CPR (cardiopulmonary resuscitation) and an AED (automated external defibrillator) are two emergency tools used together to keep a person alive during cardiac arrest. CPR manually pumps blood through the body using chest compressions, while an AED delivers an electrical shock to reset a dangerously irregular heart rhythm. When both are used before paramedics arrive, roughly 1 in 3 people with cardiac arrest outside a hospital survive.
What Happens During Cardiac Arrest
Cardiac arrest is not the same as a heart attack. A heart attack means blood flow to part of the heart is blocked, but the heart usually keeps beating. In cardiac arrest, the heart stops pumping effectively, often because its electrical system malfunctions and the heart quivers chaotically instead of contracting. Blood stops flowing. Without intervention, brain damage begins within minutes.
The signs are unmistakable: the person suddenly collapses, has no pulse, stops breathing, and loses consciousness. If you see this, call emergency services immediately and start CPR.
How CPR Keeps Blood Moving
When the heart stops, CPR acts as a manual replacement. Pressing hard and fast on the center of the chest compresses the heart between the breastbone and spine, squeezing blood out into the arteries. When you release, the chest rebounds and the heart refills. Each compression cycle mimics a heartbeat, pushing oxygenated blood toward the brain and other organs.
Current guidelines from the American Heart Association recommend compressing the chest at least 5 cm deep (about 2 inches) at a rate of 100 to 119 compressions per minute. Studies of over 13,700 patients found that this rate and depth produce the best survival outcomes. Compressing too shallowly doesn’t generate enough blood flow, while pushing deeper than 6 cm can cause injury without improving survival.
Minimizing pauses is critical. Animal research shows that 30 or more uninterrupted compressions are needed to build up enough pressure in the arteries to actually perfuse the heart and brain. Every time compressions stop, even briefly, that pressure drops and must be rebuilt from scratch. This is why current protocols emphasize continuous compressions over frequent pulse checks or rescue breaths.
What an AED Does
An AED is a portable device that analyzes the heart’s electrical activity and, if it detects a rhythm that can be corrected with a shock, delivers one automatically. The most common treatable rhythm is ventricular fibrillation, where the heart’s lower chambers quiver uselessly instead of contracting. The shock essentially resets the electrical system, giving the heart a chance to resume a normal rhythm.
The key point: CPR alone cannot fix the underlying electrical problem. It buys time by circulating blood. The AED is what actually has the potential to restart a functional heartbeat. Without defibrillation, the chance of survival drops roughly 7 to 10 percent with every passing minute.
How to Use an AED
AEDs are designed for untrained bystanders. Once you power one on, it talks you through every step with voice prompts, and some models include a visual screen with diagrams. You don’t need to diagnose anything or make medical decisions. The device does that for you.
The general sequence works like this:
- Turn it on. Open the case and press the power button. The device begins giving instructions immediately.
- Expose the chest. Remove or cut away clothing so the skin is bare. Remove anything metallic from the area, including underwire bras, as metal can interfere with the pads or cause burns.
- Attach the pads. The pads have diagrams showing exactly where to place them: one on the upper right chest below the collarbone, the other on the lower left side below the armpit.
- Let the AED analyze. The device reads the heart’s rhythm and determines whether a shock is appropriate. Everyone must stop touching the patient during analysis.
- Deliver the shock if prompted. The AED will tell you to press the shock button and to stay clear of the patient. Not every rhythm is shockable, so the device may tell you no shock is needed. In that case, resume CPR immediately.
- Continue CPR. After a shock, go right back to chest compressions. Keep going until the person responds, starts breathing normally, or paramedics take over.
One common concern is using an AED near water. While the recommended practice is to move the person to a dry surface first, simulation studies have found no significant shock hazard to rescuers even when defibrillation was performed in a wet environment. If moving the person isn’t possible, using the AED is still far better than not using it at all.
Why CPR and AED Work as a Team
Early AED protocols called for up to three consecutive shocks with pulse checks in between. This approach left gaps of 60 seconds or more without any chest compressions, and those pauses were deadly. Research also revealed that the first shock successfully converts the heart rhythm about 85% of the time, making repeated stacking of shocks unnecessary.
These findings led the American Heart Association to redesign the protocol: deliver one shock, then immediately resume CPR for about two minutes before rechecking the rhythm. This maximizes the time spent compressing the chest and maintaining blood flow to the brain.
There’s another reason to keep compressing right after a shock. Even when the heart converts back to a normal electrical rhythm, the muscle is often “stunned” and too weak to pump effectively on its own right away. Continued chest compressions bridge that gap, supporting circulation while the heart recovers mechanical function.
Where to Find AEDs
AEDs are placed in many public locations: airports, shopping centers, gyms, schools, office buildings, and sports arenas. They’re typically stored in wall-mounted cabinets marked with a green heart symbol, sometimes with an alarm that sounds when the cabinet is opened (this is just to alert staff, not to discourage use). Despite widespread placement, AEDs are retrieved before paramedics arrive in only about 8% of out-of-hospital cardiac arrests. Many bystanders either don’t know the devices are nearby or hesitate to use them.
Legal Protections for Bystanders
Every U.S. state has some form of Good Samaritan law that limits liability for people who voluntarily provide emergency care without expecting payment. These laws protect against claims of “ordinary negligence,” meaning the kinds of mistakes a reasonable person might make while trying to help in an emergency. They do not cover gross negligence or intentional harm, but performing CPR or using an AED in good faith on someone in cardiac arrest falls well within the protection these laws provide.
In practical terms, there has never been a successful lawsuit against a bystander in the United States for using an AED on someone in cardiac arrest. The legal system and public policy strongly favor bystander intervention because the alternative, doing nothing, is almost always fatal.