When a rescuer arrives with an automated external defibrillator (AED) during ongoing CPR, they should turn the device on immediately, expose and dry the chest, apply the pads while a second rescuer continues compressions, then clear everyone for rhythm analysis and deliver a shock if the AED prompts one — resuming compressions within seconds afterward. The AED pads go on bare, dry skin, one below the right collarbone and one on the left side of the chest below the armpit. Every second the compressor keeps working while pads are placed is a second of preserved coronary perfusion pressure. The device, not the arriving responder, decides whether a shock is indicated.
Why Rhythm Analysis Demands Interrupted Compressions
The AED's job is to answer one binary question: is this rhythm ventricular fibrillation or pulseless ventricular tachycardia (shockable), or is it something else (nonshockable)? Chest compressions generate electrical signals and motion artifact that a consumer-grade defibrillator cannot reliably filter out. That is why the device demands a clear field — hands off, no oxygen flowing across the chest, nobody touching the patient — before it will analyze.
The mechanism runs deeper than artifact, though. Fibrillating myocardium has disorganized electrical activity in which no coordinated contraction occurs. A shock works by depolarizing a critical mass of myocardial cells simultaneously, which stuns the chaotic circuits and lets the heart's natural pacemakers reassert an organized rhythm. The probability of that "stunning" succeeding depends heavily on myocardial energy state, which compressions preserve. Each pause in compressions drops coronary perfusion pressure, and the pressure takes time to rebuild after resumption. A 2019 analysis of resuscitation pauses found that even 5-second interruptions depress blood flow for roughly 20 to 30 seconds afterward.
The practical consequence: the arriving responder should stage the AED work so compressions stop only for the analysis-and-shock interval, typically under 10 seconds, then restart immediately. Hands-off pad placement, hands-off charging during pauses (unless the device requires it), and hands-off "clear" calls all follow from this physiology.
Quantified Context: The Numbers Behind Prompt AED Use
Survival from out-of-hospital cardiac arrest (OHCA) hinges on time to defibrillation. When the arrest is witnessed and the initial rhythm is shockable, each minute from collapse to shock reduces the chance of survival by roughly 7 to 10 percent when CPR is not in progress. Bystander CPR flattens that decline considerably, and the combination of early CPR plus early defibrillation produces the best outcomes.
Temporal anchors worth knowing:
| Time since collapse | Typical status without CPR | Rough survival trajectory with CPR + AED |
|---|---|---|
| 0-3 min | Electrical activity intact | Highest probability of shock success |
| 3-5 min | Fibrillation coarsening | Defibrillation still highly effective |
| 5-10 min | Transition toward asystole | Survival falls sharply; CPR buys time |
| 10-15 min | Metabolic exhaustion | Shock success declines markedly |
| Beyond 15 min | Minimal viability | Survival rare without exceptional circumstances |
A public-access AED programs depends on retrieval speed. Placement targets usually aim to get a defibrillator to a victim within 3 minutes of collapse in high-risk venues like airports, casinos, and gyms. That 3-minute figure is not arbitrary — it approximates the window in which shock success remains high.
The AED's own prompts are status updates, not diagnoses. "Shock advised" means the device's algorithm has classified the rhythm as shockable; it does not guarantee a shock will restore circulation. The device may advise no shock for a rhythm that has already deteriorated into asystole, and no shock can reverse asystole.
Edge Cases the Arriving Responder Must Recognize
The arriving responder must adapt the standard sequence for a small set of situations:
Water or sweat: move the patient out of standing water and dry the chest before applying pads; moisture can shunt current and cause pad failure. Transdermal medication patches: remove patches from the pad sites and wipe the skin, because some drug-delivery patches can arc or interfere with pad contact. Implanted pacemaker or ICD: place pads at least 8 inches (about 20 cm) from the device generator, usually identifiable as a firm bulge under the collarbone; the device itself may deliver internal shocks that the rescuer can feel. Hairy chest: if pads will not adhere, the device often includes a razor; shaving takes seconds and prevents pad lift-off during the shock. Pediatric patient: use pediatric pads or the pediatric key if available; adult pads may be used if the device lacks a pediatric setting, with attention to pad-to-pad spacing so pads do not touch. Pacemaker/ICD visible bulge: do not delay the shock to interrogate the device; the external shock is what the patient needs now.
None of these adjustments justify a long pause. Each is a few-second fix that the arriving responder can perform while the compressor keeps working.
Misconceptions That Cost Seconds
Several beliefs cause rescuers to act too slowly or confidently. One is that the AED must be fully charged before pads are placed. In most modern devices, pads and battery insert together, and the device begins analyzing as soon as it is on and pads are attached. Another is that the AED will tell you to start CPR — it will, but only after the analysis cycle, and the compressor should already be going.
A more insidious misconception: that a shockable rhythm means the patient has a pulse. Ventricular tachycardia can be perfusing or pulseless, and the AED cannot reliably distinguish them in the field. If the AED advises a shock and the patient has no signs of life, the shock is appropriate. If the patient shows signs of life, lay rescuers should not use an AED intended for pulseless arrest until a clinician re-evaluates.
The counterintuitive professional caveat: an AED that advises no shock does not mean the patient is dead or that nothing can be done. Nonshockable rhythms are common early in arrest and even more common late. The correct response to "no shock advised" is to resume high-quality compressions immediately and continue until advanced help arrives — not to slow down or assume futility.
Another misconception about the arriving responder is that they must take over compressions. They do not. Their most valuable contribution is delivering the defibrillator; the compressor should keep working until the device forces a pause. Role clarity at handoff matters. The arriving rescuer should announce device arrival, take over pad placement and device operation, and let the compressor continue until told to clear.
Practical Actions When the AED Arrives Mid-Resuscitation
Sequence these steps in order, and hold the compressor on task:
Power on the AED immediately and follow its voice prompts. Do not read the manual. Expose the chest and dry it if needed. Apply pads per the device's diagram: right upper chest below the collarbone, left lower chest lateral to the nipple line, avoiding the sternum. Keep compressions going during pad application. The compressor pauses only when the device says "clear" or when the analysis cycle begins. Clear everyone — announce "clear," visually check that no one touches the patient, and confirm the compressor has lifted hands before the shock is delivered or analysis runs. Shock if advised, then resume compressions instantly, within 10 seconds of the shock or the no-shock prompt. Do not wait to check a pulse before restarting. If no shock is advised, resume compressions immediately and keep them going; let the device re-analyze at its programmed intervals.
Red flags during AED use:
Pads touching each other or the patient lying in water when the shock is delivered. Anyone still touching the patient during analysis or shock delivery. Compressions paused longer than 10 seconds for pad placement or pocket checks. The device advising no shock and the rescuer slowing down instead of continuing CPR. Improvised pad placement that ignores the device's diagram, which can send current through the wrong axis.
The arriving responder should also send a second bystander to meet emergency medical services, confirm the address, and report the AED's shock count and any rhythm changes when advanced providers arrive. That handoff detail helps advanced teams decide next steps.
Limitations and What the Literature Cannot Settle
AED algorithms are highly specific for shockable rhythms but not perfect; no consumer device guarantees correct classification under every artifact condition, and motion, moisture, and poor pad contact remain real failure modes. The survival figures cited above are population averages and vary enormously with bystander training, EMS response intervals, and underlying cause of arrest. A 2020 review of public-access defibrillation noted that outcomes depend on the whole chain of survival, not the device alone, and that AEDs deployed without trained responders or maintained batteries deliver little benefit.
The limits of this explainer: it describes the standard sequence for lay and first-responder use, not the decisions of advanced life support providers who may have monitor-defibrillators and drug interventions. It does not address in-hospital code teams, mechanical CPR devices, or termination-of-resuscitation criteria, which belong to clinical protocols. Where evidence is thin — for example, the exact optimal pad-to-device distance for every implant type — the guidance is conservative rather than precise, and readers should defer to their local protocol and device labeling.
This article is general educational information, not a diagnosis, treatment plan, or substitute for certified training. AED use, CPR quality, and post-arrest care are clinical decisions governed by current guidelines from authoritative resuscitation bodies and local protocols; consult a qualified clinician or your regional emergency medical services for patient-specific decisions. For an actual emergency, call local emergency services immediately and follow the AED's prompts and dispatcher instructions.