An Automated External Defibrillator (AED) is a portable electronic device designed to treat sudden cardiac arrest by automatically analyzing a person’s heart rhythm and delivering an electrical shock if necessary. The device is a key part of the chain of survival for individuals experiencing a cardiac emergency. A common question is whether this machine can sense the physical surge of blood flow, or a pulse, to determine treatment necessity. The direct answer is that AEDs are not engineered to detect a pulse, relying instead on a fundamental measurement of the heart’s electrical function.
What AEDs Analyze: Focusing on Electrical Rhythms
The core function of an AED is to analyze the heart’s intrinsic electrical signaling through electrode pads placed on the chest. The device interprets this electrical activity to decide whether the pattern is one that a therapeutic shock can correct. This analysis focuses entirely on the heart’s rhythm, which is the sequence of electrical impulses that regulate the heartbeat. The AED identifies life-threatening electrical chaos that prevents the heart from effectively pumping blood.
The two primary rhythms an AED is programmed to identify as “shockable” are Ventricular Fibrillation (VF) and Pulseless Ventricular Tachycardia (VT). Ventricular Fibrillation is a condition where the heart’s electrical signals are disorganized and chaotic, causing the ventricles to quiver uselessly instead of contracting to pump blood. Pulseless Ventricular Tachycardia is a very rapid, organized electrical impulse that still results in an ineffective heartbeat with no resulting blood flow.
If the AED detects either of these two unstable electrical patterns, it advises a shock to momentarily stop all electrical activity. This brief interruption, or “reset,” allows the heart’s natural pacemaker to potentially regain control and establish a normal rhythm. Conversely, if the AED determines a non-shockable rhythm, such as Asystole (a complete absence of electrical activity), it instructs the rescuer to continue Cardiopulmonary Resuscitation (CPR). The AED’s analysis is a purely electrical assessment, distinguishing between rhythms that require defibrillation and those that do not.
The Critical Distinction: Electrical Activity Versus Mechanical Function
The inability of an AED to determine pulselessness stems from the difference between the heart’s electrical system and its mechanical pumping action. The electrical signal is the impulse that tells the heart muscle to contract. The pulse is the mechanical outcome—the physical wave of blood flow generated by the muscle contraction. The AED’s electrodes only sense the electrical impulse and have no sensors to measure the physical pressure wave of a pulse.
This distinction is evident with a condition known as Pulseless Electrical Activity (PEA). PEA occurs when the electrocardiogram (ECG) shows an organized electrical rhythm that would typically be associated with a pulse. However, the heart muscle is mechanically failing to contract effectively enough to produce a palpable pulse or blood pressure. In a patient experiencing PEA, the AED registers the organized electrical rhythm and advises “No Shock Advised.”
The electrical signals are present in PEA, but the mechanical function is absent, meaning the patient is in cardiac arrest and requires immediate intervention. Since the AED’s programming recognizes that a shock will not correct the organized electrical rhythm, it withholds the electrical therapy. This scenario illustrates the technological boundary of the AED, as it cannot detect the lack of mechanical blood flow that defines pulselessness.
Operator Responsibilities: The Human Role in Determining Pulselessness
Since the AED cannot assess the mechanical function of the heart, the human operator’s role in determining pulselessness remains necessary in the emergency protocol. Before attaching the AED pads, the rescuer must confirm that the person is unresponsive and not breathing normally. This initial assessment is the basis for determining the need for intervention, not the device itself.
If the person is unresponsive and not breathing normally, the rescuer should immediately initiate the first steps of the emergency response sequence. This includes activating emergency medical services and starting high-quality Cardiopulmonary Resuscitation (CPR) with chest compressions. The AED should be applied as soon as it becomes available, even while CPR is ongoing.
The decision to continue or resume CPR is made independently of the AED’s shock recommendation. If the AED advises “No Shock Advised,” the rescuer must immediately continue chest compressions. The rescuer’s recognition of unresponsiveness and abnormal breathing is the indicator to begin and maintain life-support measures until the AED prompts an analysis or a shock is delivered.