Defibrillation is the delivery of a controlled electrical shock to the heart to terminate a life-threatening cardiac arrhythmia and restore the potential for a normal heart rhythm. It is the definitive treatment for ventricular fibrillation (VF) and pulseless ventricular tachycardia (VT), the heart rhythms responsible for the vast majority of sudden cardiac arrests.
During VF, the heart’s electrical activity is completely disorganised: millions of cells fire chaotically, causing the ventricles to quiver rather than contract. No blood is pumped. A defibrillation shock passes a large electrical current through the heart, simultaneously depolarising (resetting) all the cardiac cells. If the heart’s natural pacemaker (the sinoatrial node) then takes over, a coordinated rhythm can be restored. Defibrillation does not ‘restart’ the heart in the way a car engine restarts; it briefly stops all electrical activity, giving the heart’s natural conduction system the chance to resume control.
Defibrillation can be delivered in three ways. An automated external defibrillator (AED) is a portable device available in public spaces that analyses the heart rhythm and delivers a shock if indicated, designed for use by bystanders with minimal training. A manual defibrillator is used by paramedics and hospital staff, allowing the operator to interpret the rhythm and select the energy dose. An implantable cardioverter defibrillator (ICD) is a device surgically implanted under the skin that continuously monitors the heart rhythm and delivers an internal shock when it detects VF or dangerous VT.
Time is critical. Each minute of VF without defibrillation reduces survival by approximately 10%. When combined with immediate CPR (which maintains partial circulation until a shock can be delivered) and early defibrillation (within 3 to 5 minutes), survival rates from out-of-hospital cardiac arrest can exceed 50 to 70%. Defibrillation is distinct from cardioversion, which uses a synchronised lower-energy shock to treat arrhythmias such as atrial fibrillation or stable VT without cardiac arrest.
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