Automatic External Defibrillators - How They Work

As technology has advanced and prices have dropped, the automatic external defibrillator has become a common site in shopping malls, convention centers, and other public facilities. Used properly, these units can treat irregular heart rhythms in the crucial moments before medical help can arrive to a scene.

An automatic external defibrillator (AED) works by correcting flawed rhythms in the heart. Unlike an internal defibrillator, the unit does not have to be implanted. Instead, it uses external electrodes to acquire electrocardiogram data for analysis. Rhythm checks are made in two to four second intervals, and irregular patterns must generally be present for three consecutive checks. If an eligible rhythm is detected, the unit is prepared for a shock by charging the capacitor from a battery in the unit.

Depending on the type of defibrillator, the operator may be asked to push a button to shock the patient. If the AED is fully automatic, it will give a warning to stand clear, and no further action is required from the operator to administer the shock. In semi-automatic units, the operator has the option to disregard the unit's advice and shock or refrain from shocking as he or she chooses. After the shock, either type of unit will continue to monitor the patient and determine if additional shocks are needed.

The charge that is delivered also depends on the type of unit. Recent units offer biphasic defibrillation, which is generally accepted as less dangerous than monophasic methods. Instead of delivering the entire charge in one direction, it switches direction halfway, allowing it to deliver less of the damaging current with the same amount of energy. In most cases, the biphasic unit is more effective at the same energy levels.

While automatic external defibrillators are very easy to use, they do have limitations. For an AED to be effective, the patient must be still and lack a pulse. Attempts to use the device on a patient that is seizing or otherwise infirmed could be a dangerous waste of time. If the patient is wet, any moisture should be dried before attaching electrodes. Failure to do so could burn the patient or cause the defibrillation to be unsuccessful.

CPR is also a tricky issue when it comes to using an AED. While CPR can be an important part of the resuscitation, it should not be performed while the unit is analyzing the rhythm. Many new units have a feature to detect when CPR is being performed.

Despite the evidence that AEDs can be effective in saving lives, the biggest obstacle to their success may be poor maintenance. Both batteries and electrodes should be monitored regularly to ensure proper working condition.