An Automated External Defibrillator (AED) is a sophisticated, yet simple-to-use, device designed to treat sudden cardiac arrest by analyzing the heart’s electrical rhythm and delivering an electric shock if needed. When the AED advises “no shock,” this is not a sign of device malfunction, but a precise electrical assessment of the heart’s condition. The device has determined the rhythm will not benefit from, or could even be harmed by, an electrical discharge. This directs the rescuer to shift focus immediately to other life-saving measures.
The Shockable Rhythms
The AED is specifically programmed to identify and treat two life-threatening electrical malfunctions: Ventricular Fibrillation (VF) and Pulseless Ventricular Tachycardia (pVT). These rhythms represent a primary electrical issue where the heart’s lower chambers, the ventricles, are unable to pump blood effectively. In Ventricular Fibrillation, the heart’s electrical signals are chaotic and disorganized, causing the muscle to merely quiver instead of contracting.
Pulseless Ventricular Tachycardia (pVT) is characterized by electrical signals that are organized but extremely rapid, often over 100 beats per minute. This rapid rate prevents the heart from filling with blood. In both VF and pVT, the electrical malfunction leads to circulatory collapse. The shock momentarily stops all electrical activity, allowing the heart’s natural pacemaker cells a chance to reset and restore a normal, pumping rhythm.
The Non-Shockable Rhythms
The AED advises “no shock” when it detects a rhythm that an electrical current cannot fix. The two main non-shockable rhythms are Asystole and Pulseless Electrical Activity (PEA). Asystole is commonly referred to as a “flatline” because it indicates a complete absence of electrical activity. Since there is no disorganized rhythm to reset, a shock would be useless and would only delay other necessary treatments.
Pulseless Electrical Activity (PEA) occurs when the heart’s electrical system generates an organized rhythm, but the muscle is not contracting strongly enough to create a pulse and pump blood. In PEA, the problem is mechanical or chemical, not primarily electrical, often due to issues like severe blood loss, oxygen deprivation, or drug overdose. Since the electrical activity is present but ineffective, a defibrillation shock would not solve the underlying mechanical failure. For both Asystole and PEA, treatment shifts to high-quality cardiopulmonary resuscitation (CPR) and addressing potential reversible causes.
How the AED Analyzes the Heartbeat
The AED’s internal process for determining shockability is guided by a sophisticated algorithm. When the electrode pads are attached, the device measures the frequency, amplitude, and regularity of the electrical signals transmitted through the chest. It first confirms proper contact by measuring impedance (the electrical resistance between the pads), ensuring that a shock, if needed, can be delivered effectively.
The device then filters the detected electrical signal to remove noise or artifact caused by movement or interference. The algorithm specifically looks for the disorganized signals characteristic of Ventricular Fibrillation, or the rapid, organized pattern of Pulseless Ventricular Tachycardia. If the analyzed rhythm falls outside these two shockable parameters, or if it detects Asystole or PEA, the AED’s safety mechanism is activated. This programming ensures a shock is only advised when it has a medical chance of restoring a normal heart rhythm.
Continuing Care After No Shock is Advised
When the AED announces “no shock advised,” the rescuer’s response must be the immediate continuation of high-quality CPR. The most important action is to minimize interruptions and focus on chest compressions and rescue breaths. Modern AEDs are designed to coach the rescuer, providing prompts to guide the rate and depth of compressions, even when a shock is not delivered.
The electrode pads should remain firmly attached to the chest throughout the rescue attempt. The AED will automatically re-analyze the heart’s rhythm every two minutes during the CPR cycle. This is crucial because a non-shockable rhythm can transition into a shockable one, or a shockable rhythm can revert to a normal rhythm, requiring reassessment. The rescuer must continue following the AED’s prompts and performing CPR until professional medical personnel arrive.