Automated External Defibrillators
Selection and Use
Aaron Rubin, MD
Department Editor: William O. Roberts, MD
THE PHYSICIAN AND SPORTSMEDICINE - VOL 28 - NO. 3 - MARCH 2000
Although sudden cardiac death is rare in sports, having an automated external defibrillator (AED) available facilitates early defibrillation and increases the chance of survival for an athlete in cardiac arrest. The decision to buy an AED involves many factors, and physicians and other personnel who work with athletes should be familiar with the use of such devices given their increasing availability and the widespread acceptance of the principle of early defibrillation.
In sudden cardiac arrest, the most frequent initial rhythm is ventricular fibrillation (VF). The only effective treatment for VF is electrical defibrillation, and the probability of success declines rapidly over time. As time passes from the initial event, the proportion of people in VF decreases and asystole, which is usually fatal, increases. Chances of resuscitation decrease 7% to 10% each minute (1).
Most neurologically intact survivors of sudden cardiac arrest had a ventricular tachyarrhythmia or VF treated by early defibrillation. Although AEDs are designed to recognize VF and defibrillate, most also detect and defibrillate monomorphic and polymorphic ventricular tachycardia if the rate exceeds a preset value.
Standard defibrillators are effective in terminating VF, but they are limited by their size, complexity, availability, and expense, and by the specialized training required for their appropriate use. AEDs provide a simpler, more available, less expensive means of defibrillation for patients suffering cardiac arrest. These devices are designed to be simple enough for those with minimal training to use. The hope is that AEDs will soon be easy enough for untrained individuals to use.
Two Easy-to-Use Types
AED operators need complete training in the use of this device and may receive this in American Heart Association cardiopulmonary resuscitation (CPR) training; however, general principles are outlined here. Two basic types of AEDs have been available: (1) "fully" automated defibrillators that, once activated, deliver shocks to patients as deemed necessary by the unit, and (2) "shock-advisory" AEDs that evaluate the patient's rhythm and advise the rescuer to push a button to deliver the shock. Those discussed in this article and currently most available are the shock-advisory types. Although the rescuer must recognize that the victim is unconscious and may be in cardiac arrest, both types of AED are easy to use. One simply needs to turn on either unit by pressing a well-labeled button. Both kinds use voice commands to talk the rescuer through proper procedure.
To function, either type of AED must be attached to the victim. Adherent electrode pads are typically attached over the right upper chest and on the left side of the chest in the midaxillary line left of the nipple. A diagram, usually located on the unit or the pads, illustrates the positioning. The pads are either preconnected to the defibrillator or come attached to a plug that the rescuer must connect. Once either type of unit is turned on, it assesses whether the pads are properly attached and prompts the rescuer to complete this step if he or she has not done so.
The AED then evaluates the patient's cardiac rhythm and determines if he or she is in VF. If the patient is in fibrillation, the fully automated AED warns bystanders to stand clear and delivers the shock; the shock-advisory AED prompts the rescuer to push a button to deliver the shock.
The shock-advisory type is theoretically safer because the rescuer can observe the victim and surroundings prior to delivery of the shock. Clinical experience, however, suggests that both devices are equally safe (2).
AEDs analyze multiple features of the electrocardiogram (ECG), including frequency, amplitude, and wave morphology, and then filter for 60-cycle interference, loose electrodes, poor electrode contact, and radio transmission. Some intermittent radio transmissions, patient movements, or contact with rescuers are interpreted by the AED as an unrecognizable rhythm, causing the unit to restart its evaluation process. AEDs evaluate multiple times to decrease these problems, but rescuers must be aware of possible interference and strive to minimize it.
Electrical current flow through the heart is what actually causes defibrillation. Current is measured in amperes (amps). The pressure pushing this current is called potential and is measured in volts. The resistance to this flow caused by the skin, muscle, and rest of the body is the impedance and is measured in ohms. One amp with a pressure of 1 volt is 1 watt of power. One watt produced in 1 second equals 1 joule (J), which is the energy set in the AED.
AEDS can differ in waveform: how the energy is provided to the patient. A monophasic-damped sine wave provides a high voltage and high peak current over a short time. A monophasic-truncated exponential wave responds to impedance by delivering the energy over more time. A biphasic-truncated exponential wave also delivers energy over time, but the energy is reversed at a predetermined time. New waveforms are designed to improve resuscitation probability while decreasing the energy and battery needs, thus making for a lighter and more effective AED. Full discussion of the benefits of the various waveforms is beyond the scope of this article.
Several companies market AEDs. Available units differ slightly in size, cost, technology, and use. The manufacturers and independent testers have extensively evaluated the technology differences, and no single company's AED has become a standard. The following four AEDs (table 1) are representative units. The first three are exclusively AEDs, and the fourth (Zoll's) is a fully functioning defibrillator with pacemaker capabilities.
Agilent Technologies, a subsidiary of Hewlett Packard, manufactures three versions of the Heartstream ForeRunner (3). The most elaborate version has an ECG display and manual override that allows the rescuer to shock based on the display. Other manufacturers have chosen not to incorporate the ECG display. The ForeRunner delivers 150 J in a biphasic waveform. It has voice prompts and can record, on an internal card, voice, rhythm, and shocks delivered.
Medtronic's Physio-Control manufactures the Lifepak 500 (4). It delivers a preprogrammed sequence of either 200, 200, 360, and 360 J or 200, 300, 360, and 360 J, in a monophasic waveform. The escalating monophasic shocks closely follow traditional advanced cardiac life support (ACLS) protocols. Lifepak 500 is also available as a biphasic model. The Lifepak also has voice prompts and recording capability. Models come in a two-button (on/off and shock) or three-button (on/off, analyze, and shock) version.
Survivalink Corp manufactures the FirstSave AED in biphasic and monophasic versions (5). The one-button operation provides monophasic shocks at 200, 200/300 and 360 J, and the biphasic at a variable rate from 140 to 360 J, depending on the impedance of the victim. As with the other units, features include voice prompts and recording capability.
Zoll Medical Corp takes a different approach with the Zoll 1700, which has programmable energy levels from 50 to 360 J (6). The unit can be converted from a fully functioning defibrillator to a pacemaker with the turn of a key.
Factors to Consider
The decision to buy an AED for a sports medicine practice involves many factors. The risk of young athletes suffering sudden death is low. The risk among spectators depends on their age, number, and individual risk factors. The availability of on-site ACLS services and defibrillators should be another consideration. Shared use may be an option at some facilities, with trained security personnel carrying the unit during the day and athletic trainers or team physician using it at high-attendance events.
Dr Rubin is director of the Kaiser Permanente Sports Medicine Fellowship in Fontana, California.