The correct answer is anterior/posterior. For an infant, place one AED pad on the front of the chest and the other on the back, directly opposite. This front-to-back position is what the American Heart Association specifies for infants under 1 year. For children 1 year and older and for adults, the standard is anterior/lateral, with one pad below the right collarbone and the other on the left side under the armpit. The distinction matters because infant chest anatomy and pad sizes change how current flows through the heart.
This article explains why the pads go front and back, how the current behaves in a small chest, and what to do when the situation departs from the textbook case. It covers pad sizing, manual defibrillation limits, and the errors that cause energy to miss the heart.
The mechanism: why front-to-back fits an infant chest
Defibrillation works by driving a large current across the heart so that disordered electrical activity stops at once. The shock does not restart the heart; it clears the field so normal pacemakers can take over again. What matters is current density in the heart muscle, not the voltage on the skin. Pad position decides whether the path of current actually crosses the ventricles or detours around them.
An infant's chest is short front-to-back and narrow side-to-side. Two adult pads placed anterior/lateral may overlap on the small torso or sit too close together. When pads almost touch, current arcs across the skin surface and bypasses the heart entirely. A front-to-back path uses the deepest available distance through the chest, so more of the current passes through the heart. That geometry is why anterior/posterior is the infant standard.
The vector also aligns with the heart's axis. With one pad over the sternum and the other on the back behind the heart, the current passes through the ventricles along their long axis. The apex of the heart sits toward the left and anteriorly, so a back pad positioned slightly left of the spine centers the field. This same front-to-back approach is used for synchronized cardioversion in infants, where precision matters even more.
Quantified context: energy, sizing, and distance in small bodies
Pelvic and thoracic anatomy scale with age, and so does the space between electrodes. A newborn chest may be only a few centimeters deep, while an adult chest is closer to 20 to 25 centimeters. Defibrillation thresholds track that geometry, which is why pediatric energy doses are far lower than adult doses.
Typical automated external defibrillator (AED) energy settings run 150 to 200 joules for adults and 50 to 75 joules for small children, with some devices using 50 to 86 joules in pediatric mode. Infant shocks in a hospital setting often start around 2 joules per kilogram for the first shock, with escalation to 4 joules per kilogram and then higher. Manual defibrillation is preferred for infants; an AED is a fallback.
Pad size matters as much as pad placement. Infant pads are smaller, roughly 4 to 5 centimeters in diameter, versus adult pads at 8 to 12 centimeters. The smaller pad concentrates current, raising density on the skin surface and reducing the risk of burns. When infant pads are not available, adult pads may be used, but placing them front and back reduces the chance they touch.
A professional caveat worth stating plainly: nothing in the peer-reviewed literature establishes a precise survival benefit for one pad position over another in infants. The recommendation rests on geometry, modeling, and expert consensus rather than randomized outcome trials. That does not make it arbitrary, but it does mean the standard reflects the most reliable reasoning available rather than a measured mortality difference.
Edge cases: when the rule bends
The anterior/posterior rule assumes an infant under 1 year with a typical chest. Several situations change the calculation.
Size is the first exception. A large infant approaching 1 year may accommodate adult pads without overlap, and the pediatric-to-adult transition for pad position is usually pegged to about 1 year of age or roughly 10 kilograms. For a small toddler, front-to-back placement can still be the safer geometric choice even if pediatric pads are available. Clinicians rotate pads if overlap or arcing is likely.
Mechanical devices are the second exception. An implanted pacemaker or defibrillator occupies part of the chest. Pads should not sit directly over the device housing, because the current can damage the leads or fail to reach the heart. With a device in place, an anterior/posterior arrangement often allows placement to avoid the hardware while preserving the cardiac path. The American Heart Association advises at least 8 centimeters of clearance from an implanted device where possible.
Open chests and dressings are the third. After cardiac surgery, or when a wound dressing covers the chest, pad placement may need to shift. Transdermal medication patches also belong in this category. Nitroglycerin, nicotine, and pain patches can smolder or arc under a shock, so professional guidance is to remove them from the pad location before defibrillation. If a patch covers the ideal front position, place the pad adjacent to it and keep the back pad opposite.
The fourth exception is wet skin. A sweaty or immersed infant conducts current across the surface easily. Drying the chest briefly before applying pads improves contact and reduces arcing. This is a small step that changes how the current distributes.
Red flags to watch for during pad application:
Pads positioned so their edges touch or overlap on a narrow chest. One pad placed directly over an implanted pacemaker or ICD. Medication patches left in place over a pad site. Chest not dried when wet. Adult pads used in a way that leaves no clear path through the heart.
Common misconceptions, corrected
Several ideas circulate about pediatric defibrillation that do not hold up.
One misconception is that front-to-back placement is optional or that side-to-side works as well in infants. Anterior/lateral is the adult standard, but it loses its advantage on a short, narrow infant torso. Overlap and surface arcing are the failure modes. The front-to-back position exists because it solves a geometry problem the adult position creates.
Another misconception is that an AED should not be used on an infant at all. An AED is not the first choice, because manual defibrillation allows tailored energy and rhythm recognition. But if a manual defibrillator is unavailable and the infant is in a shockable rhythm, current guidance supports using an AED, ideally with pediatric pads or a pediatric attenuator.
A third misconception is that the shock restarts a stopped heart. A flatline, called asystole, does not respond to defibrillation. Pads deliver a shock only for ventricular fibrillation or pulseless ventricular tachycardia. Cardiopulmonary resuscitation with compressions comes first and continues around any shock.
A fourth misconception is that pad placement alone guarantees the current reaches the heart. Energy level, pad size, skin contact, and chest geometry all interact. Placement is necessary but not sufficient. The professional caveat from earlier applies here: neither the anterior/posterior nor the anterior/lateral standard has been validated by outcome trials specifically in infants.
Pad position comparison
| Population | Recommended pad position | Pad type | Primary reason |
|---|---|---|---|
| Infant under 1 year | Anterior/posterior (front/back) | Infant or pediatric pads preferred | Short chest; avoids overlap and arcing |
| Child 1 year and older | Anterior/lateral | Pediatric pads, then adult pads | Established pediatric standard; adequate torso length |
| Adult | Anterior/lateral | Adult pads | Standard vector across the heart |
| Infant with implanted device | Anterior/posterior, clear of device | Pediatric pads | Avoids hardware while keeping cardiac path |
| Infant with chest dressing or patch | Anterior/posterior, pad shifted off obstruction | Pediatric pads | Maintains contact and prevents smoldering |
| Large infant near 1 year | Anterior/posterior or anterior/lateral if pads fit without overlap | Pediatric to adult pads | Geometry determines feasibility |
The table shows the pattern: position follows torso anatomy first, then hardware and skin conditions. It is a decision tree, not a single fixed rule.
Practical actions: applying infant pads in sequence
The steps below are the concrete actions for a responder, in order. They are general educational steps, not a prescription for a specific patient; follow local protocol and device labeling.
Confirm the rhythm is shockable before applying pads, when you have a monitor. Pediatric pads follow the device's pediatric mode. Dry the chest if it is wet. Wipe away moisture and any visible debris. Remove medication patches from the areas where pads will go, then wipe the skin. Place the front pad on the sternum, centered and slightly left of the midline. Place the back pad directly opposite, between the shoulder blades or slightly left of the spine over the heart. Check that pad edges do not touch. If they would overlap, use smaller pads or reposition to keep a clear gap. Keep the front pad at least 8 centimeters from an implanted device if one is present. Follow the device prompts, clear everyone from the patient, and deliver the shock. Resume compressions immediately after.
The 8-centimeter clearance figure and the "dry the chest" step are the two details most often skipped. Both are cheap and both reduce the chance of a failed shock or a burned patient.
Timeline: what happens around a pediatric shock
| Time point | Event | What it means for pad placement |
|---|---|---|
| Before arrest | Pads may be pre-attached during transport | Front/back positioning must be set early |
| At recognition | Rhythm check identifies VF or pulseless VT | Only shockable rhythms get a shock |
| Pad application | Front and back pads placed | Contact quality decides current path |
| Immediately before shock | Skin dried, patches removed, clearance checked | Reduces arcing and burns |
| Shock delivery | Energy set to device pediatric mode | Energy and position act together |
| Within seconds after | Compressions resume | Position is unchanged; no delay |
| During transport | Pads may remain attached | Recheck contact if the patient moves |
The sequence shows that pad placement is a single moment inside a larger protocol. It matters most at the instant before the shock, when contact quality and geometry combine.
Limitations and what the evidence supports
What can be concluded is modest but useful. The anterior/posterior position is the body of expert guidance for infants, and the reasoning behind it is sound: short chest, risk of pad overlap, and the need for a deep path through the heart. What cannot be concluded is that front-to-back positioning produces measurably better survival than anterior/lateral in infants, because no adequate trial has tested that.
Another limit is the boundary at 1 year. The transition from infant to child pad position is not a sharp biological line but a practical convention tied to age and weight. A large infant and a small toddler are physically similar, and the choice should follow which position fits the chest without overlap.
Energy settings also vary by device and by setting. The figures quoted here are typical ranges, not universal constants. Pediatric attenuators, manufacturer defaults, and hospital protocols may all differ. Any responder should use the device's labeled pediatric settings and local protocols rather than memorize a single number.
The final limit is the human one. The most common reason a shock fails in an infant is not the pad position; it is a gap, a wet chest, a patch left in place, or a delay in compressions. Position is the framework, and execution is the outcome.
For medical topics: this article is general educational information about AED pad placement, not a diagnosis, treatment plan, or dosing instruction. It does not promise any clinical outcome and should not replace the device labeling or the directions of a qualified local clinician or emergency responder. In an emergency, call local emergency services and follow their instructions.