CPR and an AED aren’t competing options. You use both, in sequence. When someone collapses and isn’t breathing, you start CPR immediately and use an AED as soon as one is available. CPR keeps blood flowing to the brain and heart through chest compressions, while an AED delivers an electrical shock that can restart a normal heart rhythm. Neither one fully replaces the other.
Why You Need Both, Not One or the Other
The most common cause of cardiac arrest is a chaotic electrical malfunction in the heart called ventricular fibrillation. The heart quivers uselessly instead of pumping. Chest compressions alone cannot fix this electrical problem. They physically push blood through the body, buying time by keeping oxygen flowing to the brain and vital organs. But the heart needs an electrical shock to reset its rhythm, and that’s what an AED provides.
Here’s the critical relationship between them: during the first four minutes of cardiac arrest, the heart still has some residual energy and blood flow. A shock delivered in this window has the highest chance of restoring a normal rhythm. After four minutes, blood flow stops completely and the heart muscle starts relying on backup energy sources. At this point, chest compressions become essential for “priming” the heart before a shock can work. Good compressions increase blood flow to the heart muscle, restoring enough energy for it to respond to defibrillation. Without that preparation, a shock delivered to an energy-depleted heart is far less likely to succeed.
The survival numbers reflect this. For every minute without CPR, survival from a witnessed cardiac arrest drops by 7 to 10%. When a bystander provides CPR, that decline slows to 3 to 4% per minute. And when CPR is combined with an AED, survival to hospital discharge is about 39% higher than with CPR alone.
The Step-by-Step Sequence
The American Heart Association’s Chain of Survival lays out the order clearly:
- Recognize and call 911. If someone collapses, is unresponsive, and isn’t breathing normally (or is only gasping), that’s cardiac arrest. You don’t need to check for a pulse. Call 911 immediately or have someone else call while you act.
- Start CPR right away. Begin chest compressions. Push hard and fast in the center of the chest, at least two inches deep, at a rate of 100 to 120 compressions per minute. Don’t stop.
- Use an AED as soon as one arrives. If someone brings an AED, turn it on and follow its voice prompts. It will tell you exactly what to do, including where to place the pads. You do not need training to operate one.
- Continue CPR between shocks. After the AED analyzes the heart rhythm and delivers a shock (or advises no shock), resume chest compressions immediately. Keep going until emergency medical services take over.
If you’re alone, call 911 first. If an AED is nearby (within a minute or two of walking distance), grab it before starting CPR. If it’s farther away, start compressions and wait for someone else to bring the device. Every second without blood flow matters.
What the AED Does Automatically
An AED analyzes the heart’s electrical activity and decides whether a shock will help. Not every cardiac arrest involves a rhythm that responds to defibrillation. The device checks for specific patterns of electrical chaos and only allows a shock when one is likely to work. You cannot accidentally shock someone who doesn’t need it.
If the AED announces “no shock advised,” that doesn’t mean the person is fine. It means their heart rhythm won’t benefit from a shock at that moment. You should immediately resume CPR. After one to two minutes of compressions, the AED will re-analyze. Sometimes compressions restore enough cardiac energy that a previously unshockable rhythm becomes shockable. Leave the pads attached and the device on regardless of what it advises.
If the AED detects signs of circulation returning, it will guide you to check breathing. If the person starts breathing on their own, place them on their side in a recovery position but keep the AED pads in place until paramedics arrive.
Hands-Only CPR Is Enough for Most Bystanders
If you’re not trained in CPR, skip the rescue breaths entirely. Compression-only CPR (pushing hard and fast on the chest without mouth-to-mouth breathing) is actually more effective when performed by untrained bystanders. Every pause for a rescue breath interrupts blood flow, and the positive pressure from blowing air into the lungs can reduce how much blood returns to the heart. For adults who collapse suddenly from a cardiac cause, uninterrupted chest compressions provide better outcomes.
Rescue breaths still matter in certain situations: children, drowning victims, and cases where the arrest was caused by a breathing problem rather than a heart problem. They also become more important if the arrest has lasted longer than about four minutes, since the body’s oxygen reserves deplete over time. But for the typical witnessed adult collapse, hands-only CPR combined with an AED gives the best chance of survival.
Using an AED on Children
AEDs work on children, but the setup changes depending on age and size. For children 8 years old or younger, or those weighing under 55 pounds, use pediatric pads if available. These deliver a lower energy shock appropriate for a smaller body.
Pad placement is also different. Instead of both pads going on the front of the chest (as with adults), pediatric pads go on the front and back. Place one pad on the center of the chest near the breastbone and the other between the shoulder blades. A practical approach is to roll the child over, place the back pad first, then roll them back and attach the front pad.
If pediatric pads aren’t available, use adult pads. A shock with higher energy is far better than no shock at all. Just make sure the two pads don’t touch each other on a small chest. If they would overlap, use the front-and-back placement instead.
Safety Concerns With AEDs
People often worry about using an AED in wet conditions. Research testing defibrillation on a simulated patient in pooled water found that the maximum voltage a nearby bystander would be exposed to was 30 volts in salt water, which could cause a mild tingling sensation but poses no real danger. That said, best practice is to move the person to a dry surface and towel off their chest before placing pads. If you absolutely cannot move them, using the AED in a wet environment is still safer than not using it at all.
A few other quick considerations: if the person has a medication patch on their chest where a pad needs to go, peel it off and wipe the area. If you can see or feel a lump under the skin indicating an implanted pacemaker or defibrillator, place the AED pad at least an inch away from it. Remove any jewelry or clothing that sits between the pads and bare skin. The AED’s voice prompts will walk you through most of this.
The Real Risk Is Doing Nothing
About 90% of people who experience cardiac arrest outside a hospital do not survive. The single biggest factor in changing that outcome is whether a bystander acts before paramedics arrive. CPR buys time. An AED fixes the underlying problem. Together, they roughly double the odds of survival compared to waiting for professional help. You cannot harm someone who is already in cardiac arrest by performing CPR or using an AED on them. The device won’t shock a heart that doesn’t need it, and imperfect chest compressions are vastly better than none.