by Chris Woodford. Last updated: March 23, 2013.
Charging.... five hundred... clear! We've all watched it happen on ER dozens and dozens of times, but it's still relatively uncommon to see the same thing in real life. When someone collapses with a cardiac arrest, their heart can go into a kind of quivering limbo called fibrillation. One of the most effective ways to get it going again is with a sudden powerful electric shock—and that's the life-saving job a defibrillator does. Equipment like this used to require hours of training, but now portable, fully automatic defibrillators are being installed in public places so even amateur first-aiders can administer them with minimal instruction. Let's take a closer look at how these handy little gadgets work!
Photo: A portable external defibrillator packed away in an ambulance. Photo by Charles Larkin Sr courtesy of US Air Force and Defense Imagery.
What happens in a cardiac arrest?
You might not think of yourself as having lots of muscles, but there's one super-powerful muscle in your body you absolutely depend on: the tireless blood pump in your heart. If your heart stops beating properly and blood stops flowing, your brain starts to lose its oxygen supply and you can die within five minutes. That's why people who suffer cardiac arrest (when their heart stops or goes into a dangerously abnormal rhythm) need emergency medical treatment. CPR (cardiopulmonary resuscitation) can help maintain the flow of oxygen to the brain, but getting the heart restarted and working normally often requires defibrillation with an electric shock.
Photo: Conventional CPR (heart massage and artificial respiration) can be invaluable, but may not be enough to save a patient without defibrillation. Photo by Suzanne M. Day courtesy of US Air Force and Defense Imagery.
What is a defibrillator?
As the name suggests, defibrillation stops fibrillation, the useless trembling that a person's heart muscles can adopt during a cardiac arrest. Simply speaking, a defibrillator works by using a high-voltage (something like 200–1000 volts) to pass an electric current through the heart so it's shocked into working normally again. The patient's heart receives roughly 300 joules of electrical energy (about as much as a 100 watt incandescent lamp uses in three seconds).
The kind of defibrillator you see on TV consists of an electric supply unit and two metal electrodes called paddles that are pressed very firmly to the patient's chest using insulating plastic handles (so the person using them doesn't get a shock too). The important thing is getting the current to flow through the heart, so where the paddles are applied is crucial. One way of applying them is to put one paddle above and to the left of the heart and the other slightly beneath and to the right; another method involves placing one paddle on the front of the body and the other round the back. In order for the electric current to flow properly, and to reduce the risk of skin burns, the electrodes have to be applied close enough together. They must also make good electrical contact with the skin, so a solid or liquid conducting gel is usually applied to the patient's chest first.
In units designed to be used by less-trained people in public places, sticky, self-adhesive electrode pads are often used instead of paddles for safety and simplicity: once the pads are stuck on, the operator can stand well clear of the patient's body and that reduces the risk of their getting an electric shock.
Photo: Applying the charge: Left: Conventional paddles on a manual defibrillator. Photo by Christopher Hubenthal, US Air Force. Right: Self-adhesive electrodes (with printed graphics showing you where to stick them to the patient's body) on an automated defibrillator. Photo by William Greer, US Army. Both photos courtesy of Defense Imagery.
Types of defibrillators
The units you're likely to see in railroad stations and other public areas are called automated external defibrillators (AEDs) and they're designed to be used with little or no training. They have self-adhesive electrode pads and a built-in computer that automatically analyzes the patient's heart rhythm to figure out whether a shock will help them (defibrillation doesn't work if the heart has stopped beating altogether) and, if so, what level of shock is appropriate.
The units you see in TV hospitals and ambulances that feature handheld paddles and gel applied to the skin are usually manual external defibrillators. With these devices, the doctors, nurses, or paramedics have to figure out themselves whether defibrillation will help and also what shock voltage or energy level to use. Semi-automated defibrillators can work either automatically or in a manual override mode if the doctor prefers.
Patients who suffer regular problems with their heart rhythm sometimes have an internal defibrillator permanently implanted into their chest (a bit like a pacemaker) or worn on the surface of their skin under their clothes. Units like this constantly monitor the heart rhythm and deliver a shock whenever it's needed.