Defibrillators have been an essential part of out-of-hospital care since Pantridge showed that defibrillation could be done in the field on the streets of Belfast in 1965. Early defibrillation is the most important factor in surviving an out-of-hospital cardiac arrest. To meet this need, defibrillators have become smaller, less expensive, and easier to use. Many BLS services carry automated external defibrillators (AEDs) (Fig 2-1). These devices are more precisely called shock advisory defibrillators because they analyze the patient's rhythm, determine whether a defibrillatory shock is indicated, charge the capacitors, and then inform the operator that a shock is advised. The operator can press the appropriate button to deliver the defibrillatory shock. The arrhythmia recognition algorithm of the AED is designed to advise defibrillation only for ventricular fibrillation and very fast wide QRS complex tachycardias (usually over 180 beats per minute). Rhythm monitoring and defibrillation are performed with combination pads, one placed over the cardiac apex and the other placed over the left paraspinal area at approximately the T6 level. Automated external defibrillators are designed to be used only on pulseless and apneic patients. Because AEDs have become so easy to use and function almost always as designed, a debate is ongoing whether these devices should be available for public access defibrillation by laypersons. While layperson defibrillation is beyond the scope of this discussion, there is developing evidence that rapid first-response defibrillation before the ambulance arrives, (e.g., by law enforcement officers using an AED) results in improved survival rates.5

Automated external defibrillators are priced at about $2500 to $3500 for a basic version without a rhythm monitor display, $5000 to $7000 for versions with monitor display screens, and $9000 to $10,000 for versions with full-function defibrillation, synchronized cardioversion, and external pacing capabilities. The AEDs without rhythm display monitors may actually function better because a displayed rhythm may only serve to distract a non-ALS operator. The event-recording capabilities of AEDs facilitate later review of the cardiac arrest for medical oversight and quality assurance reasons. The medical director must be involved in the selection of these devices, the training of EMS personnel, and subsequent review of their use.

Defibrillators used by ALS personnel are more sophisticated, with monitoring screens, printout units, manual defibrillation ability, and synchronized cardioversion capability. Defibrillation is often done with combination pads (as with the AED) rather than with paddles. These pads provide better contact with the skin, resulting in decreased resistance and allowing for more current to be delivered with a higher success rate for conversion. Such pads are also safer for the operator, who does not have direct contact with the patient when the shock is delivered. The monitor screen is used by the paramedics for initial interpretation of rhythms, ongoing monitoring of patients' rhythms, synchronizing a countershock for rhythms other than ventricular fibrillation, and pacing bradycardiac rhythms. The ALS defibrillators will soon be equipped with the technology to monitor blood pressures, pulse oximetry, and end tidal CO 2. The ALS personnel will use these machines for monitoring very ill patients from emergent calls or interfacility transfers.

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