Transthoracic Biphasic Defibrillation: The Case for Non-Escalating Energy Protocols

The overwhelming clinical benefit of defibrillation as a treatment for ventricular fibrillation is so obvious that, in the past, any technological design differences among manufacturers and models has perhaps been assumed to be inconsequential. A substantial shift in the technology of more recently developed external defibrillators has led to speculation regarding the comparative clinical effectiveness of competing models and the questioning of the adequacy of some. Defibrillators whose electronics are based upon a biphasic waveform, as opposed to the monophasic waveforms that characterized earlier models, are rapidly becoming dominant in the marketplace. Clearly, differences exist in the electronics of biphasic defibrillators, depending upon the manufacturer, that may potentially have important clinical ramifications. Since few clinicians have familiarized themselves with defibrillator electronics, they may be unable to discern among the claims made by device representatives or to identify the probable strengths and weaknesses in the defibrillator that they ultimately acquire. In addition, the claims made by each manufacturer are not equally supported by the strength of orderly scientific inquiry: concept development, affirmation in a laboratory setting, and confirmation in a clinical setting. Also, unlike studies supporting pharmaceuticals and most other devices, randomized, controlled trials supporting the new defibrillator technology in the treatment of ventricular fibrillation are limited to a few manufacturers. Defibrillation may potentially occur if sufficient energy is transferred from the defibrillator to the myocardium to achieve depolarization of a critical mass. The energy transfer occurs via the flow of electrons (current) through the ventricular myocardium so that, ultimately, it is sufficient transmyocardial current that makes defibrillation possible. Transmyocardial current is affected by both transthoracic impedance and shunting. The former is a concept familiar to clinicians, in which transthoracic current is reduced as impedance rises, assuming a constant voltage. Shunting is simply the acknowledgment that most of the transthoracic current passes through non-cardiac structures as it travels between paddles or pads. Evidence from the laboratory setting¹ demonstrates that while increased transthoracic impedance decreases the transthoracic current, the proportion of that current delivered to the myocardium may increase substantially. This phenomenon occurs because the chest wall, as might be assumed, is the preponderant site of impedance. Increased transthoracic impedance occurs primarily in this non-myocardial pathway. Thus, as impedance increases, a higher proportion of transthoracic current flows transmyocardially. As a hypothetical illustration, if transthoracic impedance doubled, reducing transthoracic current but shunting decreased by 50%, the resultant transmyocardial current (and total energy delivered to the myocardium) would be unchanged. From this description of defibrillation, it is apparent that current is but one of three components of the energy equation (voltage and shock duration being the others) that makes defibrillation possible. This understanding stimulated research performed in the early 1980s that studied the plausibility of current-based, rather than energy-based, defibrillation. This was predicated on the ability to measure the transthoracic impedance immediately prior to countershock,² a concept employed in all of the newer defibrillator models (impedance compensation). Ultimately, the technological change in external (as well as internal) defibrillation that was widely adopted was a change in the waveform. Most defibrillators of the past employed a monophasic damped sinusoidal (MDS) waveform. This waveform design was established in the early 1960s, and was limited by the high-voltage electronics technology available at the time. The defibrillators themselves made no adjustment for differences in impedance, and the resultant transthoracic (and transmyocardial) current was simply a passive circuit function. A characteristic of the MDS waveform is its tendency to become degraded in the face of high impedance. The lack of defibrillator compensation for and the degradation of the waveform by higher impedance made a strong case for using higher energies if it was perceived that transthoracic impedance was high. ³ Despite this assumption, the only study ever randomly comparing two energy strategies using the same model defibrillator in both arms found no increase in defibrillation success at the higher initial energy. ⁴ The new defibrillator electronics use a biphasic truncated exponential (BTE) waveform. This waveform was initially applied to implantable defibrillators, in which setting it was found to have numerous advantages. Since the biphasic nature of the waveform achieved defibrillation successfully at lower energies, capacitors could be miniaturized. In addition, the shape of the waveform could be actively manipulated and customized to each individual patient s impedance characteristics. These same attributes have proven extremely useful in transthoracic defibrillators, especially automated external defibrillators (AED), allowing them to be lighter and more compact. One additional attribute of BTE waveforms that is very relevant to transthoracic defibrillation is, unlike MDS waveforms, they are not degraded in the face of higher impedance. This is one in a series of forthcoming arguments against the need for high energy (greater than 200 joules) to achieve the highest defibrillation success rates. Using the BTE waveform, impedance compensation can take place with some degree of precision. In the case of implantable defibrillators, the shape of the biphasic waveform could be customized to the individual patient in terms of the tilt (the rate of fall from peak current flow), the phase ratio (the percentage of total shock time allowed for each phase) and the total shock duration. Since transthoracic defibrillators will serve a number of individuals, nearly instantaneous customization of these three parameters, based upon measured transthoracic impedance, would be necessary to recreate the implantable defibrillator circumstance. This is, indeed, the intent of the impedance compensation technologies in the newer biphasic defibrillators. Elegant laboratory work has identified the best ranges for tilt, phase ratio and shock duration using a BTE waveform. ⁵ Based upon animal and clinical research, engineers from Heartstream developed the first commercially available impedance compensating biphasic defibrillator (in this case, an AED). The parameters for their proprietary waveform were selected to achieve efficacious transthoracic currents at lower than traditional energy levels, regardless of the transthoracic impedance encountered. The same electronics have been adopted for the entire line of defibrillators by Philips Medical Systems, the company that ultimately acquired the Heartstream technology. The particular electronics developed at Heartstream have subsequently undergone extensive clinical testing including the only large-scale (115 patients), randomized, controlled trial6 comparing a BTE model defibrillator to monophasic defibrillators in the treatment of out-of-hospital ventricular fibrillation. In that trial, a non-escalating energy strategy (150-150-150 joules) for repeated defibrillation attempts was used in the BTE arm while the traditional escalating strategy (200-200-360 joules) was used in the monophasic arm. Defibrillation success, return of spontaneous circulation and neurologic outcome among survivors all favored the BTE arm to a statistically significant degree. The Guidelines 2000 for Cardiopulmonary Resuscitation and Emergency Cardiac Care acknowledge the superior clinical evidence supporting this energy strategy using the Heartstream electronics by declaring its use a Class IIa intervention (always acceptable, a standard of care).⁷ This same group has found insufficient scientific evidence to support any recommendation for any other defibrillator technology or energy strategy, old or new. While other manufacturers use a BTE waveform and employ some form of impedance compensation, the waveforms and nature of impedance compensation are substantially different from that developed at Heartstream. Evidence exists that each of the newer defibrillators using BTE waveforms are superior to MDS technology using defibrillation success rates as a measure. However, there is currently no evidence demonstrating the superiority of one technology over another. Even with knowledge of all the foregoing, clinicians can be misled when they seek to acquire defibrillators or use them clinically. Misunderstanding exists regarding the need for high-energy options and circumstances where employing high energy as a defibrillation strategy would be sensible. Note that with earlier technology, clinicians were prompted to use increasing energy discharges in the event of unsuccessful defibrillation attempts. Because the duration of ventricular fibrillation is the strongest predictor of outcome, many clinicians concluded that the highest energy discharge available should be used for the first attempt at defibrillation. Since this high-energy culture has become so prevalent, non-escalating, low-energy recommendations for ventricular fibrillation appear even more at odds with conventional wisdom. Five arguments support the decision made by Heartstream and are upheld by Philips to recommend a non-escalating, low-energy strategy for ventricular defibrillation using their devices. The first has already been stated. High energy is necessary for MDS waveforms, since the waveform shape degrades in the face of high impedance. A second argument relates to the shape of the biphasic waveform. The particular features of the Heartstream waveform provide transthoracic currents at low-energy levels that can only be equaled by other manufacturers at higher energy levels. Again, we are reminded that current, not energy, is the agent of defibrillation. The third argument relates to probability. Both protocol and practice lead to the use of increased energy following an unsuccessful defibrillation attempt. In laboratory investigation, it is well known that a second attempt at the same energy level may very well be successful. In fact, Guidelines 2000 for Cardiopulmonary Resuscitation and Emergency Cardiac Care states even a failed shock at one energy may be successful if simply repeated…repeated shocks, even at the same energy level add to the probability of successful defibrillation. This implies a variable susceptibility of the myocardium to countershock over time. A possible explanation for what might otherwise seem like random variation comes from a report in Circulation by Hsu et al.⁸ They noted that defibrillation success is higher and the threshold for success is lower when internal shocks were timed to the upslope of the shocking lead electrogram. Regardless, with application of the traditional 200-300-360 joule sequence of defibrillation attempts, there are no data from human trials to help determine the relative benefit of energy increase versus simple repetition. The obvious question for clinicians is whether repeating defibrillation attempts at higher, as opposed to the same energy, will improve the likelihood of successful defibrillation. Traditional escalating energy protocols were based on the notion that a failed defibrillation attempt indicated the presence of high transthoracic impedance. Advancing to a higher energy setting for the next attempt was the recommended action based on that rationale. Using such a protocol, one would eventually reach an efficacious energy level while avoiding current overdose. In contrast, out of hospital studies have shown that 96% of patients in ventricular fibrillation with considerable downtime were defibrillated by the first low-energy BTE shock and 100% were ultimately defibrillated with repeated shocks of the same energy level.⁶ This experience argues for new defibrillation protocols that are appropriate for the improved electronics. Finally, there is the issue of potential harm attributable to the use of high-energy shocks. It is obvious that post-resuscitative harm is preferable to unsuccessful defibrillation. However, could a low-energy strategy result in a similar resuscitation rate and an increased rate of ultimate survival? A number of studies have demonstrated more prolonged and severe myocardial dysfunction following high-energy shocks compared to lower energies. 9 This appears to be a stunning effect rather than tissue necrosis per se. Nonetheless, such dysfunction may lead to circulatory instability following resuscitation. This, in turn, could result in further vital organ dysfunction or the need to employ inotropic agents with their attendant arrhythmogenic potential. Animal studies support this concept; while effective defibrillation may occur at high energy levels, survival actually declines. This concern can be most logically addressed by using technology that yields high defibrillation rates at lowest energy settings possible. Ultimately, one cannot assert that a non-escalating low-energy, an escalating higher-energy or an initial highest energy strategy is most effective for the treatment of ventricular fibrillation. Since each defibrillator has its own technologic specifications, it is to be expected that each might be best used according to a unique strategy. What the Guidelines for Cardiopulmonary Resuscitation and Emergency Cardiac Care are trying to tell purchasers and users of external defibrillators is that to the present time, only one technology has been sufficiently tested to determine its effectiveness when used according to the manufacturer's recommendation. Whether we are prepared for it or not, protocols for the treatment of ventricular fibrillation have forever become more complicated.