Sudden cardiac arrest (SCA) causes thousands of deaths every year. Ventricular fibrillation (VF) is the presenting rhythm of SCA in many situations. As each minute passes, the chances of survival for a person suffering SCA drop by 10%. Even the best cardiopulmonary resuscitation cannot reverse this deadly heart rhythm. The only effective method of treatment is to deliver electric shocks using a defibrillator. Although the first commercial defibrillator used a biphasic waveform for the treatment of ventricular fibrillation, commercial external defibrillators in the Western world adopted monophasic waveforms more than 30 years ago, and these have been used almost exclusively until recently. Thus, much of our clinical experience comes from the use of monophasic waveforms. Since the introduction of the first biphasic external defibrillator in 1996, there has been a growing acceptance that this technology offers an opportunity to increase the success of the defibrillation process.
Conventional defibrillators produce monophasic shocks where the current flows in one direction. Biphasic waveform technology developed from electrophysiological work on the design of implantable defibrillators. With biphasic shocks, the direction of current flow is reversed at some point near the halfway point of the electrical defibrillation cycle during the discharge from the defibrillator. External defibrillators that use biphasic waveforms are available for EMS applications, and the number of biphasic waveform technologies continues to increase.
These devices offer a number of advantages. Low-energy biphasic shocks may be as effective as higher-energy monophasic shocks, but not in all situations.1 Evidence indicates that biphasic waveform shocks of 200 joules or less are safe and have equivalent or higher efficacy than damped sinusoidal waveform shocks of 200 J or 350 J. Recent atrial fibrillation (AF) studies indicate that 200 J biphasic energies are as effective as 360 J monophasic shocks. However, the emerging trend indicates that energies above 200 J may be required to increase effectiveness over monophasic.2 This may result in less damage to the myocardium and a reduced frequency of postshock contractility and dysrhythmias.
Evolution of Defibrillation
The common use of defibrillation technology to treat ventricular fibrillation or ventricular tachycardia (VT) is a relatively new phenomenon, having been developed only 50 years ago. Over many years of study, the theory of impedance and timing of shocks using monophasic defibrillation resulted in the common practice of initially delivering three "stacked" shocks. The key has been the sequential raising of the level of energy as measured in joules (J) from 200 J to 300 J to a maximum of 360 J, then subsequent shocks at 360 J with the standard dampened sine wave monophasic shock.
Biphasic waveform defibrillation was first used commercially in implantable cardioverter-defibrillators (ICDs) and automated external defibrillators (AEDs). The American Heart Association guidelines released in August 2000 list defibrillation of 200 J, 300 J or 360 J, or the equivalent, as a Class Ia recommendation.3 To understand how biphasic technology works, it is helpful to review the basics of traditional monophasic defibrillation.
Successful defibrillation depends on the defibrillator's ability to generate sufficient current flow through the heart. Defibrillators have long used a monophasic waveform, where current flows in one direction from one electrode to the other, stopping the heart momentarily, and allowing the basic sinus rhythm to be restored. A wave of electrical current has a shape that can be drawn as a "waveform." The waveform shows how the flow of current changes over time during the defibrillation shock. All traditional defibrillators use the same waveform technology, which is a monophasic, damped sine wave or monophasic truncated exponential waveform.