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What Are Shockable Rhythms on an AED?

An automated external defibrillator (AED) is designed to analyze the heart and, if a shockable rhythm is present, deliver a shock to a victim of cardiac arrest. But what is a shockable rhythm? And what happens when one of the AED shockable rhythms isn’t present? If you are a potential AED user or have an AED on-site, it’s important to know:

  • How the heart works
  • Shockable rhythms
  • Unshockable rhythms
  • How a portable defibrillator can help
  • What to do if you’re ever in doubt

How the Heart Works

The heart is a pump that maintains the circulation of blood around the body. It beats around 115,200 times every day, 42 million times in a year, and around 3.3 billion times over a lifespan of 79 years.

Like other hydraulic pumps (those that pump fluid), the heart contains three systems:

  1. Electrical
  2. Mechanical
  3. Hydraulic

The Heart’s Electrical System

The electrical system surrounding the heart provides the energy to make it beat. This system is composed of a series of pacemakers that each fire at a different rate. If one pacemaker fails to fire, the next pacemaker takes over, and so on.

The Heart’s Mechanical System

The physical movement that pumps the blood around the body is the contracting of the heart muscle itself. The heart has four chambers. The two upper chambers are called “atriums” and the two lower chambers are called “ventricles.”

The Heart’s Hydraulic System

The hydraulic system of the heart is the fluid itself. The chambers must continuously fill with fluid so that the fluid can be oxygenated and the carbon dioxide removed through the lungs.

What Happens in a Cardiac Arrest

A cardiac arrest takes place when any of the three systems of the heart (electrical, mechanical, or hydraulic) fails to function correctly. Treating a cardiac arrest depends on working out which system has failed, identifying AED shockable rhythms, and giving the most appropriate form of advanced cardiac life support.

Cardiac Arrest Caused by an Electrical Malfunction

Most out-of-hospital cardiac arrests are the result of a problem with the heart’s electrical activity. Common causes of cardiac arrest include:

  • Heart tissue scarring (possibly from a heart attack)
  • Atherosclerotic heart disease
  • Cardiomyopathy (thickened heart muscle)
  • Heart medications
  • Wolff-Parkinson-White syndrome and Long QT syndrome
  • Blood vessel abnormalities
  • Recreational drug use

When the heart’s electrical system malfunctions, the heart muscle stops beating and the chambers stop filling with blood. As blood no longer reaches the vital organs (including the brain), these organs begin to suffer permanent damage and will eventually die.

Cardiopulmonary Resuscitation for Cardiac Arrest

In the first few vital minutes after an out-of-hospital cardiac arrest, cardiopulmonary resuscitation can help by pumping blood around the body so that it continues to reach the vital organs. This helps to prevent organ deterioration, brain damage, and death.

If the bystander is trained in CPR, they can provide ventilation breaths in addition to chest compressions. An untrained bystander should provide continuous chest compressions at a rate of 100-120 compressions per minute—roughly the tempo of the hit song Stayin’ Alive.

Defibrillation for Cardiac Arrest and Defibrillation

If an automated external defibrillator (AED) is available, it can be used to perform an electrocardiogram (ECG or EKG) and indicate whether one of the two AED shockable rhythms is detected. In the case that the heart has a shockable rhythm, the user should make sure that no one is touching the patient and press the shock button (if using a semi-automatic model like the HeartSine Samaritan PAD 350P) or wait for the defibrillator to deliver a shock (if using a fully automatic model like the Physio-Control LIFEPAK CR2).

Rather than “jump-start” the heart like you will often see in the movies, the shock actually shuts the heart down so that the heart’s own pacemakers can start up again on their own. You can think of it as the human equivalent of pressing the “restart” button on a computer.

Cardiac Arrest Caused by a Fluid Malfunction

The other two causes of cardiac arrest—loss of blood or a muscular malfunction—will not respond to an electric shock because an electrical malfunction is not to blame.

Extreme Blood Loss

If the cardiac arrest is the result of extreme blood loss, the flow of blood must first be stopped. The best course of action (while someone else calls 911) is to use a bleeding control kit to prevent traumatic blood loss. If you don’t have a kit available, you can minimize blood loss by applying constant, direct pressure and elevating the affected limb. Once the blood loss is under control, you can administer CPR to manage blood flow.

When medical professionals arrive, they can then administer a transfusion of blood or intravenous fluids to increase the volume of fluid in the body.

Heart Muscle Malfunctions

If there is electrical activity but the heart muscle is not responding, the cardiac arrest might be a result of tension pneumothorax or cardiac tamponade, among others. To treat these muscle-related conditions, emergency medical personnel will typically use a combination of:

  • Chest compressions
  • Bag-valve mask ventilation
  • Needle decompression (in the case of a pneumothorax)
  • Epinephrine

While you’re waiting for EMS to arrive, keep the AED attached and continue CPR.

Shockable Rhythms

There are two AED shockable rhythms in an electrical-related cardiac arrest:

  • Ventricular fibrillation
  • Pulseless ventricular tachycardia

Ventricular Fibrillation

Ventricular fibrillation (VF) or v-fib is a condition in which the ventricles (the lower chambers of the heart) only quiver rather than contract. You could think of this movement as a “heart seizure”—the heart still has some movement but doesn’t actually beat.

V-fib is often caused by problems with the electrical activity or “pacemakers” or physical damage to the heart muscle that prevents it from receiving the electrical impulses—such as a heart attack. Defibrillation can help by stopping the arrhythmia and allowing the heart’s primary pacemaker to restart.

Pulseless Ventricular Tachycardia

Ventricular tachycardia (v-tach) is the other shockable rhythm that can cause cardiac arrest. In this condition, the ventricle’s pacemakers don’t receive impulses from the heart’s primary pacemakers and begin to fire rapidly to compensate.

This rapid firing doesn’t allow the muscles to contract effectively or chambers to fill with blood. Eventually, the patient’s pulse cuts out.

V-tach can be caused by:

  • Coronary artery disease
  • Heart scarring
  • Drug use
  • Certain medications
  • Electrolyte imbalances

Pulseless V-Tach vs. V-Tach with a Pulse

Ventricular tachycardia (a rapid heartbeat) can occur with a pulse if the heart muscle is still able to contract. In this case, the patient would not be in cardiac arrest and would not need to receive a shock. As AEDs only check the electrical activity (and not the pulse), it’s important to feel for a pulse as well.

However, if the victim is unconscious and unresponsive and emergency medical personnel aren’t there, it’s better to assume cardiac arrest and deliver an unneeded shock than to leave the victim to their fate. You can feel for a pulse, but if the victim is unresponsive, always send for an AED.

Unshockable Rhythms

Just as there are two shockable rhythms, there are two non-shockable rhythms:

  • Pulseless electrical activity (PEA)
  • Asystole

Pulseless Electrical Activity

Pulseless electrical activity (PEA) describes the situation in which the heart’s electrical activity is working correctly and was not the cause of the cardiac arrest. As explained earlier on, the cardiac arrest, in this case, was most likely caused by a problem with the fluid (extreme blood loss) or a problem with the cardiac muscle (such as tension pneumothorax or cardiac tamponade).

In either case, an electric shock would do nothing to reverse the cardiac arrest because the electrical impulses weren’t the problem. If the patient has a clear external bleed, call 9-1-1, apply pressure to the wound and make a makeshift tourniquet from a torn-up garment.

Otherwise, leave the AED turned on and connected to the patient and continue to provide high-quality CPR. Every two minutes, the AED will continue to look for a shockable rhythm and provide a shock if the rhythm changes to a shockable one.


Asystole, also referred to as cardiac flatline, is the total cessation of electrical activity in the heart. This condition may occur after a prolonged period of v-fib or because the heart muscle has actually died.

Asystole will not respond to an electric shock because there is no electrical activity to reset. However, high-quality CPR and medications to treat the cause can sometimes bring the person back, so the best thing a bystander can do is to continue to provide high-quality CPR until emergency services arrive.

AED Shockable Rhythms: What to Do

The best thing you can do in the case of an out-of-hospital cardiac arrest is to call 9-1-1, attach a portable defibrillator to the patient’s bare chest via the adult (or pediatric) electrode pads, turn the device on, and follow the instructions.

The AED will direct you to perform chest compressions and will periodically analyze the patient’s heart rhythm.

Shock Advised

If the patient has a shockable rhythm, the AED will direct bystanders to stand clear while the machine gives a shock (for a fully-automatic model) or instructs the user to press the shock button (semi-automatic models).

No Shock Advised

If the patient has a non-shockable rhythm, the AED will give a “No shock advised” message and direct you to continue compressions. In this case, it’s important to trust the device. If AED testing requirements have been followed and the machine is working, it will have analyzed the patient’s heart and determined that the electrical pulse is either working normally or no electrical activity is detected. In both of these cases, a shock would not help.

As mentioned previously, it’s vital to keep the AED connected and turned on after a “no shock advised” message. The device will continue to analyze the patient’s heart and give directions for CPR. A non-shockable rhythm may later turn into a shockable one. In this case, the device will provide a shock.

The only reasons to remove the electrode pads from the patient are:

  1. A transfer of care (the EMS personnel or hospital staff will remove the electrode pads when it’s safe to do so)
  2. Resuscitation efforts have ceased (again, only a medical professional can make this call)

So, as far as bystander treatment goes, leave the defibrillator on and attached and continue chest compressions until the emergency services arrive. Whether the patient has AED shockable rhythms or not, the compressions will help to keep them alive.

Be Ready to Save a Life

If you are the owner or manager of a facility where people regularly gather or exercise, having a working AED on the premises could save a life in the case of cardiac arrest. In many states, having an external defibrillator and people trained to use it are requirements for schools, gyms, dental offices, and parks.

For organizations with multiple locations and/or multiple AEDs, AED Program Management can help you keep the devices up-to-date and ready for use at any moment. Shockable rhythms can be detected by an AED. Make sure your device is ready to use!

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