How Does a Defibrillator Restart the Heart?

During cardiac arrest caused by ventricular fibrillation, the heart's muscle fibres contract randomly instead of in coordinated waves. A defibrillator delivers a brief, high-energy shock (typically 120–360 joules in a biphasic waveform) through the chest wall via adhesive pads. This shock simultaneously depolarises all heart muscle cells, effectively "resetting" them. With the chaotic signals cleared, the heart's natural pacemaker (the sinoatrial node) can re-establish a regular rhythm. Automated External Defibrillators (AEDs) analyse the heart rhythm automatically and guide bystanders with voice prompts — studies show they can double survival rates from sudden cardiac arrest when used within the first few minutes.