Cardiac arrest causes a person to collapse, stop breathing normally, and become unresponsive within seconds. There is no pulse because the heart has stopped pumping blood effectively. Brain cells begin dying permanently after just four minutes without blood flow, which is why recognizing the signs immediately can mean the difference between life and death.
What Happens in the Moment
The defining signs of cardiac arrest are sudden and unmistakable in combination: the person loses consciousness, stops responding to voice or touch, and either stops breathing or begins gasping abnormally. Unlike fainting, where someone typically recovers within seconds, a person in cardiac arrest will not wake up on their own. Their skin may quickly turn pale, bluish, or grayish, especially around the lips and fingertips, as oxygen stops circulating.
The underlying cause is almost always an electrical malfunction in the heart. In 65 to 85 percent of out-of-hospital cardiac arrests, the heart’s electrical system has deteriorated into a chaotic rhythm called ventricular fibrillation, where the heart quivers uselessly instead of pumping. This often starts as a dangerously fast heartbeat that spirals out of control within seconds. The heart doesn’t stop beating so much as it stops beating in any organized way.
Agonal Breathing Fools Bystanders
One of the most dangerous misunderstandings about cardiac arrest involves breathing. In the first few minutes, many people in cardiac arrest will gasp, snore, gurgle, or moan. This is called agonal breathing, and it is not real breathing. It’s an automatic reflex from the brainstem as it loses oxygen. Bystanders frequently mistake these sounds for normal respiration. When a 911 dispatcher asks “Is the person breathing?” they often say yes, which delays CPR and lowers the chance of survival.
Agonal breathing typically stops within about four minutes. If someone has collapsed, is unresponsive, and is making irregular gasping or snoring sounds, treat it as cardiac arrest. That gasping is actually one of the clearest signals that chest compressions need to start right away.
Warning Signs That Can Appear Beforehand
Cardiac arrest often strikes without any warning at all. But in some cases, the body does send signals in the minutes, hours, or even days before the heart’s electrical system fails. These warning signs include chest discomfort, shortness of breath, weakness, and palpitations (a racing, fluttering, or pounding heartbeat). The challenge is that these symptoms are vague enough to be dismissed as stress, fatigue, or indigestion.
Unexplained fainting is one of the most important red flags, particularly if it happens during physical activity. Fainting during or right after exercise can signal an underlying heart rhythm disorder that puts someone at risk for full cardiac arrest. Repeated fainting episodes, unusual seizure-like activity during exertion, and sudden drops in exercise tolerance all warrant a cardiac evaluation.
Symptoms Can Look Different in Women
Most people picture cardiac arrest or a heart attack as sudden, crushing chest pain. That image is more accurate for men than for women. Women are more likely to experience sweating, nausea, dizziness, and unusual fatigue as their primary symptoms, and these can appear while resting or even during sleep. Shortness of breath, back or jaw pain, pain in the upper abdomen, and extreme tiredness are all common presentations in women. Chest discomfort, when it occurs, is often not the most prominent symptom.
This matters because the warning signs of a heart attack can precede cardiac arrest. A heart attack (blocked blood flow to the heart muscle) is one of the most common triggers for the electrical malfunction that causes cardiac arrest. Recognizing atypical symptoms, especially in women, can lead to treatment before the heart’s rhythm collapses entirely.
Warning Signs in Children and Teens
Cardiac arrest in children is rare but does happen, and the warning signs are different enough from adults to deserve separate attention. According to Children’s Hospital of Philadelphia, the key signals include unexplained fainting during or just after physical activity, repeated episodes of fainting, dizziness or lightheadedness with exercise, and excessive fatigue or shortness of breath that seems out of proportion to the activity level. A child who has a recent viral illness followed by chest pain or a sudden drop in how much exercise they can handle also warrants medical evaluation.
In children, cardiac arrest more often results from an underlying structural or electrical heart condition that may never have been diagnosed. Seizure-like episodes during sports or exercise are sometimes misidentified as epilepsy when they are actually caused by a dangerous heart rhythm.
Cardiac Arrest vs. Heart Attack
These two terms are often used interchangeably, but they describe fundamentally different emergencies. A heart attack is a circulation problem: a blocked artery cuts off blood supply to part of the heart muscle. The heart usually keeps beating, and the person is typically conscious and in pain. Cardiac arrest is an electrical problem: the heart’s rhythm becomes so disorganized that it stops pumping altogether. The person loses consciousness almost immediately.
A heart attack can trigger cardiac arrest, but many cardiac arrests happen independently, driven by inherited rhythm disorders, drug reactions, severe electrolyte imbalances, or scarring from prior heart disease. About 3 to 9 percent of cases occur in people with no evidence of blocked arteries or reduced blood flow at all.
Why Seconds Matter
The brain is the organ most vulnerable to cardiac arrest. Permanent brain cell loss begins after roughly four minutes without blood flow. Every minute without CPR reduces the chance of survival by about 7 to 10 percent. This is why the 2025 American Heart Association guidelines emphasize that anyone who encounters a person who is unresponsive and not breathing normally should begin chest compressions immediately, even before emergency services arrive.
You do not need to check for a pulse. If someone suddenly collapses and won’t respond when you shake their shoulders and shout, and they are not breathing or are only gasping, call 911 and start pushing hard and fast in the center of their chest. The combination of recognizing what cardiac arrest actually looks like, including the misleading gasping, and acting within the first two to three minutes gives someone the best possible chance of surviving with their brain function intact.