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With substantial patent portfolios protecting various waveform designs, each manufacturer has chosen different approaches to manipulating the waveform. As a result, energy protocols are no longer standardized; some manufacturers have chosen a low-energy approach (either fixed or escalating) while others have adopted the escalating energy standard of the past. It is important to note, however, that the historical method of escalating energy was developed because the early monophasic waveforms performed relatively poorly with high impedance patients. Depending on the type of monophasic waveform, average first shock efficacy was only about 50 to 80 percent. Escalating the energy provided a mechanism to increase the current, thus increasing the probability of success despite inherently inefficient technology.
Historically, there has never been much evidence to support the practice of escalating energy.
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In fact, there is early evidence that escalation was associated with adverse consequences.11 The problem with this escalating energy approach, be it with monophasic or biphasic technology, is twofold. First, the myocardium remains in a lethal rhythm state while the device ramps up to an effective current dose. Second, while the traditional approach of escalating, high-energy is often assumed to be the appropriate standard for optimizing defibrillation efficacy, use of increasing doses of energy - particulary in the context of biphasic waveforms, whose efficacy has been reapeatedly proven using relatively lower energies - must be weighted against a growing body of evidence for toxicity with high energies.9,10,12-15
Energy, in fact, is not the whole story, as it is current that defibrillates, not energy. One recent study15 comparing a highenergy biphasic waveform with the low-energy SMART Biphasic waveform indicates that high peak current was the only significant predictor of survival (p < 0.001). By contrast, high-energy was associated with cardiac dysfunction, such as impaired ejection fraction and stroke volume. The study concludes that the key to a well-designed waveform is to combine high peak current for efficacy with low-energy for safety. This is the approach that Philips takes.
The defibrillation response curves in Figure 3 demonstrate graphically how the probability of defibrillation changes with increasing current.16 Figure 3 also demonstrates the difference between the defibrillation response curves for a typical biphasic truncated exponential (BTE) waveform and the monophasic damped sine (MDS) waveform (the most commonly used monophasic waveform).
With the gradual slope of the MDS waveform, it is apparent that as one increases the current, defibrillation efficacy is also improved. This finding led to the use of escalating energy with the MDS waveform, since peak current is increased with escalating energy, which results in a higher probability of defibrillation. For the monophasic waveform, therefore, increasing the energy can improve defibrillation efficacy. Selecting a fixed energy with the monophasic waveform that would defibrillate all patients could result in dangerously high energy and current levels, another factor supporting the use of escalating energy with traditional monophasic waveforms.
In contrast, the response curve for the biphasic waveform has a steeper slope and the probability of defibrillation changes very little once a certain current level is reached. This means that, if the energy and minimum delivered current levels are chosen appropriately (150J for defibrillation, in our case), escalating energy is not required to increase efficacy. By selecting a fixed energy dose, the current delivered to the patient can vary as the patient impedance varies (more on this later), and the probability of defibrillation remains high. So, for a well-designed biphasic waveform, increasing the energy does not improve the defibrillation efficacy, and is therefore unnecessary.
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