Defibrillators
Last updated: November 6, 2009.
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
adminster 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). 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.
Further reading
