Defibrillator

A defibrillator is a medical device that delivers an electric shock to the heart to stop a dangerous rhythm and give the heart a chance to resume a normal beat. The National Heart, Lung, and Blood Institute (NHLBI) describes defibrillators as devices that apply an electric charge or current to the heart to restore a normal heartbeat. They are used most often when the heart’s lower chambers quiver instead of pumping, a rhythm called ventricular fibrillation (V-fib). Cardiac arrest, the sudden stopping of an effective heartbeat, is fatal unless it is treated right away with cardiopulmonary resuscitation (CPR) and a defibrillator.

Background: how the defibrillator was developed

Electricity became commercially available in the 1870s, and accidental deaths among people who worked with power lines led physicians to ask how a shock can kill. In 1899 the physiologists Jean-Louis Prevost and Frederic Batelli of the University of Geneva showed in dogs that small shocks could trigger ventricular fibrillation and that larger shocks could reverse it.

In the 1930s the American engineer William Kouwenhoven and the Soviet researcher Naum Gurvich pushed the idea toward clinical use. Gurvich also developed a two-direction current, called a biphasic waveform, whose advantages Western researchers did not confirm until the 1980s.

The first human case followed in 1947. During surgery on a 14-year-old boy, the American cardiac surgeon Claude Beck found that the boy’s heart had stopped. After 35 minutes of manual heart massage, Beck applied shocks directly to the exposed heart, and the boy recovered fully. The early machines had one large drawback: paddles had to touch the heart itself, which meant opening the chest.

In 1956 the Boston cardiologist Paul Zoll performed the first successful closed-chest defibrillation, with a large machine that sent current through the chest wall. He reported four patients whose rhythm returned to normal, although only one left the hospital alive. In 1962 Bernard Lown and colleagues reported synchronized shocks of direct current delivered from a capacitor, a component that stores electrical energy and releases it in a brief pulse. Direct current caused fewer rhythm problems and less heart damage than the alternating current used before.

Machines then shrank. A portable unit of about 45 pounds appeared in 1961, and defibrillators moved from operating rooms to coronary care units, emergency departments and, by the early 1970s, ambulances. The first automatic implantable cardioverter defibrillator was invented in 1980 by a team at Sinai Hospital in Baltimore that included Michel Mirowski and Morton Mower, after 11 years of research and considerable skepticism from other experts. Automated external defibrillators appeared in the late 1980s, which allowed emergency technicians and ordinary bystanders to deliver the treatment.

Types of defibrillators

The NHLBI describes three main kinds for patients and the public.

Automated external defibrillators (AEDs) are found in many public spaces, and the unit tells the operator what to do. The NHLBI says even untrained people can use one in an emergency.

Implantable cardioverter defibrillators (ICDs) are small devices surgically placed in the chest. They are preprogrammed to detect cardiac arrest or a life-threatening arrhythmia, which is a problem with the rate or rhythm of the heartbeat, and then send a high-energy shock to stop it or to restart the heart.

Wearable cardioverter defibrillators (WCDs) are vests with a rechargeable battery. They work like an ICD but are usually meant for short-term use.

Hospitals and ambulance crews also use manual units, which let a trained clinician read the heart rhythm and choose the amount of energy. Choosing a device is a decision for a person’s health care provider.

How a defibrillator works

Every defibrillator has the same basic parts: a power source, a way of storing energy, and electrodes that carry current to the body. Lown’s 1962 design combined a battery, a capacitor and a transformer, and modern units still follow the same plan. Pads or paddles on the chest, or leads inside the body for an ICD, deliver the stored energy in a pulse that lasts only a fraction of a second.

The shock does not restart a stopped heart the way a key starts an engine. Its purpose is to interrupt the disorganized electrical activity of a rhythm such as V-fib so that the heart’s own electrical system can regain control.

Modern machines usually use a biphasic waveform, in which current flows in one direction and then reverses. Studies in the 1980s showed that this design needs less energy than the older Lown waveform. Lower energy reduces the risk of burns and muscle damage, and it allowed lighter, more portable machines.

Defibrillator vs. pacemaker

The two devices are often confused because both deliver electricity to the heart. A pacemaker sends small, regular pulses to keep the heartbeat at a steady rate, while a defibrillator sends a stronger shock to stop a dangerous rhythm. Some ICDs also act as pacemakers and give low-energy pulses to help the heart keep a normal rhythm.

Impact of the defibrillator

One historical review calls defibrillation one of the most important medical advances of the twentieth century. Its main effect was to turn a rhythm that was once fatal in minutes into one that can sometimes be reversed. The NHLBI notes that defibrillators can lower the risk of sudden death among people with a known arrhythmia or a high risk of a life-threatening one, because of causes such as genetic diseases, heart failure or a prior cardiac arrest.

The technology also changed where emergency care happens. A device once confined to surgical suites now hangs on public walls and rides in ambulances. Portable power came from advances in the electric battery, and rhythm recognition often depends on the same signal that an electrocardiogram records.

The limits are real. Early survival was poor, and the NHLBI stresses that a defibrillator works together with CPR, not in place of it. This entry is general information, not medical advice.

FAQs

Who invented the defibrillator? No single person did. Prevost and Batelli demonstrated the principle in 1899, Beck performed the first human defibrillation in 1947, Zoll achieved closed-chest defibrillation in 1956, and Mirowski and Mower developed the implantable version in 1980.

Is a defibrillator the same as a pacemaker? No. A pacemaker keeps the heart rate steady with small pulses, while a defibrillator delivers a stronger shock to stop a dangerous rhythm. Some implanted devices can do both.

Can an untrained person use an automated external defibrillator? The NHLBI says the unit tells the operator what to do and that even untrained people can use one in an emergency. Local emergency services and certified training courses give the practical guidance.

This entry explains a medical term. It is not medical advice. If you are concerned about your health, speak with a qualified health professional.

Editorially reviewed by Specialty Digest Editorial TeamLast reviewed November 4, 2026Researched and drafted with AI assistanceReport an issue