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Shockable vs. Non Shockable Heart Rhythms 

| Last Updated on October 5, 2022

To shock or not to shock? That is the question for your AED. 

In fact, that is exactly what an AED determines once its pads are attached to a patient: whether or not the detected heart rhythm can be treated with defibrillation. 

Electrical impulses in the heart initiate each of our heart beats. However, different diseases and structural problems in the heart can cause these impulses to go haywire. When that happens, irregular rhythms can occur, some of which cause Sudden Cardiac Arrest (SCA). 

If someone experiences SCA and receives high-quality CPR, their rescuer will attach an AED to their chest. If the AED detects Ventricular Tachycardia (VT) or Ventricular Fibrillation (VF), then it will deliver a shock. If it discovers Asystoleor Pulseless Electrical Activity (PEA), the AED will alert the user and say, “No shock advised.” 

While only two of these four irregular rhythms can be treated with an AED, all four of these arrhythmias can lead to SCA.

Contrary to many fear-based narrativesan AED will NOT deliver a shock until it has fully analyzed the patient’s heart rhythm and determined that a shock is medically necessary. When an AED detects VT or VF, quick defibrillation is a crucial treatment that can help save lives. 

Shockable Rhythms 

  • V-Tach is an abnormal heart rhythm marked by an uncharacteristically fast heartbeat. Instead of beating 60-100 times per minute, a heart in Ventricular Tachycardia usually beats over 100 times or more per minute. VT is caused by irregular electrical impulses within the heart’s lower chambers. Sometimes, VT, will cause blood to stop flowing through the body. 
  • V-Fib is an abnormal heart rhythm where the heart’s lower chambers, or ventricles, twitch but don’t perform an effective heartbeat. Often brought on during or shortly after a heart attackVentricular Fibrillation is the leading cause of sudden cardiac death.


Both V-Tach and V-Fib prevent the heart from delivering blood to the body’s vital organs, which is why immediate CPR is necessary. CPR is essential for both arrhythmias in order to manually keep blood circulating throughout the body in the absence of a regular heartbeat.

Dr. Allison Dupont, Cardiologist, Northside Hospital, speaks about shockable heart rhythms,
V-Tach & V-Fib and the importance of early defibrillation

Non-Shockable Rhythms 

  • An Asystole — rhythm means that the heart’s electrical system has shut down and there is no heartbeat. Asystole can be the result of untreated VT or VF. If someone experiences Asystole, CPR should be initiated immediately to provide the best chances of survival. If an Asystole rhythm is detected by an AED, it will not shock the patient, as defibrillation is not a viable treatment here. 
  • PEA — yet another type of irregular heartbeat caused by an electrical problem in the heart. In PEA, the heart’s electrical activity is too weak to continue pumping blood throughout the body. Like with Asystole, an AED will not correct this arrhythmia, and CPR should be administered as soon as possible to provide the best patient outcome.

AEDs are a vital component in response to cardiac emergencies, and they are the proven therapy for both V-Tach and V-Fib. 

Learn more about Arrhythmias and Sudden Cardiac Arrest

Content Team

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