Sudden cardiac arrest is an electrical malfunction in the heart that causes it to stop beating. Unlike a heart attack, which happens when a blocked artery cuts off blood flow to part of the heart muscle, SCA is a problem with the heart’s electrical system. The heart’s lower chambers begin quivering chaotically instead of pumping, and within seconds the person collapses, stops breathing, and loses consciousness. Without immediate treatment, it is fatal within minutes.
In the United States alone, emergency services treated an estimated 263,711 cases of out-of-hospital cardiac arrest in 2024. Only about 10.5% of those people survived to leave the hospital.
How SCA Differs From a Heart Attack
People often use “heart attack” and “cardiac arrest” interchangeably, but they are fundamentally different events. A heart attack is a plumbing problem: a clot blocks an artery, and a section of heart muscle starts to die from lack of blood. The person is usually conscious and aware something is wrong. Cardiac arrest is an electrical problem: the signals that coordinate each heartbeat become erratic, and the heart stops pumping altogether. The person loses consciousness almost immediately.
A heart attack can trigger sudden cardiac arrest, which is one reason the two get confused. But SCA also strikes people who have never had any known heart disease. It can happen to a young athlete on a playing field or an adult asleep at home.
What Happens Inside the Heart
The most common rhythm behind SCA is called ventricular fibrillation. Normally, electrical impulses travel through the heart in a precise sequence, causing the lower chambers (ventricles) to contract and push blood out to the body. In ventricular fibrillation, those signals become rapid and chaotic. The ventricles quiver uselessly instead of pumping. Blood flow to the brain, lungs, and every other organ stops within seconds.
Other abnormal rhythms can cause the same outcome. Some cause the heart to beat dangerously fast without effectively moving blood. Others cause it to slow down or stop entirely. The end result is the same: the body loses its blood supply, and without intervention, the brain begins to suffer irreversible damage within about four to six minutes.
Who Is at Risk
Coronary artery disease is the leading cause in adults over 35. Scarring from a previous heart attack, heart failure, and an enlarged or weakened heart muscle all raise the risk significantly. But SCA also affects younger people, sometimes with no prior warning.
Several genetic conditions make the heart vulnerable to dangerous rhythms:
- Hypertrophic cardiomyopathy: a condition where the heart muscle grows abnormally thick, which can trigger dangerously fast heartbeats. It is one of the most common causes of sudden cardiac death in young athletes.
- Long QT syndrome: a disorder of the heart’s electrical timing that can cause fast, chaotic rhythms and sudden fainting. It can be inherited or caused by certain medications.
- Brugada syndrome and Wolff-Parkinson-White syndrome: other inherited electrical abnormalities that raise the risk of life-threatening arrhythmias.
A forceful blow to the chest, known as commotio cordis, can also cause SCA without damaging the heart muscle itself. It disrupts the electrical signaling at exactly the wrong moment in the heartbeat cycle. This is most often seen in young athletes struck by a ball or another player.
Warning Signs That Precede SCA
Cardiac arrest is often called “sudden,” but roughly half of people who experience it report symptoms in the hours, days, or even weeks beforehand. A study published in The Lancet Digital Health found that the most common warning signs were shortness of breath (present in 41% of cases), chest pain (33%), heavy sweating (12%), and seizure-like activity (11%).
The pattern differed between men and women. Men were more likely to report chest pain, shortness of breath, and sweating before their arrest. For women, shortness of breath was the only symptom that stood out as significantly linked to an impending event. This matters because chest pain is what most people associate with a heart emergency, and women experiencing SCA may not have that classic signal.
Why Every Minute Counts
Survival after cardiac arrest depends almost entirely on how quickly the person receives CPR and defibrillation. Each additional minute without a shock from a defibrillator reduces the chance of survival by about 6%. That narrow window is why bystander action is so critical.
The numbers make this starkly clear. When a cardiac arrest is witnessed and a bystander starts CPR, survival to hospital discharge is 13.0%. When the arrest goes unwitnessed, that drops to 7.6%. Location matters too: arrests that happen in public places, where automated external defibrillators (AEDs) are more likely to be nearby and strangers can intervene quickly, have a 21.1% survival rate. In a home, where most cardiac arrests occur, it drops to 8.9%.
An AED is designed to be used by anyone, regardless of training. It analyzes the heart’s rhythm and delivers a shock only if one is needed. If you see someone collapse and become unresponsive, calling emergency services, starting chest compressions, and using an AED if one is available gives that person their best chance of surviving.
How High-Risk Patients Are Identified
For people with known heart conditions, doctors assess SCA risk using a combination of personal and family history, imaging, and electrical monitoring. Key factors include a family history of sudden cardiac death, unexplained fainting episodes, abnormal heart rhythms picked up on a 24-hour heart monitor, abnormal blood pressure response during exercise, and significant thickening of the heart wall seen on an echocardiogram (an ultrasound of the heart).
A standard electrocardiogram (ECG) can reveal electrical patterns associated with conditions like long QT syndrome or Brugada syndrome. For people with hypertrophic cardiomyopathy, European guidelines now use a composite risk calculator that combines several clinical measurements to estimate the five-year probability of sudden cardiac death, helping guide decisions about preventive treatment.
Prevention for Those at Highest Risk
For people identified as high risk, the most effective preventive tool is an implantable cardioverter-defibrillator, a small device placed under the skin that continuously monitors heart rhythm. If it detects a life-threatening arrhythmia, it delivers an electrical shock to restore a normal rhythm, essentially performing the same function as an external defibrillator but from inside the body.
Clinical trials have shown these devices reduce overall mortality by up to 54% and cut deaths from dangerous arrhythmias by 50% to 70% in high-risk patients. Newer models sit entirely under the skin without threading wires into the heart, reducing certain complications associated with older designs. The device does not prevent arrhythmias from occurring. It corrects them when they happen, buying time and saving lives in a condition where seconds determine the outcome.