Someone collapses in front of you. They do not respond when you speak to them. They are not breathing normally. Those three signs together are the core of how you recognize cardiac arrest, and recognizing them quickly is what gives the person a chance to survive.
Cardiac arrest means the heart has stopped pumping blood effectively. The brain and organs begin to lose oxygen within seconds. Call 911 and start CPR right away. Do not wait to feel for a pulse or look for an app to help you decide. In an emergency, the standard advice from the American Heart Association and the American Red Cross is to begin CPR on any unresponsive person who is not breathing normally.
What Are the First Signs of Cardiac Arrest?
The person is unresponsive, is not breathing normally, and shows no signs of life. That is the practical definition you need in the moment.
Unresponsiveness comes first in most cases. You speak to the person, tap their shoulders, and they do not answer or move. Next, look at their breathing. It may be absent entirely, or it may be slow, irregular, and gasping. Those gasps are called agonal breathing, and they are not real breathing — they are a reflex from a brain that is running out of oxygen. Agonal breathing is common in the first minutes of cardiac arrest, and mistaking it for normal breathing is one of the most dangerous errors a bystander can make.
Some people have chest pain, shortness of breath, dizziness, or feel faint in the minutes before they collapse. Others have no warning at all. About half of people who have a cardiac arrest outside a hospital have no prior symptoms, according to research on out-of-hospital cardiac arrest.
How To Know If Someone Is In Cardiac Arrest vs. Other Emergencies
Cardiac arrest is not the same as a heart attack, fainting, or a seizure, and telling them apart matters for what you do next. The table below shows the main differences.
| Sign | Cardiac arrest | Heart attack | Fainting (syncope) | Seizure |
|---|---|---|---|---|
| Responsiveness | Unresponsive | Usually awake and talking | Briefly unresponsive, then wakes | Unresponsive during, then confused |
| Breathing | Absent or gasping | Normal or short of breath | Normal after waking | May be irregular during |
| Pulse | Absent | Present | Present | Present |
| Chest pain | Sometimes before collapse | Common | Rare | Rare |
| Movement | None | None | Brief limp collapse | Jerking, stiffening |
A heart attack happens when blood flow to part of the heart muscle is blocked. The heart keeps beating, so the person is usually awake and can talk, even if they feel very unwell. A heart attack can lead to cardiac arrest, but most people having a heart attack are not in cardiac arrest.
Fainting is a brief loss of consciousness from a drop in blood pressure or a temporary change in heart rhythm. The person usually wakes within a minute or two and breathes normally. A seizure involves jerking or stiffening movements, and the person is often confused afterward. Neither fainting nor a seizure means the heart has stopped, but a seizure can sometimes be caused by a heart rhythm problem that also causes cardiac arrest. If you are unsure, treat it as cardiac arrest.
Why You Should Not Check for a Pulse
Checking for a pulse is unreliable, even for trained professionals, and it wastes time that the brain cannot spare. Studies have found that both laypeople and healthcare workers often misjudge whether a pulse is present, especially under stress.
That is why current CPR guidance for lay rescuers focuses on two things: is the person responsive, and are they breathing normally? If the answer to both is no, start CPR. The hesitation to feel for a pulse is one reason bystanders delay CPR, and delay costs lives. Every minute without CPR and defibrillation lowers the chance of survival.
What to Do the Moment You Suspect Cardiac Arrest
Act in a set order. The steps below follow standard lay-rescuer guidance.
- Check response. Tap the person’s shoulders and shout, “Are you okay?”
- Call 911 or direct someone else to call. Put the phone on speaker so the dispatcher can coach you.
- Send someone for an AED (automated external defibrillator) if one is nearby. Do not leave the person alone to get it yourself if you are the only rescuer.
- Check breathing. Look for normal breathing for no more than about 10 seconds. Gasping is not normal breathing.
- Start chest compressions. Push hard and fast in the center of the chest. The recommended rate is 100 to 120 compressions per minute, at a depth of at least 2 inches (5 cm) for an adult, allowing the chest to come back up fully between pushes.
- Use the AED as soon as it arrives. Turn it on and follow the voice prompts. The device analyzes the rhythm and tells you whether to deliver a shock.
If you are trained in rescue breaths, give two breaths after every 30 compressions. If you are not trained or are unsure, hands-only CPR — continuous chest compressions — is recommended for untrained bystanders and is far better than doing nothing.
What Does Agonal Breathing Look Like?
Agonal breathing looks like occasional gasps, snoring, or gulping, and it is a sign of cardiac arrest, not a sign of life. It often appears in the first few minutes after the heart stops.
The gasps may be noisy and irregular, with long pauses between them. A bystander may think the person is struggling to breathe and hold off on CPR. That is the wrong call. If the person is unresponsive and the breathing is not normal and steady, treat it as cardiac arrest and begin compressions.
Agonal breathing is one of the most common reasons bystanders fail to start CPR. Recognizing it for what it is — a sign the heart has stopped — is one of the most useful skills a member of the public can learn.
Can Cardiac Arrest Happen Without Warning?
Yes. Sudden cardiac arrest often happens with no prior symptoms, especially in people without known heart disease. When warning signs do appear, they can be easy to dismiss.
The most common warning symptoms in the hour before cardiac arrest, when they occur at all, are chest pain, shortness of breath, and fainting or near-fainting. Some people also report palpitations, nausea, or unusual fatigue. These symptoms are not specific to cardiac arrest and are far more often caused by something else. But new chest pain, unexplained fainting, or a racing heartbeat that makes you feel faint deserves prompt medical attention.
Cardiac arrest is not the same as a “widowmaker” heart attack or any single event. It is the final common result of many different heart rhythm problems, most often an abnormal rhythm called ventricular fibrillation. That is why the AED matters so much: a shock can stop ventricular fibrillation and let the heart’s normal rhythm restart.
Why Bystander Action Matters
Survival from out-of-hospital cardiac arrest depends heavily on what bystanders do in the first few minutes. When CPR is started immediately and an AED is used before emergency crews arrive, survival rates are substantially higher than when no one acts.
Most cardiac arrests outside a hospital happen at home. That means the person most likely to save a life is a family member or neighbor, not a paramedic. Learning CPR and knowing where the nearest AED is located are practical steps anyone can take. CPR classes are widely available through the American Red Cross and the American Heart Association.
Frequently Asked Questions
How do you know if someone is in cardiac arrest?
The person is unresponsive and not breathing normally, or is only gasping. If both are true, assume cardiac arrest and start CPR immediately.
Should I check for a pulse before starting CPR?
No. Lay rescuers are advised not to check for a pulse because it is unreliable and delays compressions. Go by responsiveness and normal breathing instead.
Can a person in cardiac arrest still be breathing?
They may make gasping sounds, but that is agonal breathing, not normal breathing. Treat any unresponsive person who is not breathing normally as being in cardiac arrest.
What is the difference between a heart attack and cardiac arrest?
A heart attack is a blockage of blood flow to the heart muscle, and the person is usually awake. Cardiac arrest is when the heart stops pumping, and the person is unresponsive and not breathing normally.
