An automated external defibrillator (AED) is important because it is the only effective treatment for the most common cause of sudden cardiac death. When someone’s heart suddenly stops beating normally, every minute without a shock from an AED reduces their chance of survival by about 6%. CPR alone keeps some blood flowing, but it cannot restart a heart that has fallen into a chaotic, quivering rhythm. An AED can.
What Happens During Sudden Cardiac Arrest
Sudden cardiac arrest is an electrical problem. The heart’s internal wiring malfunctions, causing it to quiver rapidly and chaotically instead of pumping blood. This is different from a heart attack, which is a plumbing problem where a blocked artery cuts off blood flow to part of the heart muscle. A heart attack can sometimes trigger cardiac arrest, but they are not the same event.
During cardiac arrest, the heart produces no meaningful blood flow. The brain begins suffering damage after roughly four minutes without oxygen, and consciousness is lost within about 15 seconds. Approximately 1,000 out-of-hospital cardiac arrests are assessed by emergency medical services in the United States every day. The vast majority of these people will die without rapid intervention, and “rapid” means minutes, not the 8 to 12 minutes it often takes an ambulance to arrive.
Why CPR Alone Isn’t Enough
CPR is a bridge, not a fix. Chest compressions push a small amount of oxygenated blood to the brain and heart, buying time until a defibrillator arrives. But the chaotic electrical rhythm that causes most witnessed cardiac arrests, called ventricular fibrillation, does not respond to compressions. The heart needs an electrical shock to reset.
The difference in outcomes is stark. A landmark study published in Circulation found that when bystanders used an AED before paramedics arrived, the survival rate for shockable rhythms was 44%, compared with 21% when people waited for EMS. For all cardiac arrests combined, bystander AED use tripled overall survival to hospital discharge: 10.5% versus 3.3% with EMS-only response. Those numbers reflect situations where the AED was available on-site and used by ordinary people with minimal training.
Every Minute Counts
The relationship between time and survival during cardiac arrest is brutally simple. Research from the American Heart Association shows that each additional minute a person remains in ventricular fibrillation is associated with a 6% lower probability of surviving to hospital discharge. At that rate, a person whose heart is in a shockable rhythm has roughly half the chance of surviving after just five minutes of delay compared to receiving an immediate shock.
This is exactly why public AEDs exist. Emergency medical services, even in well-funded urban areas, rarely arrive in under five minutes. An AED mounted on the wall of a gym, airport terminal, or school hallway can cut that gap to under two minutes if someone nearby knows where it is and grabs it.
How an AED Works
An AED analyzes the heart’s electrical activity through two adhesive pads placed on the chest. It determines whether the rhythm is one that will respond to a shock. If it detects ventricular fibrillation or a dangerously fast heart rhythm, it charges and either delivers the shock automatically or prompts the user to press a button. If the rhythm is not shockable (for instance, if the heart has already flatlined completely), the device will not allow a shock to be delivered.
The shock itself works by delivering a burst of electrical energy that simultaneously depolarizes the heart muscle, essentially forcing every cell to reset at the same moment. This gives the heart’s natural pacemaker a chance to resume a coordinated rhythm. The device is designed to use the lowest effective energy needed, and modern AEDs provide voice prompts that walk a user through every step. You do not need medical training to operate one correctly.
Who Can Use an AED
Anyone. AEDs are designed for bystanders, not medical professionals. The device makes the medical decision: it analyzes the rhythm, determines whether a shock is appropriate, and either delivers or withholds the shock accordingly. You cannot accidentally shock someone who doesn’t need it. Voice and visual prompts guide pad placement, and the entire process from opening the case to delivering a shock typically takes under a minute.
For children under 8 years old, pediatric pads or a pediatric setting on the device should be used if available, as these deliver a reduced shock dose appropriate for a smaller body. If pediatric pads are not available, the American Heart Association recommends using adult pads and an adult shock dose. A shock with adult pads is far better than no shock at all.
Where AEDs Are Required
All 50 states have public access defibrillation laws on the books, though the specifics vary. As of the most recent comprehensive review, 38 states had laws supporting targeted AED placement in specific locations. Schools are the most common mandated location, with 25 states requiring AEDs on site. Fifteen states require them in health clubs, gyms, or athletic facilities. Ten states mandate them in state-owned buildings.
Nineteen states go further, requiring that AEDs be placed in clearly marked, easy-to-access locations so that a bystander can find one quickly during an emergency. If you work in a school, fitness center, or government building, there is a good chance your state requires an AED to be somewhere nearby. Knowing where it is before an emergency happens is the single most practical thing you can do.
Keeping an AED Ready
An AED sitting in a cabinet is only useful if it actually works when someone opens it. The two components that expire are the battery and the electrode pads. Most AED batteries last 2 to 5 years once installed, depending on the brand. Electrode pads have a similar shelf life of 2 to 5 years. Both need to be replaced before they expire, and in many models, the battery and pads expire on different schedules, so tracking two dates is necessary.
Some manufacturers simplify this. HeartSine’s system, for example, combines the battery and pads into a single cartridge that gets swapped out all at once. Regardless of the brand, most modern AEDs run automatic self-checks (daily or weekly) and display a visible status indicator, typically a green checkmark or blinking light, confirming the device is ready. If that indicator shows a warning, the battery or pads need attention. Assigning someone in your workplace or organization to check that status light monthly takes almost no effort and ensures the device works when it matters.
The Core Reason AEDs Save Lives
Cardiac arrest kills because of time. The heart falls into a rhythm that produces no blood flow, the brain starts dying within minutes, and the only treatment that can reverse the rhythm is an electrical shock. Ambulances carry defibrillators, but they arrive too late for most victims. An AED puts that same life-saving shock in the hands of whoever is standing closest when someone collapses. The survival numbers are clear: bystander defibrillation more than doubles the chance of surviving a shockable rhythm compared to waiting for paramedics. That gap between collapse and first shock is the reason AEDs exist in public spaces, and it is the reason knowing where your nearest one is located could someday matter more than any other piece of information you carry.