CPR (cardiopulmonary resuscitation) is a technique that manually pumps blood through the body when someone’s heart stops. An AED (automated external defibrillator) is a portable device that delivers an electrical shock to restart a normal heart rhythm. Together, they form the two-part emergency response to cardiac arrest, and using both dramatically improves a person’s chance of survival. In one large study, survival was 9% with bystander CPR alone but jumped to 38% when an AED delivered a shock.
What Happens During Cardiac Arrest
Cardiac arrest is not the same as a heart attack. A heart attack occurs when blood flow to part of the heart muscle is blocked, but the heart usually keeps beating. In cardiac arrest, the heart’s electrical system malfunctions and the heart stops pumping blood entirely. The person collapses, stops breathing normally, and becomes unresponsive within seconds.
The most common electrical malfunction is called ventricular fibrillation, where the heart’s lower chambers quiver chaotically instead of contracting in rhythm. No blood moves. Without oxygen, brain cells start dying within about four to six minutes. Survival chances drop by roughly 10% for every minute that CPR and defibrillation are delayed, which is why bystander action before paramedics arrive is so critical.
How CPR Keeps Blood Moving
CPR works by using chest compressions to physically squeeze the heart between the breastbone and spine, forcing blood out into the arteries. Each time you release, the chest recoils and the heart refills. This creates a crude but effective artificial circulation that delivers oxygen to the brain and other organs. CPR does not restart the heart. Its purpose is to buy time, keeping tissues alive until a defibrillator can correct the underlying electrical problem.
Current American Heart Association guidelines (updated in 2025) recommend compressing the chest at least 5 cm deep but no more than 6 cm, at a rate of 100 to 120 compressions per minute. That pace is roughly the tempo of the song “Stayin’ Alive.” For every 30 compressions, you give 2 rescue breaths, each lasting about one second. The ratio is the same for adults whether you have one rescuer or two.
If you’re untrained or uncomfortable giving rescue breaths, hands-only CPR (compressions without breaths) is a valid option. The AHA has recommended it for bystanders since 2008. A large study of over 40,000 witnessed cardiac arrests found that traditional CPR with breaths produced slightly better neurological outcomes, but hands-only CPR is far better than doing nothing. Push hard, push fast, and don’t stop until help arrives or an AED is ready.
How an AED Corrects the Heart’s Rhythm
An AED analyzes the heart’s electrical activity through adhesive pads placed on the chest and determines whether a shock is needed. The device does the thinking for you: it will only allow a shock if it detects a rhythm like ventricular fibrillation that can be corrected electrically. You cannot accidentally shock someone who doesn’t need it.
The shock works by making a large portion of the heart muscle temporarily unresponsive to electrical signals, all at once. This essentially “resets” the chaotic, disorganized electrical waves that were preventing the heart from beating properly. Once those rogue signals are extinguished, the heart’s natural pacemaker cells can resume generating a coordinated rhythm. Think of it like rebooting a frozen computer: you clear the scrambled activity so the normal system can take over.
The shock has to be calibrated carefully. Too weak and it won’t stop enough of the chaotic signals. Too strong and it can damage heart tissue or trigger new abnormal rhythms. AEDs are pre-programmed to deliver the right energy level, which is one reason they’re designed for use by people with no medical training.
Using an AED Step by Step
AEDs are found in airports, gyms, offices, schools, shopping centers, and many other public spaces, typically in wall-mounted cases marked with a green or red heart symbol. Every AED gives voice prompts the moment you open it. You do not need certification to use one.
The basic process: turn on the device, expose the person’s bare chest, and attach the two adhesive pads. One pad goes on the upper right chest below the collarbone. The other goes on the lower left side of the ribcage, under the armpit. The pads have diagrams showing exactly where they go. Once attached, the AED analyzes the heart rhythm automatically. If a shock is advised, the device tells you to press a button and warns everyone to stand clear. After the shock (or if no shock is advised), you immediately resume CPR until paramedics take over or the AED prompts another analysis.
For small children whose chest is too small for both pads to fit without touching, place one pad on the center of the chest and the other on the center of the back. Some AEDs come with smaller pediatric pads or a switch to reduce the energy level for children.
Why CPR and an AED Work Best Together
CPR and defibrillation address two different problems. CPR maintains blood flow to prevent organ death. The AED fixes the electrical malfunction that caused the heart to stop pumping in the first place. Neither one alone is as effective as both together.
The numbers make this clear. In a study of over 4,800 out-of-hospital cardiac arrests, survival to hospital discharge was 9% when bystanders performed CPR but no AED was used. When an AED was applied, survival rose to 24%. When the AED analyzed the rhythm and actually delivered a shock, survival reached 38%. That’s a fourfold improvement over CPR alone.
The sequence matters too. Starting CPR immediately keeps oxygenated blood circulating to the heart and brain. This makes the heart more responsive to defibrillation when the AED is applied. A heart that has received several minutes of good compressions is more likely to respond to a shock than one that has been starved of oxygen.
Legal Protections for Bystanders
Every U.S. state has some form of Good Samaritan law that protects people who provide emergency care in good faith. These laws shield you from negligence claims as long as you’re acting voluntarily, not expecting payment, and using reasonable care. If the person is unconscious or unresponsive, the law assumes implied consent, meaning you don’t need permission to begin CPR or use an AED.
Good Samaritan protections cover what’s called “ordinary negligence,” meaning even if you don’t perform CPR perfectly, you’re protected as long as you’re genuinely trying to help. The only exception is gross negligence or intentional misconduct, which would mean a deliberate disregard for the person’s safety. In practical terms, a bystander who attempts CPR and uses an AED in an emergency faces virtually no legal risk.
What Training Looks Like
You don’t need a certification to perform CPR or use an AED in an emergency. But even a few hours of training significantly improves your confidence and effectiveness. The American Heart Association and Red Cross both offer courses that take two to four hours and teach you to recognize cardiac arrest, perform compressions at the correct depth and rate, deliver rescue breaths, and operate an AED. Many courses now include hands-on practice with mannequins and trainer AEDs.
Online-only courses exist but tend to be less effective for building the muscle memory needed for quality compressions. Blended courses that combine online learning with a short in-person skills session offer a practical middle ground. Certifications typically last two years before requiring renewal.