An Automated External Defibrillator (AED) is a portable device designed to deliver an electrical shock to the heart during sudden cardiac arrest. Its purpose is to help restore a normal heart rhythm. The AED is engineered to analyze the heart’s electrical activity and will only recommend a shock if it detects a specific, treatable rhythm.
When an AED Will Advise a Shock
An AED delivers an electrical shock for two types of cardiac rhythms: ventricular fibrillation (VF) and pulseless ventricular tachycardia (pVT). In ventricular fibrillation, the heart’s electrical activity becomes chaotic, causing the ventricles to quiver rather than pump blood effectively. This prevents the heart from circulating blood throughout the body.
Pulseless ventricular tachycardia occurs when the heart’s ventricles beat very rapidly and inefficiently, without generating a detectable pulse. Despite electrical activity, the heart cannot pump enough blood to sustain life. In both VF and pVT, the AED’s electrical shock aims to momentarily stop the heart’s chaotic activity, allowing its natural pacemaker to reset to a normal rhythm.
Understanding Non-Shockable Rhythms
An AED will not advise a shock if the heart’s rhythm is non-shockable. One such rhythm is asystole, commonly known as a “flatline,” which signifies a complete absence of electrical activity in the heart. In this state, there are no electrical impulses for a shock to reset, rendering defibrillation ineffective.
Another non-shockable rhythm is pulseless electrical activity (PEA). With PEA, the heart exhibits organized electrical activity, meaning the AED detects electrical signals, but these signals do not translate into effective mechanical contractions that pump blood. The heart muscle might be too weak or have other underlying issues preventing it from responding to electrical signals by contracting. Since the problem is mechanical rather than electrical, an AED shock cannot resolve the issue.
Other Reasons for No Shock Advised
Beyond specific heart rhythms, other factors can lead an AED to advise “no shock.” A patient may spontaneously regain a stable heart rhythm or consciousness before the AED completes its analysis. In such cases, the AED identifies that defibrillation is no longer necessary, as the heart has naturally resumed effective function.
User error can also lead to a “no shock” advisory. This includes improper placement of electrode pads on the patient’s chest, which prevents the AED from accurately reading the heart’s electrical activity. Failing to follow the AED’s voice prompts or moving the patient during analysis can also interfere with the device’s ability to assess the rhythm. Equipment malfunctions, such as low battery power or expired electrode pads, can also lead to a “no shock” recommendation.
What to Do When No Shock is Advised
When an AED advises “no shock,” immediately continue high-quality cardiopulmonary resuscitation (CPR). Continuous chest compressions and rescue breaths circulate oxygenated blood to the brain and other vital organs, sustaining the patient until advanced medical help arrives. Follow all voice prompts from the AED, as it will guide rescuers through the necessary steps.
Emergency medical services (EMS) should be contacted as soon as sudden cardiac arrest was suspected. The AED will periodically re-analyze the patient’s heart rhythm, typically after every two minutes of CPR. If the patient’s condition changes and a shockable rhythm develops, the AED will then advise a shock, prompting rescuers to clear the patient.