Automated external defibrillator

An automated external defibrillator (AED) is a portable electronic device that diagnoses and treats cardiac arrest by reestablishing an effective heart rhythm. This treatment is called defibrillation, which applies an electric shock to the entire heart muscle, uniformly clearing the electrical activity of the heart, hopefully allowing it to resynchronize.

The use of AEDs is taught in many basic life support (BLS) classes.

When an AED is indicated

An automated external defibrillator is used in cases of cardiac arrest. However, the device will not shock a victim in asystole ('flatline') where the heart has no electrical activity, as it is ineffective in these cases. An AED is designed to shock a victim experiencing:

Ventricular fibrillation (VF)
Ventricular tachycardia (VT)

In cardiac arrhythmia, the heart is beating, yet in an unusual pattern, which can be life-threatening if left uncorrected. In ventricular fibrillation, the electrical activity of the heart becomes chaotic preventing the ventricle from effectively pumping blood. In ventricular tachycardia, the heart beats too fast to effectively pump blood. Frequently, ventricular tachycardia leads to ventricular fibrillation.

Uncorrected, these cardiac conditions rapidly lead to irreversible brain damage and death. For every minute that a person in cardiac arrest goes without being defibrillated, the chance of survival decreases by 10 percent.

Unlike some more sophisticated defibrillators used by health professionals, if the heart rate is too slow (bradycardia), these devices do not typically pace the heart to make it beat faster.

Where AEDs are found

Automated external defibrillators can be found in corporate and governmental offices, shopping centers, airports, restaurants, hotels, sports stadiums, schools and universities, community centers, and other places where large groups of people gather and the risk of a sudden cardiac arrest incident is likely. In some cities, all police vehicles carry an AED. In order to make them highly visible, public AEDs often are bright red, green, or yellow in color, and are mounted in protective cases near the entrance of a building. When these protective cases are opened, and the defibrillator removed, most will sound a buzzer to alert nearby Staff as to their removal. Most of these alarms do not summon emergency services, so emergency services should be called as soon as an AED is sent for or used.

Typically, an AED kit will contain a face shield, for providing a barrier between patient and first aider during rescue breathing; a pair of Nitrile_rubber non-allergenic gloves; a pair of Tuff Cut scissors, for cutting through a patients clothing to expose the chest; a small towel for wiping away any moisture on the chest and a razor for shaving those with very hairy chests.

AEDs can also be kept and used in the home, particularly important for those with existing heart conditions. The number of devices in the community will continue to grow as more and more citizens begin to understand their importance in providing first aid.

Increasingly, many Ambulances are carrying AED or AED-Capable Defibrilators to allow Basic Life Support personnel such as First Responders and EMT-Bs/IVs to give electrical therapy to patients when the providers aren't trained in EKG administration or rhythm analysis. Some states, such as Tennessee and many others, are beginning to require AEDs to be carried by basic life support ambulances and first response trucks. Newer, ruggidized AEDs from makers like Defibtech, Cardiac Science, Phillips, Zoll and Physio-Control cater specifically to this market.

How an AED works

An AED is called external because the operator applies the electrode pads to the bare chest of the victim, unlike internal defibrillators, which have electrodes surgically implanted inside the body of a patient.

Once the pads are attached to the patient, the AED diagnoses the heart rhythm and determines if a shock is needed to treat fibrillation. If the device determines that a shock is necessary, it will charge in preparation to deliver the shock. When charged, the device instructs the user to ensure no one is touching the victim and then to press a button to deliver the shock. After the shock is delivered, the device again monitors the heart rhythm of the victim to determine if another shock is necessary.

In some cases, after an AED has been used, its 'event memory' is downloaded and analyzed by the installing organisation. This memory stores the ECG of the patient and carries details of the time the unit was activated and the number and strength of any shocks delivered. From the ECG, it is possible to see the effectiveness of both CPR and defibrillation on the patients heart. A successful defibrillation, in the UK, is one that causes VF to be converted to another rhythm, even if this rhythm is unshockable. In other cases, such as in older AEDs or in models which do not support cards, rhythm strips are printed which contain the shocked rhythms, a summary of the code, and the status of the machine.

There are two main types of AEDs on the market today: semi-automatic and fully-automatic. Semi-automatic AEDs prompt the user to stand clear and then to push a shock button to defibrillate. Fully-automatic units sound a stand clear voice prompt and then deliver the shock automatically without the user having to push a button.

In recent years, two sub-types of AEDs have become available. Monophasic and Biphasic technologies. Older defibrilators used a monophasic electrical waveform, which utilized a high joule energy, up to 360 to 400, depending on the model. This caused increased cardiac injury and in some cases second and third degree burns around the shock pad sites. Newer AEDs (Manufactured after late 2003) feature what is known as a Biphasic waveform. These are designed to utilize a lower-energy shock of 120 - 200 joules, and shock twice insted of once per cycle, once from the chest pad, and once from the rib or back pad. This lower energy waveform has proven more effective in clinical testing, as well offers a reduced rate of complications and reduced recovery time.

Simplicity of AED use

Unlike regular defibrillators, an automated external defibrillator requires very little training to use. It automatically diagnoses the heart rhythm and determines if a shock is needed. Automatic models will administer the shock without the user's command. Semi-automatic models will tell the user that a shock is needed, but the user must tell the machine to do so, usually by pressing a button. In most circumstances, the user cannot override a "no shock" advisory by an AED. Some AEDs can even be used on children (those under 55 lbs kg in weight, or under age 8). If a particular model of AED is approved for pediatric use, all that is required is the use of more appropriate pads. Some organizations, such as the American Heart Association, recommends that if pediatric AED pads are not available, adult pads should be used to determine if the child is in a shockable rhythm. There is insufficent evidence to suggest that a child, in a shockable cardiac arrest, can be "hurt" by an adult defibrillation energy setting.

All AEDs approved for use in the United States use a synthesized voice to prompt users through each step. Because the user of an AED may be deaf or hard of hearing, many AEDs now include a screen to provide visual prompts. Most units today are designed for use by non-medical operators. Their ease of use has given rise to the notion of public access defibrillation (PAD), which experts agree has the potential to be the single greatest advance in the treatment of out-of-hospital cardiac arrest since the invention of CPR ^*.

Liability

Most health professionals agree that automated external defibrillators are so easy to use that most, if not all, states in the United States now include the "good faith" use of an AED by any person under the Good Samaritan laws. "Good faith" protection under a Good Samaritan law means that a first aid responder cannot be held civilly liable for the harm or death of a victim by providing improper or inadequate care, given that the harm or death was not intentional (and other certain circumstances). In most states, Good Samaritan laws provide protection for the use of AEDs by trained and untrained responders, to a point. Even for trained providers, AEDs provide little liability if used correctly, in fact even the NREMT-B and many state EMT and almost all Basic Life Support/Professional CPR classes offered incorporate AED education as parts of their program.

External links

Cardiac electrophysiology | Prehospital care