An Automated External Defibrillator (AED) is a portable medical device designed to deliver an electric shock to the heart during sudden cardiac arrest. This device analyzes a person’s heart rhythm and, if necessary, provides an electrical shock to help restore a normal heartbeat. Proper use of AEDs is important in emergencies, as they can significantly improve survival rates.
The Heart’s Electrical Foundation
The heart functions as a pump, its rhythmic action governed by an intrinsic electrical system. This system generates electrical impulses that coordinate the contraction of the heart’s four chambers. The process begins in the sinoatrial (SA) node, the heart’s natural pacemaker, located in the right upper chamber. From there, the electrical signal travels through the upper chambers (atria) and then to the lower chambers (ventricles), causing them to contract and pump blood throughout the body. A healthy heart typically beats between 60 to 100 times per minute at rest, maintaining a regular, organized rhythm.
The Shockable Rhythms an AED Detects
An AED is programmed to identify and treat two life-threatening heart rhythms: ventricular fibrillation (VFib) and pulseless ventricular tachycardia (VTach). These rhythms are “shockable” because an electrical shock can effectively reset the heart’s chaotic electrical activity, allowing a normal rhythm to resume.
Ventricular fibrillation is characterized by chaotic, disorganized electrical activity in the heart’s lower chambers, the ventricles. This causes the ventricles to quiver uselessly instead of contracting effectively to pump blood, leading to sudden cardiac arrest. Pulseless ventricular tachycardia occurs when the ventricles beat very rapidly, often exceeding 180 beats per minute. However, these contractions are so fast and inefficient that they prevent adequate blood pumping. Despite some organized electrical activity, the heart cannot generate a pulse or effectively circulate blood.
Rhythms An AED Will Not Shock
The device will not deliver a shock for rhythms where defibrillation would not be beneficial or could even cause harm. These non-shockable rhythms include asystole and pulseless electrical activity (PEA). In these situations, the AED will advise “no shock advised,” prompting the rescuer to continue cardiopulmonary resuscitation (CPR).
Asystole, commonly known as “flatline,” represents the complete absence of electrical activity in the heart. Pulseless electrical activity (PEA) is a condition where the heart shows organized electrical activity, but the heart muscle is not contracting effectively enough to produce a pulse or pump blood. For both asystole and PEA, CPR and addressing underlying causes are the primary interventions, as an electrical shock would be ineffective.
How an AED Delivers Life-Saving Therapy
Once attached to a person experiencing sudden cardiac arrest, an AED uses electrode pads placed on the chest to monitor and analyze the heart’s electrical activity. This analysis typically takes between 5 to 15 seconds, determining if a shockable rhythm like ventricular fibrillation or pulseless ventricular tachycardia is present. The AED is designed to be highly accurate in this detection process.
If a shockable rhythm is identified, the AED advises the user that a shock is needed and begins charging. Modern AEDs are calibrated to deliver a specific amount of energy, usually measured in joules, typically ranging from 120-200 joules for adults. After charging, the AED provides clear voice prompts and visual cues, instructing the rescuer to ensure no one is touching the person before delivering the shock. The AED may deliver the shock automatically or require the rescuer to press a button. Following the shock, the AED will often advise continuing CPR and may reanalyze the rhythm after a set period, such as two minutes, to determine if further shocks are necessary.