During cardiac arrest, the heart suddenly stops pumping effectively, so blood flow to the brain and other organs ceases within seconds, and the person becomes unresponsive and stops breathing or only gasps. Without rapid CPR and defibrillation, this lack of circulation quickly leads to brain injury and, within minutes, death.
What happens in the body
The usual trigger is a severe abnormal heart rhythm (often ventricular fibrillation or very fast ventricular tachycardia) that prevents the heart’s ventricles from contracting in a coordinated way, so they no longer pump blood forward. Because circulation stops, oxygen is no longer delivered to the brain and vital organs, causing a “global” lack of blood flow similar to a whole‑brain stroke.
Within seconds, the person collapses, loses consciousness, and has no detectable pulse, and normal breathing stops or becomes abnormal gasping. If the heart rhythm is not restored, irreversible brain damage and death typically occur in minutes due to ongoing oxygen deprivation and cell death.
What the person may experience
Sometimes, just before collapse, a person may feel chest discomfort, palpitations, shortness of breath, dizziness, or nausea, but cardiac arrest can also occur without warning. Once unconscious and without brain activity, the person does not feel pain or awareness of ongoing resuscitation attempts.
For those who are successfully resuscitated, the sudden loss and later return of blood flow can still damage brain cells, leading to problems with memory, thinking, movement, or even coma, depending on how long the brain lacked oxygen. Quick CPR and defibrillation greatly improve the chances of survival and of waking up with good brain function.
How bystanders should respond
If someone suddenly collapses, is unresponsive, and is not breathing normally, it should be treated as cardiac arrest. The recommended actions are:
- Call emergency medical services immediately.
- Start hard, fast chest compressions in the center of the chest (hands‑only CPR) and minimize interruptions.
- Use an automated external defibrillator (AED) as soon as one is available, following its voice prompts.
If you are willing, taking a basic CPR/AED class can prepare you to act quickly in this situation and potentially save a life.