Which heart rhythms can a defibrillator shock?
When someone suffers a Sudden Cardiac Arrest (SCA), their heart is no longer pumping blood effectively. The most important intervention at this point is a combination of CPR and a shock from a defibrillator.
While CPR keeps blood and oxygen circulating, only a shock from an Automated External Defibrillator (AED) can reset the heart’s electrical system and restore a normal rhythm.
Shockable vs Non-Shockable rhythms
A defibrillator does not work on every type of cardiac arrest. It only delivers a shock if the person’s heart is in certain abnormal rhythms that can potentially be corrected by a shock. These are known as shockable rhythms.
If the heart is in a rhythm that a shock will not help, the defibrillator will simply advise you to continue CPR until emergency services arrive.
What are Shockable Rhythms?
There are two shockable heart rhythms that can be treated with a defibrillator shock:
Ventricular Fibrillation (VF)
- The heart’s electrical activity becomes chaotic.
- The ventricles quiver instead of contracting, so no blood is pumped around the body.
- A shock can stop the chaos, giving the heart a chance to reset into a normal rhythm.
Pulseless Ventricular Tachycardia (VT)
- The heart beats too fast for blood to be pumped effectively.
- Without a pulse, this is a life-threatening emergency.
- A shock can reset the rhythm back to normal.
Both VF and pulseless VT are the most common causes of sudden cardiac arrest, which is why early defibrillation is so critical.
It is vital to have a defibrillator nearby in an emergency, and one that is straightforward to use. We offer a wide range of AEDs suited for untrained and trained responders.
What are Non-Shockable Rhythms?
There are also some rhythms where a defibrillator cannot help. In these cases, CPR and advanced medical support are the only options.
- Asystole (flatline): The heart has no electrical activity at all. A shock will not restart it.
- Pulseless Electrical Activity (PEA): The heart shows some organised electrical activity on a monitor, but it is not producing a pulse or pumping blood.
In both situations, high-quality CPR and rapid medical intervention are vital to give the best chance at survival.
How a Defibrillator Knows Which Rhythms Are Shockable
One of the most reassuring things about modern AEDs is that they are designed for anyone to use. The AED device:
- Analyses the patient’s heart rhythm automatically
- Detects shockable rhythms like VF or pulseless VT and decides whether a shock is needed
- Gives clear voice prompts guiding the rescuer through every step
This means you do not need to worry about diagnosing heart rhythms yourself, the AED will do the work for you. A defibrillator will only allow or advise a shock when it detects a shockable cardiac rhythm.
Peace of mind with Defib Warehouse
Defib Warehouse has been one of the most trusted suppliers of defibrillators online since 2013, offering AED devices from all major brands, including ZOLL, Philips, Physio Control and Primedic.
Medical Disclaimer:
Any medical/health information on the Site is for general informational and educational purposes only and is not a substitute for professional medical/health advice. Always seek the advice of a qualified healthcare professional with any questions you may have regarding a medical condition.
Sources
RCUK Adult Advanced Life Support Guidelines. Published October 2025: https://www.resus.org.uk/professional-library/2025-resuscitation-guidelines/adult-advanced-life-support-guidelines
RCUK Adult ALS algorithm 2025 (PDF): https://www.resus.org.uk/sites/default/files/2025-10/Adult%20ALS%20algorithm%202025.pdf
Soar J, et al. European Resuscitation Council Guidelines 2025: Adult Advanced Life Support. Resuscitation 2025;215 (Suppl 1):110769: https://doi.org/10.1016/j.resuscitation.2025.110769
