Defibrillator – application of Capacitors in the medical field

One gadget in which capacitors play a significant role is the defibrillator. A defibrillator has a large capacitor and when it is fully charged it can store a Maximum of 360 Joule of energy in its electric field.

The defibrillator is quite fast to deliver all this energy to a patient in about 2 ms. (This is roughly equivalent to 1800 times the power delivered to a 100-W lightbulb!)

Under the proper circumstances, the defibrillator can be used to stop cardiac fibrillation (irregular contractions) in heart attack victims.

When fibrillation occurs, the heart produces a rapid, irregular pattern of beats. A fast discharge of energy through the heart can return the organ to its normal beat pattern.

Emergency medical teams use portable defibrillators (portable automatic external defibrillator or AED). AED is a device designed to shock the heart of a person who is in ventricular fibrillation.

Defibrillator - application of Capacitors in the medical field
figure 1: Schematic diagram showing where to place the hands-free electrodes.

An AED provides a pulse of electrical current intended to stimulate the heart to beat regularly. Typically, an AED is designed to analyze a person’s heartbeat automatically, determine if the person is in ventricular fibrillation, and administer the electrical pulse if required.

The operator of the AED must attach the electrodes of the AED to the chest of the person experiencing the problem and push the start button. The AED will do nothing if the person is not in ventricular defibrillation.

If the AED determines that the person is in ventricular fibrillation, the AED will instruct the operator to press the button to initiate the electrical pulse. Note that an AED is not designed to restart a heart that is not beating. Rather it is designed to restore a regular heartbeat when the heart is beating erratically.

These contain batteries capable of charging a capacitor to a high voltage. (much higher voltage than that of the battery) The stored energy is released through the heart by conducting electrodes, called paddles, that are placed on both sides of the victim’s chest.

In this case, capacitors serve as energy reservoirs that can be slowly charged and then discharged quickly to provide large amounts of energy in a short pulse.

When the AED delivers an electrical current, the capacitor is charged from a battery contained in the AED. The capacitor is then discharged through the person to stimulate the heart to beat in a regular manner. Most AEDs can deliver the electrical current many times without recharging the battery.

camera’s flash unit also uses a capacitor

A camera’s flash unit also uses a capacitor, although the total amount of energy stored is much less than that stored in a defibrillator. After the flash unit’s capacitor is charged, tripping the camera’s shutter causes the stored energy to be sent through a special lightbulb that briefly illuminates the subject being photographed.

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