DEAR MAYO CLINIC: What is the difference between sudden cardiac arrest and a heart attack? Who is at risk for both, and what safety measures can we take?

ANSWER: Sudden cardiac arrest may occur in various conditions when someone, while active (playing basketball or walking with friends), collapses and passes out. Their blood pressure drops, and often their heart stops. This may be caused by a lethal arrhythmia, which is when the heart beats abnormally and does not efficiently pump anymore. If your heart is not pumping, blood is not getting to the brain, and that's what causes the collapse.

A heart attack occurs when there is a blockage, and your heart muscle is deprived of blood flow. People experience chest pains or difficulty breathing. A heart attack can sometimes cause a sudden cardiac arrest and even induce a lethal arrhythmia due to irritation from lack of oxygen and blood flow. Ventricular fibrillation and ventricular tachycardia occur when the heart is beating too fast, and it becomes dangerous. This also causes a lack of blood flow to your brain and vital organs since your heart is not pumping efficiently.

If someone is in cardiac arrest and you witness it, call 911. When calling for help, ask for an automated external defibrillator (AED) immediately. If the situation is safe to do so, perform CPR or chest compressions. If nearby, use a defibrillator on the person's chest. If an abnormal, dangerous beat is detected, the machine will appropriately shock the patient out of that rhythm. The defibrillator will give you a prompt so that if you've never done it before, you're not a medical professional or you're nervous, you can follow the instructions and it will tell you exactly what to do.

The overall population of those who are going to get sudden cardiac arrest is very small, but it changes based on your age and underlying medical conditions. Under 35, it is much more common to occur from an underlying cardiomyopathy (abnormal heart muscle). The most common type is hypertrophic cardiomyopathy, where the heart muscle is thickened in regions and is more prone to arrhythmia (an irregular heartbeat). The risk of this is 1 in 500. This is what can cause abnormal heartbeat in young athletes in high school and college.

Over 35, the concern switches to coronary artery disease and heart attack risk. Issues can arise with the heart arteries because of hypertension, family history, diabetes, tobacco use and other risk factors that may cause cholesterol-rich plaques in your heart arteries. If that plaque ruptures, there is no blood flow to the muscle, and the muscle goes into abnormal heartbeats.

People who do not exercise often and live a sedentary life can have sudden cardiac arrest. On the flip side, athletes such as triathletes and marathon runners put a large amount of pressure on their hearts. These patients are looked at more carefully. Family history of sudden cardiac arrest is an important consideration during physical examination. Exercise is critically important, not just for your overall mortality but also your mental well-being — the question is how to do it safely and who is at higher risk.

A 65-year-old who has been running marathons their whole life but has had open-heart surgery for coronary artery disease can continue running safely by keeping speed and capacity at a much lower heart rate. We talk to patients about how to do that. During a physical exam, doctors listen for unusual murmurs and conduct an electrocardiogram (EKG or ECG) and chest X-rays in case cardiomyopathy can be detected. Cardiopulmonary stress testing also can be performed, where a special mask is put on the face to measure oxygen and carbon dioxide. This aids healthcare teams in picking up very subtle irregularities that may signal a heart or lung abnormality.

Exercise plans are then tailored accordingly, but be sure to monitor your symptoms. If you have difficulty breathing during minimal exercise or tightness in the chest, dizziness or passing out spells, these things are concerning and warrant further testing. You can get back to exercising and doing the things you enjoy again — whether that's running a 5K or playing softball with your kids — the goal is to always do so safely.— Brian Shapiro, M.D., Cardiovascular Medicine, Mayo Clinic, Jacksonville, Florida

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