An Automated External Defibrillator (AED) is a portable electronic device designed to deliver a controlled electrical shock to restore a normal heart rhythm during sudden cardiac arrest. This device is a crucial tool in the chain of survival. However, not all irregular heart rhythms benefit from a shock, as the device specifically targets certain electrical dysfunctions.
Heart Rhythms that Receive a Shock
An AED delivers a life-saving electrical shock for two specific abnormal heart rhythms: ventricular fibrillation (VFib) and pulseless ventricular tachycardia (pVT). These are chaotic electrical activities that prevent the heart from pumping blood effectively, leading to cardiac arrest. Immediate defibrillation is crucial for these conditions, as it can significantly enhance survival rates.
Ventricular fibrillation occurs when the heart’s lower chambers, the ventricles, quiver uselessly instead of contracting in an organized manner. This disorganized electrical activity means the heart cannot pump blood to the brain and other vital organs. An AED shock works by momentarily stopping all electrical activity in the heart, allowing its natural pacemaker to reset and potentially resume a normal, effective rhythm.
Pulseless ventricular tachycardia is characterized by an uncharacteristically fast heartbeat originating from the ventricles, resulting in no detectable pulse. The heart beats so rapidly that it cannot fill with blood or pump it efficiently. Similar to VFib, an AED shock aims to interrupt this rapid, ineffective rhythm, giving the heart an opportunity to re-establish a functional beat. Both rhythms are emergencies that require prompt intervention, as the chance of survival decreases significantly with each passing minute without defibrillation.
Heart Rhythms That Do Not Receive a Shock
AEDs are programmed not to deliver a shock for certain heart rhythms, including asystole and pulseless electrical activity (PEA). In these cases, a shock would not be beneficial and could potentially cause harm.
Asystole, commonly known as “flatline,” indicates no electrical activity in the heart. The heart has completely stopped. Since defibrillation works by resetting chaotic electrical activity, it cannot correct a heart that has no electrical activity at all. For asystole, primary interventions focus on high-quality cardiopulmonary resuscitation (CPR) and the administration of medications like epinephrine, aiming to restart some electrical activity in the heart.
Pulseless electrical activity (PEA) is a condition where the heart’s electrical system shows organized activity, but this activity is too weak to produce a palpable pulse or effectively pump blood. An AED will not deliver a shock for PEA because the issue is not chaotic electrical activity, but rather the heart’s inability to respond mechanically to those signals. Treatment for PEA primarily involves continuous CPR and addressing any underlying causes, such as severe blood loss or blockages, to restore effective circulation.
How AEDs Analyze Heart Rhythms
An AED utilizes technology to accurately detect and analyze a person’s heart rhythm before advising a shock. When electrode pads are applied to the chest, they act as sensors, picking up the electrical signals generated by the heart. These pads must be placed on the bare skin, typically with one on the upper right chest and the other on the lower left side.
Once the pads are in place, the AED automatically begins its analysis. The device evaluates the heart’s electrical waveform, determining if a shockable rhythm like ventricular fibrillation or pulseless ventricular tachycardia is present. During this analysis phase, it is important that no one touches the person to avoid interfering with the AED’s reading.
The AED then provides clear voice prompts and sometimes visual cues to guide the user through the process. If a shockable rhythm is detected, the device will advise a shock and begin charging. For semi-automatic AEDs, the user will be prompted to press a button to deliver the shock, while fully automatic models will deliver the shock on their own after a warning. This safety mechanism ensures that a shock is only delivered when medically appropriate, preventing accidental shocks to non-shockable rhythms or a heart that is beating normally.