How Does an AED Affect the Rhythm of the Heart?

An Automated External Defibrillator (AED) is a portable device that addresses sudden cardiac arrest (SCA), which occurs when the heart’s electrical system malfunctions and stops pumping blood. An AED analyzes the heart’s electrical activity and, if needed, delivers a controlled shock to help restore a normal rhythm. These devices provide verbal instructions to guide a rescuer, making them common in public spaces like offices, schools, and airports.

Heart Rhythms an AED Can Treat

An AED is programmed to treat two life-threatening heart rhythms that cause sudden cardiac arrest. The first is Ventricular Fibrillation (VF), a state where the heart’s lower chambers, the ventricles, quiver chaotically instead of pumping. The second rhythm is pulseless Ventricular Tachycardia (pVT), where the ventricles contract too rapidly to effectively fill with and eject blood.

Both VF and pVT are dangerous because they render the heart unable to function as a pump. Without blood flow, a person will lose consciousness within seconds and survival decreases for every minute that passes without intervention. An AED will not advise a shock for every type of heart problem, only those it is designed to correct.

This specificity is a built-in safety feature. The device is designed to intervene only when it detects the electrical signatures of VF or pVT. For other conditions, like a normal rhythm or a state with no electrical activity (asystole), the AED will not deliver a shock because it would be ineffective and potentially harmful.

How an AED Assesses the Heart’s Rhythm

Once the electrode pads are placed on the chest, the AED begins to analyze the heart’s electrical activity. The pads contain sensors that transmit data about the heart’s rhythm to a processor inside the device, which uses an algorithm to interpret the signals.

The algorithm is designed to distinguish between shockable and non-shockable rhythms. It compares the detected rhythm against programmed patterns for VF and pVT. The analysis is automated, removing the need for the user to interpret the rhythm themselves, which allows untrained individuals to use the device.

If the AED identifies a shockable rhythm, it will alert the user and prepare to deliver a shock. If the detected rhythm is not VF or pVT, the device will state that a shock is not advised and will instruct the user to perform Cardiopulmonary Resuscitation (CPR). This automated process ensures a shock is only delivered when appropriate.

Delivering the Shock to Restore Normal Rhythm

When an AED determines that a shock is necessary, it charges its internal capacitor to deliver a controlled electrical current. The user is instructed to ensure no one is touching the person and to press the shock button, though some models may deliver the shock automatically. This electrical energy passes from one pad, through the heart, to the other to stop the chaotic electrical storm of VF or pVT.

The shock works through a process called defibrillation. It delivers a simultaneous wave of electricity that depolarizes, or resets, the heart’s muscle cells at once. This depolarization halts the erratic and uncoordinated electrical firing that characterizes these arrhythmias, creating a momentary pause in the heart’s electrical system.

This brief pause is the goal of the intervention. It provides an opportunity for the heart’s natural pacemaker, the sinoatrial (SA) node, to resume its function. If the SA node re-establishes control, it will generate organized electrical impulses for an effective heartbeat. The shock reboots the heart’s electrical system, giving it a chance to return to a coordinated rhythm.

Outcomes After an AED Shock

Immediately following the delivery of an electrical shock, the AED will re-analyze the heart’s rhythm. A successful shock terminates the dangerous arrhythmia, though it does not guarantee an immediate return to a normal sinus rhythm. The objective is to stop the electrical chaos so a more organized rhythm can take over.

In some cases, the initial shock may not be sufficient to convert the rhythm, or the heart may revert to VF or pVT. The AED is programmed to recognize this and may advise that another shock is needed. The device will guide the rescuer through subsequent analysis and shock cycles.

Whether a shock is delivered or not, the AED will instruct the rescuer to resume CPR immediately. If the shock was successful, chest compressions help circulate oxygenated blood while the heart muscle recovers and attempts to re-establish a stable rhythm. If the AED advised “no shock” for a non-shockable rhythm, CPR is the main intervention to manually circulate blood until advanced medical help arrives.