Defibrillators, Portable

Definition

A portable defibrillator is a device, often automated and generally weighing less than 5 lbs (2.2 kg), that is commonly used in non-hospital settings to administer a shock and re-establish a regular heart-beat to treat sudden cardiac arrest.

Purpose

Rapid defibrillation is the most significant factor in the survival of the abrupt disruption of the heart function known as sudden cardiac arrest (SCA). In non-hospital settings, the most common method of administering such a shock is by using a portable defibrillator, also known as an automated external defibrillator (AED). AEDs are now common equipment in airports, malls, casinos, golf courses, businesses, hotels, schools, on airplanes, and through various emergency response groups such as the police, fire departments, and paramedic squads.

Contrary to the perception portrayed by television and movies, defibrillation is not effective in treating hearts showing a flat-line electrocardiogram (ECG). Instead, defibrillation is most effective in reversing arrhythmias (abnormal rhythms of the heart) such as those that can cause SCA. This condition is often characterized by ventricular fibrillation, a rapid, nonproductive contraction of the ventricles. To treat this problem, the shock stops the chaotic electrical activity and allows the natural pacemaker of the heart, the sinoatrial node, to regain control of the beat.

SCA is the cause of death of more than 350,000 Americans per year, striking persons of all ages and in both sexes equally. Unlike a heart attack, SCA often occurs without warning symptoms. The American Heart Association estimates that nearly 300 persons per day would be saved if everyone who suffered from SCA had access to treatment with an AED within 10 minutes. AED models are now available for treatment of adults, children, and infants.

Because many AEDs include an ECG display, the device can also be used during cardiac emergencies to monitor and record the heart's electrical signals before, during, and after any given treatment. This is true even when the person suffers from a heart condition that does not warrant administering a shock, as determined by the device's automatic analysis of the heart problem.

Description

AED devices include an ECG to monitor the heartbeat of the patient, software and voice prompting to guide the operator, other software to analyze the advisability of administering a shock, and a shock generator to administer a shock of set duration and power. The device itself is enclosed in a case that includes a display, a speaker, leads running to two pad-shaped electrodes, and two buttons (power and shock). More sophisticated versions of this device can include manual over-rides (for control by trained medical personnel of the power and duration of the shock) and pacing abilities.

Generally, defibrillators are available that produce two types of shocks, monophasic and biphasic. Monophasic shocks move from one electrode to the other, while biphasic shocks move from one electrode to the other, and then reverse direction. Biphasic shocking is usually more effective than monophasic in taking a heart out of fibrillation and is associated with less post-treatment heart or brain dysfunction. Biphasic shocking, combined with pre-shock impedance measurements, is a method particularly successful in treating obese patients, a situation that can be a challenge to other types of defibrillation therapies.

The analysis of the heartbeat is done using an algorithm (a sequence of mathematical steps) that compares the electrical output of the patient's heart to known heartbeats to determine if a shock should be administered. The four characteristics of the output examined are rate, conduction, stability, and amplitude. Rate is measured in beats per minute (bpm), with normal being between 60 and 100. Increased rate is characteristic of many common arrhythmias. Conduction is evaluated by looking at the characteristics of the R wave of the ECG, which is the portion of the electrical signal of the heart where there is a tall, narrow spike. Rounded wide R waves can indicate problems with conduction.

Stability of the heartbeat is evaluated by comparing one heartbeat signal to the next. In healthy hearts, the beats repeat themselves with a regular pattern. Unhealthy hearts have varied beat signals, an indication of instability. Finally, the algorithm looks at the amplitude (height) of the electrical signal put out by the heart. Lowered amplitude is a characteristic of an unhealthy heart. Additionally, many AEDs have systems to filter out artifacts, electrical signals that do not come from the heart of the patient being evaluated and that may interfere with the evaluation process.

Operation

The operation of an AED is as straightforward as possible so a person can use the machine correctly even without training. When in automatic mode, the machine provides voice and display prompts to guide the operator through its use. Many models have only two buttons, a green power button and an orange or other brightly colored shock button.

The use of a defibrillator to treat SCA is one part of a four-step program known as the chain of survival. The four critical steps include early access to emergency care (by calling 911 or another emergency number), early cardiopulmonary resuscitation (CPR), early defibrillation, and early advanced cardiac support. Thus, before using an AED, the operator should have some-one call 911 and then begin CPR, if possible.

AEDs should only be used on individuals who are unresponsive, not breathing, and have no pulse. On activation, the machine asks the operator to confirm that these conditions are present. Next, the operator is directed to place the electrode pads on the collapsed person. The two pads are placed on the chest, one on the patient's upper right chest area, and the other below the ribs on the patient's left side. Often, there is a diagram on the machine to aid in electrode placement. At this point, the machine directs the operator to plug in the electrodes, guided by a light. Some machines skip this step, as the electrodes are permanently attached.

The machine then directs everyone to stand clear of the patient so that the heartbeat can be evaluated. If the condition can be treated with a shock, the operator is directed to both visually and verbally clear the area around the patient and then push the shock button. After the shock is delivered, the machine automatically checks the heartbeat for change and, if further shocks are needed, the machine directs their delivery.

Maintenance

Older AEDs require periodic maintenance and calibration by using external devices, sending the machine to the factory, or service contracts with outsource providers of maintenance. However, newer machines have internal maintenance programs that carry out the required routine performance checks and function adjustments. Many will check the ability of the machine's shock function on a daily basis. The internal maintenance functions, combined with heavy-duty, easy-to-carry external casings, facilitate the use of AEDs as emergency equipment in non-health care settings. Wall mountings such as those that are used for fire extinguishers are available to keep the equipment in sight but protected.

Health care team roles

AEDs are specifically designed to be used by people who have no medical training. Indeed, a study using AEDs showed that sixth grade students, without the benefit of training or ability to ask questions, took only 30 seconds longer to administer a shock than trained emergency technicians.

In a hospital setting, portable defibrillators are often available for use by hospital personnel in order to assure quick access to defibrillation equipment in intensive care units and other areas of the hospital where SCA might occur. The manual settings found on more sophisticated units can be used by health care providers such as doctors, nurses, paramedics, and emergency medical technicians who have training in advanced life support techniques. Their training allows them to identify specific abnormal rhythms that may require different energy settings for the most effective correction.

Training

When an AED program is implemented, for example, by a business, training for the employees is often accomplished using a four-hour course, with refreshers suggested every three months. Training programs are available from the company that manufactures the AED or from organizations such as the American Red Cross.

KEY TERMS

Algorithm— A sequence of mathematical steps used by an automated external defibrillator to evaluate whether a shock is appropriate treatment for the patient attached to the machine's electrodes.

Biphasic— A term to describe a defibrillation shock that travels from one electrode to another and back again.

Monophasic— A term to describe a defibrillation shock that travels from one electrode to the other.

Resources

BOOKS

Every, Nathan R., and W. Douglas Weaver. "AEDs: Automatic and Advisory Transchest Defibrillation." In Defibrillation of the Heart, ed. W.A. Tacker, Jr. St. Louis: Mosby, 1994.

PERIODICALS

Gundry, J.W., et al. "Comparison of Naive Sixth-Grade Children with Trained Professionals in the Use of an Automated External Defibrillator." Circulation 100 (October 19, 1999): 1703-07.

Page, Richard L., et al. "Use of Automated External Defibrillators by a U.S. Airline." The New England Journal of Medicine 343 (October 26, 2000): 1210-16.

Valenzuela, Terence D., et al. "Outcomes of Rapid Defibrillation by Security Officers after Cardiac Arrest in Casinos." The New England Journal of Medicine 343 (October 26, 2000): 1206-09.

ORGANIZATIONS

National Center for Early Defibrillation. University of Pittsburgh, 230 McKee Place, 4th Floor, Suite 911, Pittsburgh, PA 15213-4910. (866) AED-INFO. (412) 647-2694. 〈http://www.early-defib.org〉.