An AED (automated external defibrillator) delivers a controlled electrical shock through the chest that forces all heart muscle cells to reset at once, giving the heart’s natural pacemaker a chance to restart a normal rhythm. The shock typically ranges from 120 to 200 joules in adults and passes through the chest in milliseconds. While the goal is to fix a life-threatening heart rhythm, the electrical current also affects skeletal muscles, skin, and pain-sensing nerves along the way.
How the AED Reads Your Heart Rhythm
Before delivering any shock, the AED analyzes the heart’s electrical activity through adhesive electrode pads placed on the chest. The device essentially reads a simplified version of the same signal a hospital heart monitor would display. It runs this data through an algorithm designed to distinguish between rhythms that need a shock and those that don’t.
The two rhythms an AED is looking for are ventricular fibrillation, where the heart quivers chaotically instead of pumping, and ventricular tachycardia, where the heart beats dangerously fast without moving blood effectively. Modern AED algorithms detect ventricular fibrillation with 81 to 94% sensitivity. About 70% of the time an AED is applied to a cardiac arrest patient, the device determines no shock is needed, either because the rhythm isn’t shockable or because the heart has already flatlined completely. In those cases, the AED will not fire.
What the Shock Does to the Heart
The electrical pulse depolarizes a critical mass of heart muscle cells nearly simultaneously. In plain terms, it forces the heart’s electrical system to go briefly silent, wiping out the chaotic signals that were preventing the heart from pumping. This momentary pause gives the sinoatrial node, the heart’s built-in pacemaker located in the upper right chamber, a window to take back control. If that pacemaker is still viable, it fires an impulse that travels through the heart’s normal conduction pathways, restarting coordinated contractions and restoring a pulse.
This doesn’t always work on the first attempt. The heart may slip back into a chaotic rhythm, requiring additional shocks. Each cycle of analysis and shock delivery gives the sinoatrial node another opportunity to regain control. Timing matters enormously: when bystander CPR is being performed, survival drops by 3 to 4% for every minute defibrillation is delayed.
The Physical Jolt You Can See
The most visible effect of an AED shock is the sudden, involuntary contraction of the chest and abdominal muscles. Short-duration biphasic pulses cause intense, rapid chest-wall muscle contraction that produces a noticeable body jolt. Researchers measuring this movement have recorded peak accelerations as high as 10 g (ten times the force of gravity) in the limbs during a standard biphasic shock.
This muscle contraction is forceful enough that it occasionally causes injuries. Case reports have documented rotator cuff tears, hip fractures, scapular muscle detachment, and even splenic rupture, all likely caused by the intensity of the skeletal muscle contraction during shock delivery. These complications are rare, and in the context of cardiac arrest, the alternative to shocking is almost certain death. Still, the physical force involved is substantial.
For conscious patients receiving cardioversion shocks (a related procedure using the same type of energy), the experience is extremely painful. The rapid contraction of chest skeletal muscles activates both fast and slow pain-sensing nerve fibers simultaneously. This is why cardioversion in a hospital setting is done under sedation. During cardiac arrest, however, the patient is unconscious and does not feel the shock.
Effects on Skin and Tissue
Skin burns beneath the electrode pads are a well-documented side effect. A survey of coronary care units found that nearly 99% had observed defibrillation burns in their patients. The most common cause was repeated shocks during prolonged resuscitation efforts. Other contributing factors include poor pad contact with the skin, moisture or sweat under the pads, and air pockets that concentrate the electrical current into smaller areas.
These burns typically appear as reddened, irritated patches matching the shape of the electrode pads. In most cases they are superficial and heal without significant treatment. The electrical current itself generates heat as it passes through tissue with resistance, and the skin, being the first layer the current encounters, absorbs a disproportionate share of that thermal energy.
Impact on Internal Organs
The electrical current travels between the two electrode pads, passing through the chest wall and heart. The lungs and other structures in the current’s path do receive some electrical exposure, but at the energy levels used in standard defibrillation, significant internal organ damage is uncommon. The shock is calibrated to be strong enough to reset the heart’s electrical system but brief enough (measured in milliseconds) to minimize collateral tissue damage.
Repeated shocks at high energy levels can cause direct myocardial damage, meaning injury to the heart muscle itself. This is a particular concern in pediatric patients, where adult-level energy doses have been shown to cause more frequent myocardial damage and worse heart function after resuscitation in animal studies. This is why pediatric electrode pads exist: they reduce the delivered energy to roughly 50 joules, compared to the 150 to 200 joules an adult receives.
How Energy Levels Differ for Children
Children’s smaller bodies require less electrical energy to achieve defibrillation. The American Heart Association recommends starting at 2 joules per kilogram of body weight, escalating to 4 joules per kilogram if the first shock fails. The European Resuscitation Council takes a slightly different approach, recommending 4 joules per kilogram from the start without escalation.
The lower starting dose has been questioned by research. In one study, a shock of 2 joules per kilogram terminated ventricular fibrillation in fewer than 10% of cases. With a biphasic waveform at the same dose, success rates reached only 25% for infant-sized subjects and 32% for child-sized subjects. Increasing to 4 joules per kilogram pushed success rates above 80%. Another review found that only 18% of children defibrillated at 2 joules per kilogram had their dangerous rhythm terminated on the first shock, with 38% needing more than two shocks.
Most public-access AEDs come with optional pediatric pads or a pediatric mode that attenuates the energy output. If pediatric pads aren’t available, guidelines support using standard adult pads on a child in cardiac arrest, since the risk of withholding defibrillation far outweighs the risk of delivering a higher-than-ideal dose.
What Happens After the Shock
If the shock successfully resets the heart’s rhythm, the patient may still be unconscious and will almost certainly need continued medical care. A successful shock restores electrical activity, but the heart may be sluggish and weak after being in a non-pumping state. Blood pressure is often low, and the heart muscle may be temporarily stunned from both the arrest itself and the defibrillation energy.
CPR should continue immediately after the shock until emergency medical services confirm a stable pulse. Even when the AED restores a rhythm, the first few minutes afterward are fragile. The heart’s pacemaker may falter again, and the AED will continue monitoring and advise another shock if the rhythm deteriorates. The pads stay on the patient until hospital staff take over.