Do defibrillators really restart a stopped heart? The story of shocks and CPR

No, not in the way films show it. A defibrillator does not restart a heart that has stopped. It shocks a heart that is in a dangerously disordered rhythm, such as ventricular fibrillation, where the muscle quivers instead of pumping. The shock depolarizes a large amount of heart muscle at once, ending the arrhythmia, and the heart's natural pacemaker, the sinoatrial node, can then re-establish a normal rhythm. A heart in asystole, a true flatline, cannot be restarted by defibrillation. This lesson describes how it works; it is not medical advice.

CPR is the other half of the story, and it does not usually restart the heart either. It is an emergency procedure of chest compressions, often combined with artificial ventilation, to preserve brain function and maintain circulation until spontaneous breathing and heartbeat can be restored. Its main purpose is to restore a partial flow of oxygenated blood to the brain and heart. In practice, CPR buys time and defibrillation resets the rhythm.

The shock has a long history. In 1899 two Geneva physiologists, Jean-Louis Prévost and Frédéric Batelli, found that small shocks could induce ventricular fibrillation in dogs and larger charges would reverse it. The first use on a human came in 1947, when the surgeon Claude Beck, who believed fibrillating hearts were often fundamentally healthy, "Hearts that are too good to die", used one. In 1962 Bernard Lown introduced the external DC defibrillator. Today automated external defibrillators are designed for use by untrained or briefly trained laypeople.

Television has set expectations high. A 1996 study found that CPR in television shows had a 75 percent success rate for immediate circulation and 67 percent survival to discharge. In US hospitals in 2017, 26 percent of patients who received CPR survived to hospital discharge.