Defibrillators, Portable Health Article

Definition

A portable defibrillator is a device, often automated and generally weighing less than 5 lbs (2.2 kilograms), that is commonly used in non-hospital settings to administer a shock and re-establish a regular heartbeat 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.