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What Are Shockable Rhythms on an AED?

What Are Shockable Rhythms on an AED?

What Are Shockable Rhythms on an AED?

An automated external defibrillator (AED) is designed to analyze the heart of a sudden cardiac arrest victim and, if it’s a shockable rhythm, the AED will deliver a shock. But what is a shockable rhythm? And what happens when one of the two AED-shockable heart 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
  • What happens in cardiac arrest
  • Which rhythms are shockable
  • Which rhythms are not shockable
  • What to do in the event of a cardiac arrest

How the Heart Works

To understand why only some heart rhythms are shockable, it’s essential to know how the heart works. The heart is a pump that maintains the circulation of blood around the body. It works very hard—beating around 100,000 times every day, around 40 million times in a year, and around 3 billion times over an average lifetime.

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

  1. Electrical: The heart’s electrical system 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.
  2. Mechanical: This is the contraction 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.”
  3. Hydraulic: 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 dispersed throughout the body. At the same time, carbon dioxide is removed through the lungs.

What Happens in a Cardiac Arrest

A cardiac arrest happens when any of the heart’s three systems (electrical, mechanical, or hydraulic) fails to function correctly. Treating cardiac arrest depends on assessing which system has failed and providing the most appropriate intervention to keep the patient alive until emergency medical services arrive on the scene.

Cardiac Arrest Caused by an Electrical Malfunction

Most out-of-hospital cardiac arrests result from a problem with the heart’s electrical activity. 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 start to suffer permanent damage with every passing minute 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 oxygen 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

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 restarts the heart’s natural pacemakers. You can think of it as the human equivalent of pressing the “restart” button on a computer.

Non-Electrical Causes of Cardiac Arrest

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 9-1-1) 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

Now that you understand how the heart works and what happens in sudden cardiac arrest, it will make much more sense why some rhythms are shockable and others are not. There are two AED shockable rhythms in an electrical-system-related cardiac arrest:

  • Ventricular fibrillation
  • Pulseless ventricular tachycardia

Ventricular Fibrillation

Ventricular fibrillation (VF) or V-fib is the leading direct cause of sudden cardiac death. When V-fib occurs, 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 or pump.

V-fib is often caused by problems with the heart’s electrical activity (or “pacemakers”) or results from 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 abnormal rhythm and allowing the heart’s primary pacemaker to restart.

Pulseless Ventricular Tachycardia

Pulseless ventricular tachycardia (V-tach) is the other shockable rhythm that can cause cardiac arrest. When this poorly perfusing rhythm occurs, the ventricles’ 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 the chambers to fill with blood. Eventually, the patient’s pulse cuts out.

Pulseless V-tach can be caused by:

  • Coronary artery disease
  • Valvular heart disease
  • Cardiomyopathy
  • Long QT syndrome
  • Brugada syndrome
  • Certain drugs and 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.

However, seeing as it’s difficult for laypeople to determine accurately whether or not there is a pulse, the updated 2020 CPR guidelines from the American Heart Association recommend commencing CPR on an unresponsive patient who is not breathing normally without feeling a pulse. The risk of the victim receiving chest compressions when they are not needed is much lower than the risk of withholding CPR from a pulseless victim. The same applies to defibrillation.

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 system is working correctly and is not the cause of the cardiac arrest. Cardiac arrest, in this case, could be 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 conduction isn’t the problem.


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 itself has 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 high-quality CPR until emergency services arrive.

What to Do in the Event of Cardiac Arrest

The best thing you can do in the case of an out-of-hospital cardiac arrest is:

  1. Call 9-1-1
  2. Attach a portable defibrillator to the patient’s bare chest via the adult electrode pads or pediatric pads for an infant or small child
  3. Turn the device on
  4. Follow the instructions

The AED will direct you to perform chest compressions (to maintain blood flow to the vital organs) 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 (for 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 all of the AED testing requirements have been followed and the machine is working, it will have analyzed the patient’s heart rhythm 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 a shockable rhythm or not, the compressions will help to keep them alive until they can receive advanced cardiac life support in a hospital.

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. Shockable rhythms can be detected by an AED. Make sure your device is ready to deliver that much-needed lifesaving shock!

Disclaimer for information purposes only:

Our website provides information for general knowledge and informational purposes only. We do not offer medical advice, diagnosis, or treatment. Readers should consult with qualified healthcare professionals for personalized medical advice.

While we endeavor to ensure the accuracy and reliability of the information provided, we do not guarantee its completeness or suitability for any specific purpose. The use of this website is at the reader’s own risk.

By accessing and using this website, you agree to indemnify and hold harmless the website owners, authors, contributors, and affiliates from any claims, damages, liabilities, losses, or expenses resulting from your use of the information presented herein.

Picture of Michelle Clark, RN ICU/CCU
Michelle Clark, RN ICU/CCU
As a seasoned Nurse (RN) in Critical Care, CCU (Cardiac Care Unit), and ICU (Intensive Care Unit) with nearly three decades of experience, specializing in Cardiopulmonary care, I've embarked on a new path as a trusted figure in the realm of sudden cardiac arrest and first aid. With a profound dedication to patient well-being honed throughout my nursing career, I now utilize my expertise to enlighten and empower others in life-saving methods. Leveraging my comprehensive understanding and proficiency in critical care, I endeavor to leave a lasting imprint in healthcare by promoting awareness and offering practical guidance.

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