An automated external defibrillator (AED) is a portable electronic device designed to treat sudden cardiac arrest (SCA), a life-threatening electrical malfunction of the heart. The AED analyzes the patient’s heart rhythm and delivers an electrical shock, or defibrillation, to stop chaotic electrical activity and allow the heart to re-establish a normal rhythm. While AEDs are simple to use and are a powerful tool for survival, knowing the specific circumstances when they should not be used is important for safety and efficacy.
When the Patient is Conscious or Responsive
The fundamental requirement for using an AED is that the person is in sudden cardiac arrest (SCA), meaning they are unresponsive and not breathing normally, or only gasping. If a person is conscious, moving, talking, or has a detectable pulse, they are not in cardiac arrest, and the AED must not be used. Administering an electrical shock to a heart that is beating normally can induce a fatal arrhythmia.
It is essential to distinguish between a heart attack and SCA, as the AED is only for the latter. A heart attack is a blockage of blood flow to the heart muscle, where the patient is typically conscious. SCA is an electrical problem that causes the heart to stop beating effectively, leading to immediate unconsciousness. If the patient is unresponsive, check for breathing; abnormal, gasping breaths (agonal respirations) should not be mistaken for normal breathing.
Situations That Prevent Safe Application
Several environmental and physical factors can interfere with the safe and effective delivery of the defibrillation shock. Since electricity follows the path of least resistance, any substance that blocks pad-to-skin contact or conducts the electrical current away from the heart reduces the shock’s effectiveness. The patient must be moved away from standing water, and their chest must be dried quickly before applying the pads, as water conducts electricity and could harm the patient or the rescuer.
The patient’s chest must be clear of any metallic jewelry or objects that might interfere with the current. Transdermal medication patches, such as those for nitroglycerin or pain relief, must be removed and the skin wiped clean before pad application. Placing an AED pad over a medication patch can reduce energy transfer and may result in minor skin burns.
Excessive chest hair can prevent the adhesive pads from making sufficient contact with the skin, leading to poor electrical conduction and potential burns. Many AED kits include a disposable razor to quickly shave the area for pad placement to ensure firm adhesion. If a razor is unavailable, a common technique is to use the first set of pads to rapidly clear the hair, then apply a fresh set of pads for the shock.
Device Failure and Pediatric Considerations
An AED must be fully functional and regularly maintained to ensure readiness. The AED should not be relied upon if the device displays an error message, the battery is dead or low, or the electrode pads are expired. Expired pads compromise the electrical connection due to poor adhesive quality, and a dead battery prevents the device from charging for a shock. If the device malfunctions, the immediate focus must shift entirely to performing high-quality cardiopulmonary resuscitation (CPR) until emergency medical services (EMS) arrive.
For children under eight years old or weighing less than 55 pounds, pediatric-specific pads or a reduced-energy setting should be used. These pads contain a built-in attenuator, which reduces the energy delivered from the adult dose to a lower, safer level. If pediatric pads are not immediately accessible, the latest guidelines state that adult pads should be used instead of delaying the life-saving shock. For infants under one year of age, a manual defibrillator is preferred, but an AED with pediatric pads is used if a manual device is unavailable.
When the AED Advises “No Shock”
The AED’s primary function is to analyze the heart’s electrical rhythm and determine if a shock is necessary. If the device announces “No Shock Advised,” it is functioning correctly and must not be overridden by the user, as the detected rhythm does not respond to defibrillation. The two primary non-shockable rhythms are Asystole (an electrical flatline) and Pulseless Electrical Activity (PEA), where electrical activity exists but the heart muscle is not contracting effectively.
In both Asystole and PEA, the underlying issue is a systemic problem, not a chaotic electrical storm that a shock can reset. For these non-shockable rhythms, high-quality chest compressions are the only immediate action that can keep oxygenated blood circulating to the brain and vital organs. When the AED advises against a shock, the rescuer must immediately resume chest compressions and continue CPR until the device re-analyzes the rhythm or EMS takes over.