Where Should You Place AED Pads on an Adult: The Anterolateral Default and the Anatomy Behind It

Place AED pads on an adult in the anterolateral position: one pad on the upper right chest below the collarbone, the other on the lower left chest beside the breastbone, just under the armpit. Keep both pads clear of any implanted device by at least 8 inches (about 20 cm). The AED's pictures show pad placement, and the device analyzes the heart rhythm itself, so pad position affects current flow through the heart, not the shock decision.

How pad position steers current through the heart

The heart sits in the center-left of the chest, angled so its long axis runs from the upper right shoulder area toward the lower left flank. The left ventricle, the chamber that pumps blood to the body, is the largest muscle mass and the usual source of a lethal rhythm called ventricular fibrillation. An AED works by pushing a brief current between two pads, and that current must cross the ventricles to depolarize enough muscle at once.

Pads act like endpoints of a circuit. Current leaves one pad, spreads through tissue between them, and returns to the other. The path of least resistance runs through the chest wall, lungs, and heart. Positioning one pad above and right of the heart and the other below and left means the current crosses the heart diagonally, roughly parallel to its long axis. That routing captures both ventricles in the current field.

Tissue between the pads matters as much as the pads themselves. Fat, bone, and air-filled lung resist current. A pad on the upper right chest sits over ribs and the right lung; the lower left pad sits over the left lower rib cage near the heart's apex. The heart lies closer to the chest wall on the left, so the lower left pad sits relatively near the ventricular muscle. Removing clothing and drying the skin lowers resistance and improves current delivery. This is also why pad contact quality changes how much energy reaches the heart, a point the device adjusts around but cannot fully fix.

One layer deeper: the shock must synchronize muscle fibers that are firing chaotically. A current spanning the ventricles resets a critical mass of cells at once, giving the heart's natural pacemaker a chance to resume organized rhythm. A current that misses part of the ventricle leaves organized tissue behind. That is the mechanical reason pad geometry matters, and why a mispositioned pad can reduce the odds of successful defibrillation even when the device delivers a shock.

In plain terms: the two pads are the two ends of a path, and the current does the most good when the heart sits squarely in the middle of that path.

Quantified context: energy, resistance, and time

Defibrillation succeeds when enough current reaches the heart muscle, not just enough voltage at the pads. Transthoracic impedance, the resistance of the chest to current flow, typically ranges from about 25 to 180 ohms in adults. Energy delivered to the heart falls as impedance rises. Modern biphasic AEDs, the dominant design in the United States, deliver shocks in the range of roughly 120 to 200 joules and adjust output based on measured impedance. Older monophasic devices used higher fixed energies, often 200 to 360 joules.

Pad contact is the single largest controllable factor in impedance. Dry, clean, fully adherent pads on bare skin lower impedance; hair, sweat, chest cream, or a partly peeled pad raise it. The AED measures impedance across the pads before shocking and will prompt you if contact is poor. Follow that prompt. Pressing the pads firmly and smoothing them from the center outward improves adhesion.

Timing matters more than most people realize. Survival from out-of-hospital cardiac arrest falls by roughly 7 to 10 percent for each minute without CPR and defibrillation. That number frames every choice: if pad placement takes longer than correct compressions, the tradeoff usually favors getting the pads on the standard anterior positions quickly. American Heart Association guidance treats early, high-quality CPR and early defibrillation as the two interventions with the clearest survival benefit. Pad position is important, but it is a smaller lever than speed and compression quality.

The counterintuitive detail: more joules is not automatically better. Excess energy can cause skin burns and myocardial injury without improving success, which is why manufacturers tune biphasic waveforms and why the AED, not the rescuer, decides the dose. Your job is contact and position, not power.

The standard positions and their alternatives

The anterolateral placement is the default because it is reliable, easy to teach, and works for nearly all adults. Two other configurations exist for specific situations, and professional responders sometimes switch to them when the standard layout is blocked.

The anteroposterior layout can be useful when a wound, bandage, or pacemaker pocket blocks the standard left-chest spot. Its tradeoff is time: rolling the patient or reaching the back slows the sequence. For a lay rescuer with an AED, the anterolateral spots are the ones to reach for first. If the left lower chest is unavailable, a nearby spot on the same side, such as the mid-axillary line, is an acceptable compromise. Do not skip the anterior pad; both pads must be placed for the device to analyze and shock.

One professional caveat: some clinical settings use a shortened pad-to-pad axis to reduce impedance in patients with very large chests. Fewer clinical data exist for these alternatives than for the standard layout, so they are responder choices, not lay-rescuer defaults.

Timeline: the shock sequence and where placement fits

Pad placement is one step in a fixed sequence. The following table shows the typical flow from collapse to shock, with the actions that matter at each stage.

The sequence is not rigid; rescuers can place pads while a second rescuer does compressions. The professional practice is to minimize hands-off time, which is why some teams apply pads during ongoing CPR and pause compressions only for analysis and shock. For a single rescuer, follow the AED's prompts and prioritize compressions between them.

Edge cases and special situations

Certain conditions change pad placement or require extra caution. Handle these before applying pads when time allows.

Implanted pacemaker or ICD: place pads at least 8 inches (about 20 cm) from the device, which usually sits in the upper left chest. Keep the lateral pad clear of the generator and, if possible, position the pad so current does not flow directly through the device. Devices are designed to tolerate external shocks, but direct current across the generator can damage it. Transdermal medication patches: remove the patch and wipe the skin before placing a pad. Some patches contain metal or drug that can interfere with contact or cause a small burn. Wet or sweaty chest: dry the skin thoroughly. Water on the skin can create a conductive path between pads, shunting current away from the heart and risking arcing. Excessive chest hair: if pads will not adhere, clip hair quickly with the device's razor if one is included, or press firmly. Do not shave with a blade under time pressure; clipping is faster and safer. Pregnancy: use standard anterolateral placement. Defibrillation is not withheld because of pregnancy; maternal survival is the priority. Trauma, burns, or dressings: avoid wounds and dressings if possible; move to the nearest intact skin. Do not place pads over a burned area or over medication-infused dressings.

A high-stakes warning: never place a pad directly over an implanted device, and never let pads touch each other or bridge across the breastbone. Touching pads can arc current across the skin instead of through the heart, and the AED may refuse to shock or deliver wasted energy.

The counterintuitive professional caveat: for patients with a left-sided implanted device, responders sometimes shift to an anteroposterior layout to keep current away from the generator. That is a judgment call, and studies comparing layouts in this group are limited. When in doubt, standard placement with the 8-inch clearance is the safer default.

Common misconceptions, corrected

Several beliefs about pad placement circulate widely and are wrong in ways that matter during an emergency.

First, many people believe pad placement determines whether the AED shocks. It does not decide the rhythm; the device analyzes the electrical signal and shocks only ventricular fibrillation or pulseless ventricular tachycardia. Placement affects how effectively the shock reaches the heart, not whether the device recommends one.

Second, some believe the pads must go exactly where the pictures show, to the millimeter. The diagrams show the standard spots; small adjustments for anatomy, hair, or devices are acceptable as long as both pads are on the chest and clearance rules are met. Precision helps, but usability and speed matter more.

Third, there is a belief that a shock alone restarts the heart. In reality, the shock resets chaotic electrical activity, and CPR keeps blood flowing until the heart resumes an organized rhythm. High-quality compressions before and after the shock are what buy time. Pad position supports that, but it does not replace it.

Fourth, some assume AEDs are for trained responders only. Modern public-access AEDs are designed for lay use, with voice and visual prompts that walk a rescuer through pad placement and shock delivery. Legal protection for good-faith use exists in all 50 states under various Good Samaritan statutes, though the specifics vary. The device will not shock a normal rhythm, which is the central safety design.

One more myth: that you should not use an AED on a wet chest because of electrocution risk. The risk is to shock effectiveness, not to the rescuer from a properly functioning device. Dry the chest so current flows through the heart rather than across wet skin.

Practical actions and thresholds

The following steps turn placement into a repeatable sequence. Practice them on a trainer AED so the motions are automatic.

Confirm the scene is safe, check response and breathing, and call 911. Start compressions immediately if the person is unresponsive and not breathing normally. Turn on the AED as soon as it arrives and follow the voice prompts. Bare the chest completely; remove or cut clothing rather than working around it. Dry the chest if wet, and remove medication patches. Clip excessive hair only if pads will not stick. Apply the first pad to the upper right chest, below the collarbone and to the right of the breastbone. Press firmly from the center outward. Apply the second pad to the lower left chest, beside the breastbone and under the armpit, roughly at the level of the heart's apex. Keep both pads at least 8 inches (about 20 cm) from any implanted device. If the left pad would sit over a device, shift it laterally or use an alternative layout. Clear everyone from the patient during analysis and shock. Resume compressions immediately after the shock or if no shock is advised.

Thresholds to remember: 8 inches (20 cm) clearance from implanted devices; pads fully adherent with no bridging; dry skin; 30 compressions to 2 breaths for single-rescuer adult CPR, or continuous compressions with a second rescuer. The AED will tell you when to stand clear and when to press the shock button. If the device says pads are poorly connected, press them down again and check for hair, moisture, or peeling edges.

Limitations and what the evidence cannot settle

The core mechanism, current crossing the ventricles along the heart's long axis, is well established and stable. The relative superiority of one alternative layout over another in unusual anatomy is less settled. Most comparisons of pad positions come from small studies and simulation work, and the differences in shock success are modest at best. No large trial has shown that a nonstandard layout saves more lives in routine adult cardiac arrest. That is why guidelines keep the anterolateral layout as the default.

Device design also limits what a rescuer can control. The AED chooses energy and decides whether to shock. Pad placement and contact quality are the rescuer's main levers, along with CPR quality and speed. Do not overthink position at the expense of compressions. If pads are on, the chest is dry, and the device is analyzing, you have done the placement part correctly.

Finally, this article describes general principles, not a treatment protocol for a specific person. It is general educational information, not a diagnosis or treatment plan, and it does not promise any outcome. In a real emergency, follow the AED's prompts, call 911, and follow instructions from dispatchers or qualified local emergency responders.