How can something so small be so effective?
How can something so small be so effective?
AED shock technology improves
About eight inches by ten inches in size, automated external defibrillators (AED) are portable computerized devices that read and analyze a heartbeat. They tend to be brightly colored and require little maintenance. Most feature an "eye" that tells you whether the battery is charged.
The device is user-friendly and walks the operator step-by-step through defibrillation. The operator puts the patches on the patient, pushes the button, and the machine does the rest.
The AED senses ventricular fibrillation, assesses whether an electrical jolt is appropriate, determines the level of shock, and delivers the shock through the chest wall to the heart. Moments later, it reanalyzes the patient’s condition and re-administers electrical current, if appropriate.
Typically, it delivers multiple short intense pulses in the standard stair step 200-300-360 joule sequence. Before it administers a shock, the AED audibly warns bystanders. If the patient remains in arrest, the machine can be restarted.
Catching up with ICDs
The technology used in implantable cardioverter-defibrillators (ICD) progressed far beyond that used in transthoracic defibrillation until about a year ago. When a patient receives a shock from a defibrillator, energy is delivered in a controlled time pattern or waveform. Waveforms determine the amount of voltage received and the time over which it’s delivered. Too little is ineffective; too much can damage the heart. An optimal wave is adjusted to the patient’s needs and chest impedance.
The size, weight, and cost of the AED are related to its defibrillation waveform shape. Older, larger models delivered shocks through the use of damped sine monophasic waveforms in which the current flows in one direction from one electrode pad through the body to the other electrode pad. (See comparison of defibrillation methods, at left.) ICDs utilize truncated biphasic waveforms in which the direction of the current is reversed midway through the pulse. The biphasic waveform requires less energy than the monophasic waveform to achieve comparable defibrillation efficacy. A year ago that technology was found to work as well for transthoracic defibrillation.1
Buying a unit
Early AEDs were bulky, expensive, maintenance intensive, and required some skill in the part of the operator. Breakthroughs in technology have addressed most of those barriers to general acceptance, and today, most units tend to have the following characteristics:
• Maintenance free.
Lithium-based battery systems are reliable, compact, powerful, and long-lasting. Units self-test and have status indicator features.
• User-friendly.
Because AEDs are used infrequently, they have to be intuitive and make use of audible prompting and icons to provide instructions to operators. Less training is required, and safeguards are built in.
• Portable.
AEDs have to fit into cramped spaces and be light enough to be carried easily. Some weigh as little as four pounds and are about the size of a hardcover book.
• Durable.
They remain dependable in an abusive environment.
• Affordable.
Units cost between $3,000 and $4,000. (For list of manufacturers, see box, p. 74.)
Cardiac arrest kills an estimated 350,000 people each year.
Reference
1. Bardy GH, Marchlinski FE, Sharma AD, et al. Multicenter comparison of truncated biphasic shocks and standard damped sine wave monophasic shocks for transthoracic ventricular defibrillation. Circulation 1996; 94:2,507-2,514.
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