happens to the heart during exercise? A lot of things go faster – breathing rate, heart rate, blood flow. Now, imagine an endurance athlete such as a marathon runner, a Tour de France cyclist, an Iron Man triathelete. Their body and their heart perform faster – for hours and hours almost non-stop.

This introduction to cardiac output by the Montana State University explains what is happening to an athlete’s heart during a competition.

Cardiac output (in liters) is the total blood volume pumped by the heart’s ventricle each minute and is calculated as the heart rate (beats per minute) and stroke volume (mL of blood). Below is a table where the performance of a heart at rest and during a cross country competition is compared.

Source: Montana State University; BBC

The blood pumped by the heart is distributed to different organs in the body. When resting, only 15% of cardiac output goes to the muscles. The highest proportion goes to the liver (about 27%) and the kidney (about 22%). During a cross-country skiing competition, the muscles require 60 to 70% of cardiac output, followed by the skin (about 15%) while the requirements for the kidneys and the liver drop below 10%.

The heart of a trained athlete

When the heart performs at top speed regularly such as that of an athlete, the volume of its chambers become larger, the cardiac output also increases. Thus, based on the numbers in the table above, a trained athlete’s heart during a cross-country competition performs almost 8 times more than what a normal does at rest about 3 times more than a non-athlete doing exercise.

Although the numbers will vary from sport to sport and from individual athlete to individual athlete, it is clear from the figures above that a lot of physiological changes occur during physical exercise. Marathon running, cycling, and triathlon are among the most physically demanding sports. Cycling competitions like Tour de France are especially challenging because the event goes on for days with barely time for the cyclists to recover.

The benefits of exercise

We’ve always thought that athletes are the healthiest people on earth. After all, the benefits of exercise are quite well-known (see previous post) for young and old alike, as follows:

• Lowers blood pressure
• Strengthens the heart
• Strengthens the bones
• Prevents disability
• Prolongs life

Lack of exercise on the other hand, is a major risk factor for cardiovascular disorders, diabetes, and osteoporosis among others. The fact that many athletes suddenly collapse and die of cardiac events at their prime during sports events is incomprehensible to us.

Recent research on the athlete’s heart

Studies by Belgian cardiologists show that endurance sports may trigger abnormal heart rhythms (arrhythmias) among athletes, even those without cardiomyopathy. Furthermore, these arrhythmias are not necessarily benign but can lead to more serious heart conditions.

In a first study [1], 46 high-level endurance athletes, mostly cyclists, with ventricular arrhythmia (VA) were monitored for a median of 4.7 years. Over the follow up period, a large number of the athletes developed more serious heart conditions, 18 developed a major arrhythmic event that resulted in 9 deaths. The study concludes that

complex ventricular arrhythmias do not necessarily represent a benign finding in endurance athletes. ..Endurance athletes with arrhythmias have a high prevalence of right ventricular structural and/or arrhythmic involvement. Endurance sports seems to be related to the development and/or progression of the underlying arrhythmogenic substrate.

The second study [2] looked at 22 endurance athletes diagnosed with VA, 15 athletes without VA, and 10 non-athletes without VA as control. Their results show clear differences between athletic and non-athletic hearts. In addition, the condition of VA frequently start as a mild abnormality in the right ventricle that, with prolonged performance of endurance sports, can promote ventricular changes and trigger arrhythmias.

The sudden death of athlete has always been thought to be due to pre-existing conditions such as hypertrophic cardiomyopathy or right ventricular dysplasia. Many of these conditions are genetically determined, thus, the American Heart Association strongly recommends a close look at an athlete’s family history to determine his or her risk.

There is strong indication that excessive physical exercise is linked to potentially dangerous irregular heart rhythms. The studies above report about “acquired right ventricular dysplasia” among endurance athletes that may potentially lead to atrial fribrillation and sudden death. Thus, a trained athlete’s heart may actually become a health hazard rather than the fountain of youth.

Like many things in life, too much of a good thing can be bad. It seems that this also applies to exercise.

As this editorial in the European Heart Journal [3] states:

“…athlete’s heart is a well-known consequence of sport practice, and has been considered as a kind of physiological adaptation to extreme training. It was not until very recently that athlete’s heart (dilatation, hypertrophy and enhanced vagal tone) was recognized as a possible risk factor for the development of atrial fibrillation,establishing a link between excessive training and the presence of arrhythmias.”

References:

1. Heidbüchel et al. High prevalence of right ventricular involvement in endurance athletes with ventricular arrhythmias. Role of an electrophysiologic study in risk stratification. Eur Heart J. 2003 Aug;24(16):1473-80.
2. Ector et al. Reduced right ventricular ejection fraction in endurance athletes presenting with ventricular arrhythmias: a quantitative angiographic assessment. Eur Heart J. 2007 Feb;28(3):345-53.
3. Lluís Mont and Josep Brugada. Endurance athletes: exploring the limits and beyond. Eur Heart J 2003 24(16):1469-1470.

Photo credit

cross-country skiing

cycling 2

marathon