Defibrillator is a device used to correct a dangerously abnormal heart rhythm. Isually ventricular fibrillation or to restart the heart by depolarizing its electrical conduction system and delivering brief measured electrical shocks to the chest wall or the heart muscle itself.

An implantable cardioverter defibrillator (ICD) is a pacemaker-like device. It's about the size of three silver dollars. It's placed under the skin just below the collarbone. The device uses leads (wires threaded into the heart's chambers) to monitor every single heartbeat. If a lethal rhythm appears, the defibrillator shocks the heart back to a normal rhythm.

A major technological advance was the development of portable defibrillators in 1966. These devices were developed in Belfast, Ireland by cardiologist Frank Pantridge. These portable heart defibrillators became suitable for installation in ambulances. The next great leap of innovation was the development of automated external defibrillators. This led to the mobilization of advanced cardiac life support for EMTs, paramedics, police officers, and other first responders.