It's 1985 and Jon Meyers is in ventricular fibrillation (VF). You grab the defibrillator and administer three escalating shocks: 200 joules, 300 joules, and 360 joules. Mr. Meyers' rhythm finally converts to normal sinus rhythm and he recovers. The procedure saves his life but leaves burns on his chest.
Fast-forward to 2005. Mr. Meyers is in your unit recovering from his second myocardial infarction. He again experiences cardiac arrest from VF. You apply the defibrillator pads and charge the unit's new biphasic defibrillator. Moments later, after a single shock of 120 joules, Mr. Meyers' arrhythmia has converted to normal sinus rhythm. This time, his chest isn't burned.
Wave of change
Pioneered in the 1950s, defibrillation technology remained largely unchanged until a few years ago. Today, however, external biphasic defibrillation offers equal or better efficacy at lower energies than traditional monophasic waveform defibrillators—with less risk of postshock myocardial dysfunction and skin burns.
Based on a different type of waveform, biphasic defibrillation technology was first used in implantable cardioverter-defibrillators (ICDs) and automated external defibrillators (AEDs). It's now moved to hospital-based defibrillators. The American Heart Association (AHA) guidelines released in 2000 include biphasic defibrillation (at 200 joules or less) as an intervention of choice for advanced cardiac life support (see What the AHA guidelines say about biphasic defibrillation).
Hospitals have begun to replace older monophasic defibrillators with biphasic ones, but during the transition, health care providers must be aware of which type of defibrillator they're using. Initial research has found the two biphasic waveforms to be equally effective. To understand how biphasic technology works, you first need to understand the basics of traditional monophasic defibrillation.
All traditional defibrillators use the same waveform technology, a monophasic damped sine wave. These monophasic defibrillators deliver the shock in a single direction: Current flows from one paddle (or electrode) to the other. Without another effective clinical choice, clinicians shocked countless patients without concern for the adverse effects of high-energy defibrillation. But in the 1990s, researchers began to examine whether a low-energy biphasic technology might work as well for external defibrillation with fewer risks, as it did for ICDs.
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's clear that biphasic waveforms lower the electrical threshold for successful defibrillation.
Initially, research indicated that for short-duration VF, low-energy biphasic shocks are as effective as 200-joule monophasic shocks. But results from a large out-of-hospital trial on long-term VF showed that low-energy biphasic shocks were more effective than conventional high-energy shocks. Transthoracic defibrillation with biphasic shocks also causes fewer ST-segment changes than monophasic defibrillation.
Unlike monophasic devices, biphasic defibrillators use a different waveform technology: either a biphasic truncated exponential (BTE) waveform or a rectilinear biphasic waveform (see Comparing waveforms). Clinical protocols and energy requirements for biphasic defibrillation vary depending on the waveform type.
Biphasic shocks and complications
For the patient, the benefits of biphasic technology include less myocardial dysfunction after defibrillation and a lower risk of skin burns. 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.
Skin burns can occur after defibrillation or synchronized electrical cardioversion, causing scarring in some cases. Some evidence indicates that the risk of skin burns is reduced with low-energy biphasic defibrillation (see How electrode type affects skin burns and Common electrode placement schemes).
Two types of waveforms
Let's take a closer look at the two types of biphasic waveforms approved for use in conventional external defibrillators.
The rectilinear biphasic waveform was developed specifically for external defibrillation and takes into account high and varied patient impedance levels (such as the blocking of current flow caused by large lung volume, large chest size, and poor electrode-to-chest contact). The rectilinear waveform maintains a stable shape in response to impedance, and the constant current in the first phase reduces potentially harmful peak currents.
The BTE waveform was developed for internal use in ICDs, where impedance is low. When it's used in a transthoracic device such as a defibrillator, impedance affects the waveform's shape. Research has shown that as the biphasic waveform's shape changes, its efficacy varies. The rectilinear waveform remains stable in shape, however, and current delivery dynamics are similar for patients over a wide range of impedances. This reduces the potentially adverse effect of patient impedance on successful defibrillation.
Successful defibrillation depends on the defibrillator's ability to generate sufficient current flow through the heart. Defibrillation current has two components: Average current delivery is a key determinant of successful defibrillation; high peak currents are associated with myocardial injury.
Current delivery is determined by the energy level you select and by the level of patient impedance. Patient impedance is the factor most responsible for blocking the flow of adequate current. At any energy level, current delivery decreases as patient impedance increases.
How do the different types of biphasic waveforms respond to patient impedance? When impedance is low (50 ohms), a BTE biphasic defibrillator set at 360 joules delivers more current than required, exposing the patient to potentially harmful high peak currents. At an average patient impedance of 75 ohms, the BTE defibrillator set at 360 joules and a rectilinear defibrillator set at 200 joules are equally effective. With high impedance (greater than 100 ohms), the 200-joule rectilinear shock delivers a higher average current than a 360-joule BTE shock, therefore making it more effective at lower energy levels.
A direct clinical comparison between the two types of biphasic waveforms has yet to be done in a prospective, randomized trial with appropriate controls. But the growing body of published, peer-reviewed human data points to some waveform-specific performance characteristics.
Higher energy doesn't necessarily mean you'll be raising the average current delivered. Researchers found that a high-energy BTE defibrillator needs nearly 50% more energy to deliver the same average current as a low-energy rectilinear defibrillator.
Many studies have found that biphasic waveforms outperform monophasic ones, and several studies comparing the two biphasic waveforms have found them to be equivalent in effectiveness.
What about children?
Monophasic waveforms have traditionally been used in pediatric defibrillation. The energy levels used, 2 to 4 joules/kg, are based on outcomes of a single study. Because sudden cardiac arrest is rare in children, the clinical studies of pediatric biphasic waveform use have been done in animals and suggest that the 2 to 4 joules/kg level is appropriate for children. (However, the optimum energy level hasn't been conclusively established.) The International Liaison Committee on Resuscitation has found AEDs safe and effective for children ages 1 to 8. Pediatric pads and cables should be used, if available, to deliver a dose to a child.
Which one's best?
Both types of biphasic waveforms are considered more effective at lower energy levels than monophasic defibrillation, and the AHA doesn't recommend one type of biphasic defibrillator over another or biphasic over monophasic defibrillation. When deciding which biphasic defibrillator to use, consider how easy the device is to use, whether it offers other clinical parameters (such as pacing, 12-lead electrocardiogram monitoring, and pulse oximetry monitoring), and if it can be easily upgraded should the technology change. Know the recommended energy settings for the defibrillator you'll be using and you'll be ready to intervene appropriately. By doing so, you may be able to help your patient avoid disability or death.
What the AHA guidelines say about biphasic defibrillation
At press time, the American Heart Association (AHA) was revising its guidelines, including those that would use biphasic defibrillation in emergency cardiac care. The new guidelines are expected to be released in November. At present, the AHA's Guidelines 2000 for Cardiopulmonary Resuscitation and Emergency Cardiovascular Care state that biphasic shocks of 200 joules or less are an “intervention of choice.”
In a nutshell, here's what the 2000 guidelines say about low-energy biphasic defibrillation:
* Biphasic defibrillation is now a part of advanced cardiac life support (ACLS) guidelines.
* Biphasic shocks at energy levels of 200 joules or less are at least as safe and effective as monophasic shocks with traditional energies between 200 and 360 joules.
* Biphasic shocks of 200 joules or less are now classified as a Class IIa recommendation (supported by good to very good evidence).
* Biphasic defibrillation protocols may vary depending on the specific biphasic waveform employed.
Two areas of the guidelines discuss biphasic shocks:
Automated external defibrillators: The data indicate that biphasic waveform shocks of relatively low energy (200 or fewer joules) are safe and have equivalent or higher efficacy for termination of ventricular fibrillation (VF) compared with higher-energy escalating monophasic waveform shocks.
Defibrillation: Research indicates that repeated biphasic shocks of 200 or fewer joules are as successful or more successful at terminating VF than escalating shocks, which increase the energy (from 200 to 360 joules) with successive shocks.
For example, the VF/pulseless ventricular tachycardia (VT) algorithm states that you'd defibrillate refractory VF or pulseless VT with either monophasic shocks at 200, 200 to 300, and 360 joules or biphasic shocks at energy levels documented to be clinically equivalent (or superior) to the monophasic shocks.
The standard ACLS protocol of escalating energies applies only to monophasic defibrillation. Energy recommendations for monophasic shocks can't be extrapolated for use with biphasic shocks. The guidelines don't present a protocol for biphasic defibrillation, noting that energy levels vary with the type of device and type of waveform. This suggests that protocols may need to vary depending on the device.
How electrode type affects skin burns
Many health care professionals consider skin burns to be an unavoidable adverse effect of electrical cardioversion or defibrillation. Recent improvements in the design of multifunction electrodes should change that.
Often alarming, the initial reddening that appears shortly after a shock is delivered results from blood pooling in the tissues. Although it may look like an early-stage burn, this redness often disappears within hours of the procedure.
A true skin burn resulting from cellular necrosis manifests in 24 to 48 hours postshock. Signs may include persistent redness, blistering, and scabbing.
Low-energy defibrillation may pose less of a risk for skin burns than high-energy defibrillation. Knowing the types of electrodes and when to use them can also help you reduce your patient's risk of skin burns.
Resuscitation electrodes are designed for various applications (including defibrillation, electrical cardioversion, and noninvasive external pacing) and environments. Clinical applications demand that electrodes be suitable for long- or short-term use and adult or pediatric patients, and be opaque or radiolucent. Whatever the demand or application, certain design characteristics minimize resulting skin burns. Let's look at three common designs.
Solid or wet get?
Solid gel defibrillation electrodes gained popularity because they're easy to apply and store and there's nothing to clean up. But the trade-off for convenience has been a rise in patient skin burns with elective electrical cardioversions. As a result, liquid gel electrodes are making a comeback, and advances in design and materials have largely eliminated their inconveniences.
One issue in the solid versus wet gel choice is lateral conductivity—the ability of the electrode to conduct electricity evenly. Solid gel electrodes tend to concentrate current in a narrow band at the perimeter of the conductive plate, increasing the risk of skin burns along the edge (“edge effect”). Some solid gel electrode designs attempt to alleviate this phenomenon by creating conductive plates with zones of varying conductivity. Although this approach is logical, it can also limit current flow.
Wet gel electrodes generally provide significantly better lateral conductivity. All other things being equal, these electrodes have a wider band of energy concentration near the perimeter of the electrode (see illustration A). This wider band of energy transmission reduces insult at any one point on the patient's skin.
The second difference between solid and wet gel electrodes is skin coupling—the ability of the gel to wet the skin surface. Effective, even coupling with the patient's skin reduces the frequency and incidence of skin burns. Solid gels have a tendency to sit on the skin's peaks, not fully wetting the skin surface. Wet gel flows into the valleys and maximizes coupling (see illustrations B and C).
Some clinicians believe that solid gels with strong adhesive characteristics produce a superior result for defibrillation, pacing, or monitoring. Solid gels with strong adhesive might give the appearance of good coupling but may be adhering to the peaks and not coupling with the entire skin surface. Wet gels, with their low viscosity, tend to wet the skin surface very quickly and provide good electrical coupling almost immediately.
Air pockets are another concern. With solid gel electrodes, small pockets of air can be trapped between the gel and the patient's skin when the electrodes are applied. Air pockets can also occur during patient repositioning or with obese patients. Air pockets cause current to concentrate around the circumference of the air bubble and increase the probability of skin damage in these areas. A wet gel electrode's low viscosity minimizes air pockets.
At Women and Infants' Hospital in Providence, R.I., Elaine Amato-Vealey is manager of surgical services and Patricia A. Colonies is critical care clinical educator.
The authors have disclosed that they have no significant relationship with or financial interest in any commercial companies that pertain to this educational activity.
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