Biphasic Defibrillation: The Shape of Resuscitation Today

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.

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.

Defibrillation current has two components. The highest part of the waveform, the peak current, is a key determinant of successful defibrillation. There must be enough current to reach the heart to defibrillate (terminate the lethal rhythm), but not so much peak current that the heart is damaged.

Current delivery is determined by the energy level selected and by the level of patient impedance. Impedance is the body's resistance to the flow of current. At any energy level, current delivery decreases as patient impedance increases. If impedance is high, the heart may not receive enough current for defibrillation to be successful.

Biphasic Defibrillation

Unlike conventional monophasic defibrillators, biphasic defibrillators deliver current in two directions. In the first phase, the current moves from one paddle to the other as with monophasic defibrillators. During the second phase, the current flow reverses direction. The underlying physiologic mechanisms aren't fully understood yet, but it is clear that biphasic waveforms lower the electrical threshold for successful defibrillation. The shape of the current's delivery is determined by the current, direction of energy and duration of the delivered energy. Animal research has demonstrated that the most effective waveforms maintain their shape and duration regardless of patient impedance. Biphasic waveforms adjust for impedance by varying the characteristics of the waveforms. This is intended to ensure that high-impedance persons will have the same chance for survival as those who are of low impedance.

Biphasic defibrillation offers equal or better efficacy at lower energies than traditional monophasic waveform defibrillators, with less risk of post-shock complications such as myocardial dysfunction and skin burns. This is why almost all manufacturers of external defibrillators are now using biphasic waveforms in their devices.4 The precise waveforms used in biphasic shocks varies between models.5

Biphasic Waveforms

The manufacturers of several other devices, such as AEDs, use a variety of methods of fixed or escalating energy levels. There is some evidence from animal studies that these higher biphasic energies may be more effective than lower energies if transthoracic impedance is high, but this requires confirmation in human clinical studies. Overall, several studies in animals and humans have shown that defibrillators using biphasic waveforms are more effective for terminating ventricular fibrillation (VF) than those using monophasic waveforms.6

Defibrillator manufacturers can use lower-energy biphasic waveforms to achieve a number of technical advantages. Lower-energy devices can be smaller, lighter, less expensive and less demanding of batteries, with fewer maintenance requirements. These technical advantages help fulfill the need for lighter defibrillators with additional capability for patient monitoring.

Clinicians must be alert to the possibility of confusion caused by the fact that some biphasic defibrillators are designed to deliver shocks with varying energy levels. This potential for confusion is compounded because at present there is no "standard energy sequence" that can be applied to all defibrillators that use biphasic waveforms; the energy levels recommended by the various manufacturers are different. Therefore, the 200 J, 300 J, 360 J sequence of shocks recommended by the American Heart Association (AHA) for use with monophasic defibrillators is not appropriate as a generic approach for all biphasic devices.7 The equivalent biphasic dose, as recommended by the AHA, has not yet been determined.

Two of these devices commonly used in prehospital care use a biphasic truncated exponential waveform and deliver shocks with an escalating energy level to 200 J (Philips, Andover, MA) or an escalating energy level (Medtronic Physio-Control, Redmond, WA).8 The other uses a rectilinear biphasic waveform and, when treating VF, the manufacturer recommends shock delivery with escalating energy levels of 120 J, 150 J and 200 J (Zoll Medical, Burlington, MA).9

EMS providers who use new biphasic defibrillators in the field should use the energy levels indicated in the relevant manufacturer's instructions and local protocols.

Reducing Myocardial Damage in Defibrillation

The potential side effects of defibrillation include cell membrane damage, postshock dysrhythmias and postdefibrillation ST-segment depression on the electrocardiogram. Skin burns can occur after frequent defibrillation or cardioversion, causing scarring in some cases. Some evidence indicates that the risk of skin burns is reduced with low-energy biphasic defibrillation; however, recent studies show no statistical difference between 200 J and 300 J biphasic shocks for the conversion of atrial fibrillation.10

Any of these side effects is minor compared to the benefit of a life saved. The risk of side effects is expected to be less with biphasic waveforms because they achieve defibrillation with less energy. One experimental study that compared low-energy biphasic and high-energy monophasic protocols found that left ventricular ejection fraction and mean arterial pressure returned to baseline more quickly when low-energy biphasic shocks were used. In fact, it took up to 72 hours for ejection fractions to return to preshock levels following high-energy shocks. Other studies refute the potential for clinically significant damage at energy levels below 360 J.11 More studies are needed to determine if the biphasic device is as effective as the monophasic device and has fewer adverse effects.

Increasing Efficacy and Reducing Post-Resuscitation Complications

As demonstrated in the figures, a wave of electrical current has a shape that can be drawn as a "waveform," showing how the flow of current changes over time during the defibrillation shock. The highest part of the current waveform is called "peak current." Defibrillation requires a true middle-of-the-road approach. There must be enough current to reach the heart to defibrillate (stop the lethal rhythm), but not so much peak current (remember, the peak is one part of the current waveform) that you risk damaging the heart.

More current may be delivered by increasing the energy selected on the defibrillator.

Biphasic waveforms adjust for impedance by varying the characteristics of their waveforms, and are intended to ensure that high-impedance persons will have the same chance for survival as those who are of low impedance.

Clinical studies demonstrate the success of low-energy biphasic waveforms, but they are reliant on many factors that affect the chance of defibrillation success: time elapsed before the first shock is given, placement of electrode pads, the person's impedance level and certain health conditions. Therefore, it may take more current, a longer shock duration and/or increased energy to ensure success.

Clinical Efficacy of Biphasic Defibrillation for EMS

The objective in defibrillation is to achieve the highest efficacy with the lowest energy and current. Biphasic waveforms could improve therapy and affect outcomes when compared to monophasic waveforms.

Although the latest research shows biphasic defibrillation to be more effective than monophasic, international guidelines state that the care given using monophasic devices is neither unsafe nor ineffective. Significant research into the safety, efficacy and success in terminating episodes of VF has been completed since the release of these guidelines. Biphasic waveforms are now becoming more popular for use in external defibrillators, due in part to these studies demonstrating improvements in patient care.

Biphasic defibrillators are now available in many EMS systems throughout North America. Correlation of studies performed in-hospital may be difficult in out-of-hospital SCA. The out-of-hospital scenario is significantly different from the in-hospital time to delivery of the first shock. Out-of-hospital VF is most often due to myocardial ischemia, frequently in the absence of CPR, within a rapidly developing hypoxia and acidosis.

Resuscitations are dynamic events with multiple rescuers and interventions and intermediate outcomes. Defibrillation has several outcomes: persistent VF or conversion to a perfusing rhythm, asystole or pulseless electrical activity. Furthermore, patients who have had defibrillation may later refibrillate and require more shocks. Shock delivery and outcome occur in combination with many other interventions, such as CPR and the arrival of ACLS personnel who provide endotracheal intubation and intravenous medications. Clinical treatment data are difficult to correlate with data recorded by the event documentation components of the biphasic defibrillator.

Challenges for EMS

Biphasic defibrillation waveforms increase the rate of successful conversion of ventricular fibrillation, reduce the myocardium's exposure to high peak current and have the potential to improve outcomes. Emerging data show that postshock dysfunction, cellular injury, transmembrane effects, recovery time and skin effects are reduced and outcomes improved with clinically relevant defibrillation energies.11 The challenge remains in translating these enhancements to patient care into clear improvements in patient outcomes following out-of-hospital SCA.

Several factors will optimize the application of biphasic defibrillators in EMS systems. Great training and patient preparation for defibrillation will improve outcomes in the use of this tool. The EMS system can minimize the time from accessing the patient to delivery of the first shock with training and review of the defibrillator, easy access to all needed equipment and supplies (including a razor and electrodes), preconnection of the multifunction pads, if available, and rapid patient assessment.12


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