A defibrillator is a device that reads the rhythm of a failing heart and, if needed, gives it an electric shock so that a normal rhythm can return. It is used in hospitals and ambulances, and is also mounted on the walls of public places such as airports and metro stations.
The shock is for one particular problem. In ventricular fibrillation, the heart's lower chambers quiver in a chaotic way instead of pumping. Blood stops moving round the body, and the person collapses within seconds. This is called sudden cardiac arrest. A shock stops the chaotic electrical activity for a moment. That gives the heart's own natural pacemaker a chance to start a normal beat again. Every minute without a shock lowers the chance of survival.
There are two main kinds. A manual defibrillator is used by doctors and nurses in hospitals. It shows the ECG on a screen. The doctor reads the rhythm, chooses the energy, such as 200 joules, and presses the shock button. Many hospital units combine a patient monitor and a defibrillator in one machine. An automated external defibrillator (AED) is made for anyone to use, even without training. It gives spoken instructions, analyses the rhythm itself and decides whether a shock is needed.
The most important part of an AED is software. Its rhythm analysis algorithm reads the ECG through two sticky pads on the chest. In about ten seconds, it decides whether the rhythm is one that a shock can treat. If it is not, the AED does not allow a shock. A wrong decision could kill. So the algorithm is tested on large collections of recorded ECGs, and certified as part of the device, as described in safety certification and software as a medical device.
Take, for example, a passenger who collapses at a departure gate in Kochi airport. An airport employee takes the AED from the wall and opens the lid. A voice tells her to stick one pad below the right collarbone and one on the left side of the chest. The AED analyses the rhythm and says, "Shock advised. Stand clear." She presses the button. The AED then tells her to press hard on the chest for two minutes, and beeps to set the pace, before it analyses the rhythm again. Paramedics arrive and take over.
The AED stores every ECG trace, every decision and every shock, with the time of each. At the hospital, staff download this record and add it to the patient's hospital EHR. A device implanted in the chest can give the same kind of shock from inside the body. That device is one of the cardiac implants.