Sudden cardiac arrest is an abrupt loss of heart function caused by an electrical malfunction in the heart. Within seconds, the person loses consciousness and stops breathing normally because the heart can no longer pump blood to the brain, lungs, and other organs. Without immediate treatment, survival chances drop by about 10% for every minute that passes.
SCA is not the same as a heart attack, though the two are often confused. Understanding what happens during cardiac arrest, what causes it, and how bystanders can respond makes a real difference in whether someone survives.
What Happens Inside the Heart
Your heart beats because of a coordinated electrical system that tells the muscle when to contract. In sudden cardiac arrest, that system short-circuits. The most common malfunction is a chaotic, rapid quivering of the lower chambers (called ventricular fibrillation) where the heart muscle fires in uncoordinated waves instead of producing a unified pumping action. Blood stops circulating almost instantly.
This electrical failure typically requires two things happening at once: a triggering event, like a surge of adrenaline or a brief interruption in blood flow, combined with heart tissue that’s already vulnerable. That vulnerability might come from scarring after a previous heart injury, thickened heart muscle, or inherited defects in the heart’s electrical wiring. The combination creates the conditions for the heart’s rhythm to spiral out of control.
Less commonly, cardiac arrest results from the opposite problem: the electrical system slows dramatically or stops altogether, and the heart simply ceases to beat.
SCA vs. Heart Attack
A heart attack is a plumbing problem. A blocked artery cuts off blood supply to a section of heart muscle, which begins to die. The person is typically conscious, often experiencing chest pain, and their heart is still beating. A heart attack is serious, but it unfolds over minutes to hours, giving time to seek help.
Sudden cardiac arrest is an electrical problem. The heart stops pumping effectively, consciousness is lost within seconds, and death follows within minutes without intervention. A heart attack can sometimes trigger cardiac arrest if the damage to heart tissue disrupts the electrical system, but many cardiac arrests happen without a heart attack occurring at all.
Common Causes and Risk Factors
Coronary artery disease is the leading cause. Most people who experience cardiac arrest outside of a hospital have some form of heart disease, even if they were never diagnosed. Cholesterol buildup in the arteries gradually damages the heart over years, creating the kind of vulnerable tissue that makes lethal rhythms more likely.
Other structural heart problems also raise the risk. Cardiomyopathy (a disease of the heart muscle itself) and heart valve disorders can alter the heart’s shape and function enough to set the stage for electrical chaos. Some forms of cardiomyopathy can trigger cardiac arrest during exercise or even during sleep.
In younger people, the picture looks different. The main culprits are inherited conditions, particularly hypertrophic cardiomyopathy (HCM), a genetic condition where the heart muscle is abnormally thick. HCM is the most commonly identified cause of sudden cardiac death in young athletes in the United States. The second most common cause is a congenital abnormality in how the coronary arteries are positioned, accounting for roughly 17% of cases in young athletes. Heart inflammation and substance use are also significant risk factors in this age group.
Warning Signs Before Cardiac Arrest
SCA is often described as striking “without warning,” but research tells a more nuanced story. A study tracking patients aged 35 to 65 found that 51% experienced warning symptoms in the four weeks before their cardiac arrest. The most common was chest pain, reported by 46% of those with symptoms, often in the form of intermittent episodes of pressure or tightness. Shortness of breath was the second most common, affecting 18%. Fainting, palpitations, flu-like symptoms, and abdominal complaints made up most of the rest.
Critically, 93% of people who had warning symptoms experienced them again in the 24 hours before their cardiac arrest, and in 80% of symptomatic patients, symptoms had started more than an hour beforehand. That window of time represents a real opportunity. New or unexplained chest pain, episodes of feeling faint, or sudden shortness of breath in someone with heart disease risk factors deserve urgent medical attention, not a “wait and see” approach.
What to Do If Someone Collapses
Survival depends almost entirely on how quickly the right steps happen. The American Heart Association outlines a Chain of Survival with six links, and the first three are actions any bystander can take.
- Recognize it and call for help. If someone suddenly collapses, is unresponsive, and isn’t breathing normally, call emergency services immediately. Don’t waste time checking for a pulse if you’re not trained to do so.
- Start CPR right away. Push hard and fast on the center of the chest at a rate of 100 to 120 compressions per minute, pressing down at least 2 inches with each compression. This manually circulates blood to the brain and vital organs, buying time until a defibrillator arrives.
- Use an AED as soon as one is available. Automated external defibrillators are designed for untrained users. The device analyzes the heart rhythm and delivers a shock only if needed. Early defibrillation is the single most important factor in restarting a normal heartbeat during ventricular fibrillation.
The remaining links in the chain involve paramedics providing advanced care, hospital-based treatment after the heart restarts, and long-term recovery including rehabilitation and psychological support. But the outcome is largely determined before the ambulance arrives. Bystander CPR combined with rapid defibrillation can double or triple the chance of survival.
Recovery and Long-Term Management
Surviving cardiac arrest is only the beginning. The brain is highly sensitive to interrupted blood flow, and the duration of the arrest determines how much neurological recovery is possible. Some survivors return to full function, while others face lasting cognitive or physical challenges depending on how long the brain went without adequate circulation.
After a cardiac arrest, doctors work to identify and treat the underlying cause. If coronary artery disease triggered the event, procedures to restore blood flow to the heart may be necessary. For people with structural or electrical heart conditions, an implantable cardioverter-defibrillator (ICD) is often recommended. This small device monitors heart rhythm continuously and delivers a corrective shock if it detects a dangerous arrhythmia, functioning as a personal, internal version of the AED that saved them initially.
Recovery also includes cardiac rehabilitation, medication to manage underlying heart conditions, and often psychological support. Post-traumatic stress, anxiety, and depression are common among cardiac arrest survivors and their families. The adjustment is significant, but many people return to active, fulfilling lives with appropriate ongoing care.
SCA in Young Athletes
Cardiac arrest in a young, apparently healthy athlete is rare but devastating, and it draws attention precisely because it seems so unexpected. The causes in people under 35 are overwhelmingly genetic or congenital rather than the result of lifestyle-related heart disease. HCM, the most common cause, often produces no symptoms before the fatal event. The thickened heart muscle creates disorganized tissue that is prone to electrical instability, especially during intense physical exertion.
Pre-participation screening with a physical exam and personal and family health history is standard practice for competitive athletes. Some countries add an electrocardiogram (ECG) to screening protocols, which can detect HCM and certain electrical disorders before they cause problems. The debate over universal ECG screening continues, but any young person with unexplained fainting during exercise, a family history of sudden death before age 50, or known heart conditions in close relatives should be evaluated thoroughly before participating in competitive sports.