The irony of exercise is that while it works wonders for heart health, it actually increases short-term risk for experiencing sudden cardiac arrest (SCA) while you’re working out. The acute risk of SCA is higher when you’re unfit, when you’re engaging in high-intensity exercise, and you have underlying cardiovascular disease.

How does fitness protect the heart?

Endurance athletes (think long distance runners) have large coronary arteries that prevent them from developing low oxygen levels or an interruption of blood flow (ischemia) by preserving blood supply to the heart. This was famously noticed in Clarence DeMar, who was known as “Mr. Marathon” in the early 20th century. The diameter of DeMar’s  coronary arteries were two to three times larger than normal.

Studies have also shown that exercise training cured dogs that were susceptible to a fatal heart arrythmia caused by ischemia. This improvement in heart health was caused by better autonomic nervous system function. The good news is you don’t need to be an athlete for exercise to boost your nervous system function. Healthy young and older adults saw improvements after three months of moderate intensity running three to four days a week at 75% to 85% of maximal heart rate.

What’s the cause of SCA and SCD?

Researchers don’t know the exact cause of SCA and SCD because no two cases are exactly the same. And even though exercise protects the heart, sudden cardiac death (SCD) is the leading cause of death in athletes except for accidental death.

There are some distinctions between younger competitive athletes under the age of 35 and older recreational athletes.

• Competitive athletes. People in this group are more likely to have inherited cardiac diseases that can be structural or electrical. Competitive athletes have a higher incidence of SCD—and these cases get more media attention—but the total number of deaths is greater in recreational athletes.
• Recreational athletes. Atherosclerosis is the most common underlying risk factor for recreational athletes. Middle-aged men make up the overwhelming majority of cases. The increased participation in mass athletic events (e.g., Boston Marathon) and amateur athletes starting rigorous training later in life may contribute to the number of SCA and SCD cases. Marathon runners had a higher risk of SCA versus half-marathoners.

Can SCA and SCD be prevented?

Since 2010, the number of half-marathoners and marathoners in the U.S. who died from a SCA has decreased from 71% to 34% due to faster initiation of CPR and use of an automated external defibrillator (AED) to shock the heart back into a normal rhythm. Proper training in giving CPR and using an AED is paramount to preventing death once sudden cardiac arrest occurs.

Preventing SCA and SCD is surrounded by a bit of controversy. Electrocardiogram (ECG) screening to assess the heart’s electrical signals and diagnose underlying (and mainly genetic) cardiac disorders can be done before an athlete begins rigorous training. Regions with high incidence of certain conditions, such as Italy, have found enormous reductions in SCD with their screening program.

But mass screening increases false positives, may not be cost-effective and can mistakenly disqualify young people from participating in sports for life. ECG screening is also less likely to be useful for recreational athletes with early signs of atherosclerosis. In many scenarios, clincians propose a shared decision-making approach with a collaborative team (athletes, family, physicians) to navigate these issues.

The bottom line

The reward of chronic exercise far outweighs the risk of SCA and SCD, but it is important to acknowledge that a risk does exist. Always consult with your health care team if you experience exercise-induced cardiac symptoms.