An automated external defibrillator (AED) is a portable device used to analyze heart rhythms and deliver an electrical shock to restore a normal heartbeat. It is specifically designed to treat sudden cardiac arrest caused by two chaotic electrical rhythms: ventricular fibrillation (VF) and pulseless ventricular tachycardia (pVT). While AEDs are life-saving tools, their use is limited or contraindicated in certain situations for safety and efficacy. Understanding when an AED should not be used is as important as knowing how to operate one.
When the Patient is Conscious or Responsive
The fundamental rule of AED use is that the device is only for a person who is unresponsive, not breathing normally, and shows signs of sudden cardiac arrest. If an individual is awake, talking, moving, or breathing normally, they are not a candidate for defibrillation. For example, a person experiencing a heart attack is typically conscious and breathing, and applying an AED would be unnecessary and potentially harmful.
The AED is programmed with an analysis system that will not deliver a shock if a person has a pulse or a normal heart rhythm. However, placing the pads on a conscious patient is inappropriate because the underlying issue is not sudden cardiac arrest. The device’s primary function is to interrupt a chaotic, life-threatening electrical pattern, not to treat other medical issues.
Environmental Safety Risks
External factors surrounding the patient can create safety hazards that prohibit the immediate use of an AED. Water and electricity are incompatible, and using a defibrillator when the patient is submerged or lying in a large puddle can conduct the electrical current away from the heart. This not only makes the shock ineffective but also poses a severe electrocution risk to the rescuer and others nearby.
Before applying the AED, move the person to a dry area and quickly dry their chest to ensure the pads adhere and the electrical current passes correctly through the heart. While the AED can be used on a person lying on a metal surface, the electrode pads must not contact large metal objects, as this could interfere with the electrical pathway. Since an AED delivers an electrical spark, it must not be used in environments with spilled gasoline, industrial gases, or other flammable or explosive substances due to the risk of ignition.
Patient Specific Physical Conditions
Certain physical conditions or medical devices on the patient’s body require adaptation of the AED procedure to ensure safety and effectiveness. Many people have implanted cardiac devices, such as pacemakers or implantable cardioverter-defibrillators (ICDs), which appear as a small lump or scar on the upper chest. The AED pad must be placed at least one inch away from these devices to prevent damage to the implant or blocking the electrical shock from reaching the heart.
Transdermal medication patches, such as those for nitroglycerin or pain management, must be removed before placing the AED pads. Delivering a shock directly over a patch can cause a thermal burn to the skin or block the energy transfer. After removal, the area should be quickly wiped dry to eliminate any residual medication before attaching the pads. In cases of severe trauma or massive bleeding, the immediate priority is controlling the hemorrhage, and AED application is secondary.
When the AED Advises No Shock
Even when an AED is correctly applied to an unresponsive person, the device may still advise against delivering a shock. The message, “No Shock Advised,” means the AED’s analysis has determined the heart rhythm is not ventricular fibrillation or pulseless ventricular tachycardia. The heart may have stopped completely, a rhythm known as asystole, or it may have pulseless electrical activity (PEA), where electrical activity is present but the heart is not pumping blood effectively.
Neither asystole nor PEA are considered shockable rhythms, so the AED will not charge to deliver therapy. Hearing “No Shock Advised” does not indicate recovery; it is an instruction to immediately continue high-quality cardiopulmonary resuscitation (CPR). The pads must remain attached, as the AED will continue to monitor the patient and will advise a shock later if the rhythm changes to a shockable one.