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Sudden Cardiac Arrest: Symptoms, Causes, Diagnosis, and Treatment Explained

Writer: Dr. Baraa Alnahhal
Dr. Baraa Alnahhal
Oct 7
13 min read

Updated: 4 days ago

Medically reviewed by Dr. Baraa Alnahhal, MD · Last reviewed: October 2026

Quick Answer

What is sudden cardiac arrest?

Sudden cardiac arrest is the sudden loss of all heart activity caused by an irregular heart rhythm. It is not the same as a heart attack, although a heart attack can trigger it. Symptoms are immediate: sudden collapse, no pulse, no breathing, and loss of consciousness. If you see someone who is unconscious and not breathing, call 911 or your local emergency number, then start CPR: push hard and fast on the chest, about 100 to 120 pushes a minute, and use an AED if one is available. The most common cause is an irregular rhythm called ventricular fibrillation. It often occurs in people with coronary artery disease, a past heart attack, an enlarged heart, valve disease, congenital heart defects, or conditions such as long QT syndrome. Survival is possible with fast, appropriate medical care.

What the research says

  • Survival is low overall, but better when bystanders act. A meta-analysis of 141 studies of adults who received CPR for out-of-hospital cardiac arrest found that 8.8% survived to hospital discharge (95% CI 8.2%–9.4%), rising to 11.3% (95% CI 9.3%–13.2%) when a bystander performed CPR (Yan et al., Crit Care, 2020).

  • Using an AED before paramedics arrive is linked to better outcomes. A meta-analysis of 6 observational studies found that bystander AED use was associated with higher odds of survival to hospital discharge (odds ratio 1.73, 95% CI 1.36–2.18) and of a good neurological outcome (odds ratio 2.12, 95% CI 1.36–3.29). The authors rated the quality of the evidence as low to very low (Holmberg et al., Resuscitation, 2017).

  • Community CPR programs help. A meta-analysis of 9 studies including 21,266 patients found that community interventions to improve bystander CPR were linked with higher survival to discharge or 30 days (odds ratio 1.34, 95% CI 1.14–1.57) and a higher bystander CPR rate (odds ratio 1.28, 95% CI 1.06–1.54) (Yu et al., JAMA Netw Open, 2020).

What Is Sudden Cardiac Arrest?

Sudden cardiac arrest (SCA) is the sudden loss of all heart activity due to an irregular heart rhythm. Breathing stops, and the person becomes unconscious.

Without immediate treatment, sudden cardiac arrest can lead to death. Emergency treatment includes cardiopulmonary resuscitation (CPR) and shocks to the heart from a device called an automated external defibrillator (AED). Survival is possible with fast, appropriate medical care.

Is sudden cardiac arrest the same as a heart attack?

No. A heart attack happens when blood flow to part of the heart is blocked. Sudden cardiac arrest is not caused by a blockage. However, a heart attack can change the heart's electrical activity in a way that leads to sudden cardiac arrest.

What Are the Symptoms of Sudden Cardiac Arrest?

Symptoms of sudden cardiac arrest are immediate and severe. They include:

  • Sudden collapse

  • No pulse

  • No breathing

  • Loss of consciousness

Sometimes other symptoms come first, such as chest discomfort, shortness of breath, weakness, and a fast, fluttering, or pounding heartbeat called palpitations. But sudden cardiac arrest often happens with no warning.

When should you call 911?

When the heart stops, the lack of oxygen-rich blood can quickly cause death or permanent brain damage. Call 911 or emergency medical services if you have any of the following:

  • Chest pain or discomfort

  • A pounding heartbeat

  • Rapid or irregular heartbeats

  • Unexplained wheezing

  • Shortness of breath

  • Fainting or near fainting

  • Lightheadedness or dizziness

If you see someone who is unconscious and not breathing, call 911 or your local emergency services first, then start CPR. The American Heart Association recommends CPR with hard and fast chest compressions. Use an AED if one is available.

How do you do CPR?

Do CPR if the person is not breathing. Push hard and fast on the center of the chest, about 100 to 120 pushes a minute. These pushes are called compressions. If you have been trained in CPR, check the person's airway and give rescue breaths after every 30 compressions. If you have not been trained, just keep doing chest compressions. Let the chest rise fully between pushes, and keep going until an AED is available or emergency workers arrive.

Portable AEDs are available in many public places, including airports and shopping malls, and you can buy one for home use. AEDs give voice instructions, and they are programmed to allow a shock only when it is appropriate.

What Causes Sudden Cardiac Arrest?

A change in the heart's electrical activity causes sudden cardiac arrest. The change makes the heart stop pumping blood, so no blood flows to the body.

A typical heart has two upper and two lower chambers. The upper chambers, the right and left atria, receive incoming blood. The lower, more muscular chambers, the right and left ventricles, pump blood out of the heart. The heart valves act as gates at the chamber openings and keep blood moving in the right direction.

Electric signals control the rate and rhythm of the heartbeat. Faulty or extra signals can make the heart beat too fast, too slowly, or in an uncoordinated way. These changes are called arrhythmias. Some are brief and harmless, but others can lead to sudden cardiac arrest.

Diagram comparing a normal heartbeat with the ventricular fibrillation that causes sudden cardiac arrest.

What is ventricular fibrillation?

The most common cause of sudden cardiac arrest is an irregular rhythm called ventricular fibrillation. Rapid, erratic electrical signals make the lower heart chambers quiver uselessly instead of pumping blood. Sudden cardiac arrest can also happen in people who have no known heart disease.

Which heart conditions can lead to sudden cardiac arrest?

Heart condition

How it can lead to sudden cardiac arrest

Coronary artery disease

Arteries clogged with cholesterol and other deposits reduce blood flow to the heart.

Heart attack

A heart attack, often from severe coronary artery disease, can trigger ventricular fibrillation. It can also leave scar tissue that changes the heartbeat.

Enlarged heart (cardiomyopathy)

The heart muscle becomes bigger or thicker, usually when its walls stretch.

Heart valve disease

Leaking or narrowed valves can stretch or thicken the heart muscle, and the strain on larger or weaker chambers raises the risk of an irregular heartbeat.

Congenital heart defect

In children and adolescents, sudden cardiac arrest is often caused by a heart condition they were born with. Adults who have had repair surgery for a congenital defect also have a higher risk.

Long QT syndrome and other heart signaling conditions

Conditions such as long QT syndrome and Brugada syndrome make the heart beat in an unorganized way. If the rhythm is not restored quickly, sudden death can occur. Young people with long QT syndrome are especially at risk.


Who Is at Risk of Sudden Cardiac Arrest?

The same things that raise the risk of heart disease can raise the risk of sudden cardiac arrest:

  • A family history of coronary artery disease

  • Smoking

  • High blood pressure

  • High blood cholesterol

  • Obesity

  • Diabetes

  • An inactive lifestyle

Other things that might raise the risk include:

  • A previous episode of sudden cardiac arrest, or a family history of it

  • A previous heart attack

  • A personal or family history of other heart disease, such as heart rhythm disease, heart failure, or heart conditions present at birth

  • Growing older

  • Being male

  • Using illicit drugs such as cocaine or amphetamines

  • Low potassium or magnesium levels

  • Obstructive sleep apnea

  • Chronic kidney disease

What Complications Can Sudden Cardiac Arrest Cause?

During sudden cardiac arrest, less blood flows to the brain. If the heart rhythm is not restored quickly, complications may include brain damage and death.

How Is Sudden Cardiac Arrest Diagnosed?

Sudden cardiac arrest happens suddenly and needs emergency care at a hospital. If the heart is restored quickly, survival is possible. Once you are stable, the hospital team runs tests to find the cause. These tests show how well the heart pumps blood and look for diseases that affect the heart.

Test

How it works

Blood tests

Check for heart proteins that leak into the blood after heart damage, plus potassium, magnesium, hormones, and other chemicals that affect how the heart works.

Electrocardiogram (ECG or EKG)

A quick, painless test that records the heart's electrical activity through sensors on the chest and sometimes the arms and legs. It shows how fast or slow the heart beats and can reveal changes that raise the risk of sudden death.

Echocardiogram

Sound waves create moving images of the heart, showing how blood flows through the heart and valves and revealing valve problems and heart muscle damage.

Ejection fraction

Measured during an echocardiogram, it is the percentage of blood leaving the heart with each squeeze. A typical ejection fraction is 50% to 70%, and a value below 40% raises the risk of sudden cardiac arrest.

Chest X-ray

Shows the size and shape of the heart and lungs and may show heart failure.

Nuclear scan

Usually done with a stress test. A small amount of radioactive tracer is given by IV, and special cameras follow it through the heart and lungs to show changes in blood flow.

Cardiac catheterization

A thin, flexible tube is guided to the heart through a blood vessel, usually in the groin or wrist. Dye shows blockages in the heart arteries on X-ray. Balloon angioplasty and a stent can be placed during the test if a blockage is found.


Flowchart of the tests used to find the cause of sudden cardiac arrest, from blood tests and ECG to echocardiogram and cardiac catheterization.

How Is Sudden Cardiac Arrest Treated?

Treatment includes immediate CPR to prevent death, resetting the heart rhythm (defibrillation) with an AED if one is available, medicines to treat irregular heartbeats and manage symptoms, and procedures or surgery to place heart devices or treat a blockage. In the emergency room, the team looks for the cause, such as a heart attack, heart failure, or changes in electrolyte levels, and treatment depends on what they find.

What medicines are used?

Anti-arrhythmic drugs can help restore the heart rhythm. Other medicines that might treat the causes of sudden cardiac arrest or lower the risk include beta blockers, angiotensin-converting enzyme (ACE) inhibitors, and calcium channel blockers.

What procedures and surgeries are used?

Surgery and other procedures may be needed to correct an irregular heartbeat, open a blockage, or place a device to help the heart work better.

Treatment

How it works

Implantable cardioverter-defibrillator (ICD)

A battery-powered unit placed under the skin near the collarbone, similar to a pacemaker. It continuously checks the heart rhythm and sends shocks to reset it if it finds an irregular heartbeat, which can stop a life-threatening change.

Coronary angioplasty (percutaneous coronary intervention)

Opens blocked or clogged heart arteries. A tube with a tiny balloon is moved to the blockage, and the balloon is widened to open the artery. A metal mesh tube called a stent may be left in place to keep it open.

Coronary artery bypass surgery (CABG)

Creates a new pathway for blood to flow around a blocked artery to the heart, restoring blood flow.

Radiofrequency catheter ablation

Blocks a faulty heart signaling pathway. Catheters guided to the heart use heat to create small scars that block the irregular signals.

Corrective heart surgery

Corrects heart conditions present at birth, heart valve disease, or diseased heart muscle.


Overview of sudden cardiac arrest care, from the emergency response of calling for help, CPR, and an AED to hospital tests and treatments that help prevent another event.

How Can Sudden Cardiac Arrest Be Prevented?

Prevention starts with keeping the heart and blood vessels healthy. These steps may help:

  • Eat a heart-healthy diet

  • Stay active and get regular exercise

  • Do not smoke or use tobacco

  • Keep a healthy weight

  • Limit alcohol: if you drink, do so in moderation, which means no more than one drink a day for women and men older than 65, and no more than two drinks a day for younger men

  • Manage stress

  • Have regular checkups and get screened for heart disease

  • Control blood pressure and cholesterol

Genetic tests can show whether you have long QT syndrome, a common cause of sudden cardiac death. Check with your insurer to see whether the test is covered. If you carry the long QT gene, your healthcare professional may recommend testing for other family members.

If you have a known risk of cardiac arrest, your healthcare professional might recommend an implantable cardioverter-defibrillator (ICD), which is placed under your collarbone. You might also consider buying an AED for home use and discuss it with your healthcare team. AEDs can be expensive and are not always covered by health insurance.

Should people around at-risk individuals learn CPR?

Yes. If you live with someone who is at risk of sudden cardiac arrest, it is important to be trained in CPR. The American Red Cross and other organizations offer courses in CPR and defibrillator use. Training helps your loved one and may help others too: the more people know what to do in a heart emergency, the greater the survival rate is likely to be.

Conclusion

Sudden cardiac arrest is sudden, often silent, and severe. The heart's electrical activity changes, the heart stops pumping, breathing stops, and the person collapses without a pulse. It is not a heart attack, although a heart attack can set the stage for it. The most common mechanism, ventricular fibrillation, makes the lower chambers quiver instead of pump. Coronary artery disease, a previous heart attack, an enlarged heart, valve disease, congenital heart defects, and conditions such as long QT syndrome all raise the odds, and some cases strike people with no known heart disease.

Speed makes the difference. Calling for emergency help, pushing hard and fast on the chest at 100 to 120 compressions a minute, and using an AED are the actions that matter in the first minutes. Hospitals then work to find the cause with an ECG, echocardiogram, blood tests, imaging, and catheterization, and to prevent the next event with medicines, an ICD, angioplasty with stents, bypass surgery, catheter ablation, or corrective surgery. For people at long-term risk, a heart-healthy lifestyle, regular screening, genetic testing for conditions such as long QT syndrome, and CPR training for family members complete the protection.

Next Steps

Learn hands-only CPR today. It takes minutes to learn and can be the difference between life and death. Find out where the AEDs are in your workplace, gym, and community center, and consider one for your home if a family member is at risk. If you or a loved one has fainting episodes, rapid or irregular heartbeats, unexplained shortness of breath, or a family history of sudden cardiac events, ask your doctor about screening. And keep your heart's risk factors in check: don't smoke, stay active, eat well, and control your blood pressure and cholesterol.

Frequently Asked Questions

Sudden cardiac arrest is the sudden loss of all heart activity due to an irregular heart rhythm. Breathing stops and the person becomes unconscious. Without immediate treatment, it can lead to death.

No. A heart attack happens when blood flow to part of the heart is blocked, while sudden cardiac arrest is not caused by a blockage. A heart attack can, however, change the heart's electrical activity and lead to sudden cardiac arrest.

Symptoms are immediate and severe: sudden collapse, no pulse, no breathing, and loss of consciousness. It often happens with no warning.

Sometimes. Chest discomfort, shortness of breath, weakness, and a fast, fluttering, or pounding heartbeat (palpitations) can come first. Often there is no warning at all.

A change in the heart's electrical activity makes the heart stop pumping blood. The most common cause is an irregular rhythm called ventricular fibrillation.

It is a rhythm in which rapid, erratic electrical signals make the lower heart chambers quiver uselessly instead of pumping blood. It is the most common cause of sudden cardiac arrest.

Yes. It can happen in people who have no known heart disease.

Coronary artery disease, heart attack, an enlarged heart (cardiomyopathy), heart valve disease, congenital heart defects, and heart signaling conditions such as long QT syndrome and Brugada syndrome can all cause it.

In children and adolescents it is often caused by a heart condition they were born with, called a congenital heart defect. Young people with long QT syndrome are also especially at risk.

The same things that raise the risk of heart disease: a family history of coronary artery disease, smoking, high blood pressure, high cholesterol, obesity, diabetes, and an inactive lifestyle.

A previous cardiac arrest or a family history of it, a previous heart attack, other heart disease, growing older, being male, illicit drug use such as cocaine or amphetamines, low potassium or magnesium levels, obstructive sleep apnea, and chronic kidney disease.

Less blood flows to the brain. If the heart rhythm is not restored rapidly, complications may include brain damage and death.

Yes. Survival is possible with fast, appropriate medical care, including CPR and defibrillation.

Call 911 or your local emergency number, then start CPR. Use an automated external defibrillator (AED) if one is available.

Push hard and fast on the center of the chest, about 100 to 120 pushes a minute. If you are trained, check the airway and give rescue breaths after every 30 compressions. If you are not trained, just continue chest compressions until an AED is available or emergency workers arrive.

An automated external defibrillator is a portable device that shocks the heart to reset its rhythm. It gives voice instructions and allows a shock only when appropriate. AEDs are found in many public places and can be bought for home use.

Once the person is stable, doctors run tests such as blood tests, an ECG, an echocardiogram with ejection fraction, a chest X-ray, a nuclear scan, and cardiac catheterization.

An ICD is a battery-powered unit placed under the skin near the collarbone. It continuously checks the heart rhythm and sends shocks to reset it if it detects a dangerous irregular heartbeat.

Keep your heart healthy: eat a heart-healthy diet, stay active, don't smoke, keep a healthy weight, limit alcohol, manage stress, have regular checkups, get screened for heart disease, and control blood pressure and cholesterol.

Yes, especially if you live with someone at risk. The American Red Cross and other organizations offer CPR and defibrillator courses, and the more people who know what to do, the greater the survival rate is likely to be.

References

Medical Disclaimer

This article is for informational purposes only and does not constitute medical advice. Sudden cardiac arrest is a medical emergency, so always call emergency services immediately. Always seek the advice of your physician or other qualified health provider with any questions you may have regarding a medical condition.

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