Where Should the AED Pads Be Placed When Using the Anterior/Posterior Pad Placement on an Infant: One on the Chest, One on the Back

When using anterior/posterior placement on an infant, put one pad on the front of the chest (anterior) and the other on the back (posterior), directly opposite it, so the shock current passes through the heart from front to back. The anterior pad goes on the left side of the chest, below the nipple line and clear of the sternum. The posterior pad goes on the upper back, between the shoulder blades and left of the spine.

This front-to-back arrangement is a recognized option for infants younger than 1 year when the standard front-and-side configuration cannot be used, such as when the chest is too small for two pads to sit without touching. It is not the default. Defibrillator manufacturers and resuscitation guidelines describe it as the fallback, and the reason is geometry: on a body the size of a loaf of bread, pad spacing is the limiting factor, not the heart's position.

What follows explains why the current path matters, how small the infant thorax actually is, where the edge cases bite, and which placement mistakes are common enough to have earned their own warnings.

How the shock current reaches the heart

An automated external defibrillator does not "aim" at the heart. It applies a voltage between two electrode pads, and current distributes through the tissue by the path of least resistance. The heart sits in the middle of that field, and the fraction of current that crosses the myocardium depends on the geometry between the electrodes. Pads placed so the heart lies between them produce a straighter, denser current path through the ventricles. Pads placed side by side on the same plane produce a more tangential field, and less current crosses the muscle you are trying to depolarize.

Anterior/posterior placement puts the heart roughly on the line connecting the two pads. The anterior pad sits over the left ventricular territory, and the posterior pad sits behind it. Current flows front to back, through the ventricular mass. This is the same logic behind the anterior/posterior configuration used for cardioversion in older patients and for some implanted-device testing.

The configuration also changes pad-to-pad distance. Two pads on the same side of a small chest sit close together, and close pads risk arcing across the gap (a "bridge") or overlapping. Moving one pad to the back roughly doubles the separation on a tiny torso. That is why the placement exists: it solves a spacing problem more than a targeting problem.

Why infant geometry forces the choice

Infant chests are small enough that pad footprint, not anatomy, decides placement. Adult pads are typically about 8–13 cm across depending on model, and pediatric pads or attenuators are smaller but still substantial relative to a newborn's thorax. A newborn's chest circumference is roughly 30–35 cm at birth, and the front surface available for a pad is a fraction of that. Two adult pads on an infant chest can touch or leave no gap, which degrades the current path and can cause arcing.

That is the mechanical reason guidelines push pediatric pads or an attenuator first, and alternative pad positions second. The American Heart Association's pediatric advanced life support guidance supports anterior/posterior placement when pads would otherwise be too close. The default for infants remains anterior/lateral (front and side) with appropriately sized pads; anterior/posterior is the documented alternative, not the preferred first move.

A practical threshold: if you cannot fit the pads with a clear gap between them and no overlap, the pads are too large for that position, and you should switch to the back placement or use smaller pads. Arcing between touching pads can injure the skin and divert energy from the heart.

Where each pad actually goes

Anterior pad: left side of the chest, below the nipple line, lateral to the sternum. Keep it off the breastbone itself and off any implanted device, though implants are rare in infants.

Posterior pad: upper back, between or just left of the shoulder blades, at the level of the heart. Smooth the pad flat against the skin. The back is often easier on an infant than on an adult because the scapulae are small and the surface is relatively flat.

The two pads should face each other across the chest. "Directly opposite" matters: a posterior pad sitting high near the neck or low near the waist moves the current axis away from the ventricles. Aim for the mid-back, not the upper neck or the lumbar area.

For infants, both pads go on bare, dry skin. Wipe away moisture, dry the skin, and remove any medication patch. If the chest is wet from rain or vomit, dry it first; water on the skin spreads current along the surface and can shunt energy away from the heart or cause arcing.

Placement comparison

The table's takeaway: anterior/posterior is a legitimate alternate, front-front is not, and the pad size question outranks the position question.

When to switch to anterior/posterior

Switch when the standard front-and-side position fails a spacing test. Try anterior/lateral first with pediatric pads. If the pads touch, overlap, or leave no visible gap, reposition. Move the lateral pad to the back and keep the anterior pad on the front-left chest. That is the moment anterior/posterior earns its place.

Red flags and stop-and-fix conditions:

Pads touching or overlapping: reposition before delivering any shock. Pad over a wet chest or a medication patch: dry the skin and remove the patch first. Pad over an implanted device or a pacemaker bulge: shift the pad away, though this is uncommon in infants. Infant in water or on a metal surface: move the infant to a dry, nonconductive surface before using the AED. Pads expired or damaged: use a fresh set; expired adhesive can fail to conduct.

If the AED voice prompt or screen offers a pediatric mode, select it. If not, use pediatric pads or an attenuator. If neither exists, use adult pads in the anterior/posterior position as a last resort, because the alternative is no shock at all.

Common misconceptions

The most common error is treating anterior/posterior as the first choice for infants. It is not. Guidelines list it as an alternative when the standard position will not work. Defaulting to it wastes seconds during a cardiac arrest and can complicate CPR because the infant must be turned to place the back pad.

A second misconception is that pad position alone determines energy delivery. It does not. The AED's energy setting, pad size, and skin contact quality all matter, and on an infant the pad-size problem usually dominates. Correct position with oversized pads still risks arcing and excess energy.

A counterintuitive caveat from resuscitation practice: turning an infant to place a posterior pad interrupts chest compressions. The interruption is usually brief, but compressions are the intervention with the strongest evidence in pediatric arrest. Place the back pad during a planned rhythm check or when another rescuer is available to keep compressions going, rather than stopping compressions mid-cycle to flip the infant.

A third misconception is that the back pad must be perfectly centered. It should sit at heart level, which on an infant is roughly the mid-back, but a few centimeters of error is tolerable. Overlap and contact quality are the errors that matter most.

The sequence during an arrest

Timing matters because every step competes with compressions. The practical sequence for an infant in cardiac arrest with an AED available:

Confirm the infant is unresponsive and not breathing normally, and call for help and the AED. Start CPR immediately, with compressions at the guideline rate for infants, while the AED is retrieved and pads are readied. Expose and dry the chest, then apply pediatric pads. Try anterior/lateral first. If pads touch or overlap, move the lateral pad to the upper back, at heart level. Let the AED analyze the rhythm, clear everyone, and deliver the shock if advised. Resume compressions immediately after the shock or after the "no shock advised" prompt.

The sequence is a loop, not a one-time checklist. Each analysis pause is a chance to check pad contact and spacing, and each shock is followed by immediate compressions.

Timeline of an infant AED response

The numbers are approximate and depend on the AED model and the rescuers' speed. The point of the table is that pad placement is a small slice of a fast-moving process, and it should not delay compressions.

Practical thresholds and actions

Concrete steps and numbers to act on:

Use pediatric pads or an attenuator for infants younger than 1 year whenever available. Place the anterior pad on the left chest below the nipple line, lateral to the sternum. Place the posterior pad on the upper back at heart level, between or just left of the shoulder blades. Require a visible gap between pads; if they touch, reposition one to the back. Dry the skin and remove medication patches before applying pads. Do not delay compressions for more than a few seconds to place a back pad.

If the AED has a pediatric mode, engage it. If the pads come in pediatric and adult sizes, choose pediatric. If the only pads available are adult pads, place them anterior/posterior to maximize separation, and expect the AED to deliver adult energy, which is why pediatric pads or an attenuator are preferred.

The one number that reconciles the thresholds: pad separation. Adult pads need enough gap to avoid arcing, and on an infant that gap is the binding constraint across every guideline. Position serves separation; separation protects the current path.

Limitations of the evidence

The evidence for anterior/posterior versus anterior/lateral in infants is thin. Most pediatric AED guidance extrapolates from adult defibrillation data and from manikin studies, not from large infant trials, because infant cardiac arrest is uncommon and randomized data are scarce. Guidelines describe both configurations as acceptable, with anterior/lateral as the usual default and anterior/posterior as the alternative when spacing fails. What the evidence does not support is a claim that one position is clearly superior for shock success in infants.

Pad size and energy attenuation matter at least as much as position, and no guideline resolves the trade-off between a suboptimal position with correct pads and an optimal position with oversized pads. The defensible rule is to use pediatric pads when available, choose the position that gives adequate spacing, and prioritize compressions over perfect pad geometry.

This article is general educational information about AED pad placement, not a diagnosis, treatment plan, or substitute for training. For an infant in cardiac arrest, call local emergency services immediately and follow dispatcher instructions and current resuscitation training. Consult a qualified clinician or certified instructor for hands-on guidance.