Using an AED (automated external defibrillator) is designed to be straightforward, but several factors can affect whether it works properly. Knowing what to watch for before you press that shock button can make the difference between a successful rescue and a wasted minute. When an AED is used within two minutes of cardiac arrest, survival rates can reach as high as 70%.
The AED Makes the Decision, but It Needs Clean Data
An AED analyzes the heart’s electrical activity and determines whether a shock is appropriate. It targets two specific rhythms: ventricular fibrillation (a chaotic quivering of the heart) and rapid ventricular tachycardia (a dangerously fast heartbeat). For any other rhythm, including a flatline, the device will not deliver a shock. You cannot accidentally shock someone who doesn’t need it under normal conditions.
The key word is “normal conditions.” Chest compressions create electrical artifacts that interfere with the AED’s ability to read the heart rhythm accurately. The motion from compressions overlaps with the electrical signals the device is trying to interpret, which can lead to either a missed diagnosis or a false one. This is why every AED will instruct you to stop CPR and stand clear during the analysis phase. Those few seconds of stillness let the device get an accurate reading.
Water, Wet Skin, and Electrical Safety
If someone collapses in a pool, lake, or ocean, you need to move them out of the water before using an AED. You cannot use the device on a person who is submerged. However, if the person is simply wet, lying in a puddle, or standing in snow, you can proceed. The person does not need to be completely dry.
What matters is the area where the pads go. Use a towel or the wipes included in most AED kits to dry the skin where you’ll place each pad. The pads need firm adhesion to work. Water between the pad and skin can disperse the electrical current across the surface instead of directing it through the heart, and a pad that won’t stick delays the entire process.
A common worry is whether using an AED on a wet or metal surface could shock bystanders. Testing by Zoll Medical found that the maximum voltage detectable on a wet surface during a shock was less than what a standard 9-volt battery produces. Metal surfaces like a helicopter floor or metal grating pose no shock hazard to rescuers or bystanders, as long as nobody is touching the patient when the shock is delivered.
Obstacles on the Chest
Thick Chest Hair
Chest hair can prevent AED pads from making full contact with the skin, which reduces or blocks shock delivery entirely. Sparse or thin hair is usually fine, as modern adhesive pads can stick through it. For thick hair, most AED kits include a small disposable razor. Shave the two areas where the pads will go. If no razor is available, press the pads down as firmly as you can to compress the hair against the skin. Direct contact is always preferable, but a firm press is better than doing nothing.
Medication Patches
Transdermal medication patches (nicotine, pain medication, hormone therapy) should be removed if they’re in the area where an AED pad needs to go. Some patches contain metal backing that can cause burns during defibrillation, and any patch between the pad and skin will interfere with electrical conductivity. Peel the patch off, wipe the area clean, then place the AED pad. Wear gloves if you have them, since the medication on the patch can absorb through your skin.
Pacemakers and Implanted Defibrillators
Look for a small, hard bulge under the skin on the upper chest or abdomen. This typically indicates a pacemaker or implanted cardiac defibrillator. You can still use the AED, but place the pad at least four finger-widths away from the bulge. Placing a pad directly over an implanted device can block the shock from reaching the heart effectively and may damage the implant.
Children Need Different Pads
For children 8 years old or younger, or weighing less than 55 pounds (25 kg), use pediatric pads if available. These pads deliver a lower energy dose appropriate for a smaller body. If you only have adult pads, use them anyway. An adult-level shock on a child is not ideal, but cardiac arrest without any intervention is fatal. For anyone over 8 or heavier than 55 pounds, standard adult pads are correct.
Pad Placement Basics
AED pads come with diagrams printed on them showing exactly where to place each one. Generally, one pad goes on the upper right chest below the collarbone, and the other goes on the lower left side of the ribcage. The goal is to position the heart between the two pads so the electrical current passes through it. Place pads on bare, dry skin. You can place pads over scars without any issue, unless the scar indicates an implanted device underneath.
Nobody Touches the Patient During a Shock
This is the single most important safety rule. Before the AED delivers a shock, it will give a voice prompt telling everyone to stand clear. Make sure no one is touching the patient, including you. Verify visually. The electrical current follows the path between the two pads through the patient’s body, and anyone in physical contact could receive part of that energy. The risk to bystanders from the surface the patient is lying on is negligible, but direct contact is a real hazard.
Batteries and Pads Expire
An AED that sits in a wall-mounted case for years might not work when you need it. Both the battery and the adhesive pads have expiration dates, and using expired components can mean a failed rescue.
Most AED batteries last two to five years in standby mode, depending on the brand. Pads generally have a shelf life of two to four years. Some specific examples: Philips HeartStart batteries last about four years, while Zoll batteries last about five years in standby. Pad lifespan ranges from two years for brands like Cardiac Science and Philips to up to five years for certain Zoll models. Pads must always be replaced after use, even if the device never delivered a shock, because the adhesive gel degrades once the sealed packaging is opened.
If you’re responsible for an AED in a workplace, gym, or school, check the expiration dates on both the battery and pads regularly. Most modern AEDs run self-checks and display a warning light when something needs replacing, but periodic manual inspection is still worthwhile.
Legal Protections for Bystanders
Every U.S. state has some form of Good Samaritan law that protects people who use an AED in an emergency. These laws are specifically designed to encourage bystanders to act. In most states, you are shielded from liability for ordinary negligence when you use an AED voluntarily and in good faith during an emergency. You do not need to be a medical professional or have formal certification to be protected. The immunity typically does not cover gross negligence or intentional misconduct, but using an AED as intended on someone in cardiac arrest falls well within the protected range. The legal system overwhelmingly favors people who try to help.