Biphasic Waveform Technology

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US Cardiology 2004;2004:1(1):1-5

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In the past few years, biphasic defibrillation waveforms have become the norm for external defibrillators, and for good reason. These waveforms have demonstrated lower defibrillation thresholds, resulting in lower energy levels for the shock delivered to the patient, which results in less stress on the patientÔÇÖs heart. This article will give some of the reasons behind this shift to biphasic and provide a basic understanding of the implications for improved patient care as well as improvements in the defibrillators themselves.

The monophasic waveforms used in commercial defibrillators over the last 40 years were either monophasic damped sine (MDS) or monophasic truncated exponential (MTE) waveforms. Both of these waveforms were detailed in the AAMI specification for defibrillators1 and both waveforms were limited to a maximum energy setting of 360J because of concerns about myocardial damage at higher energies. The specification assumed the patient impedance was 50 ohms and no attempt was made for the MDS waveform to modify the waveform if the impedance varied. The AAMI standards for these two waveforms were developed based on practice and there was no clinical data offered to support their efficacy (see Figure 1).

The biphasic truncated exponential (BTE) waveform was developed initially for implantable defibrillators and became the standard for these devices in the late 1980s. The advantage of this waveform was that it defibrillated at lower energies and could therefore allow the design of devices that were substantially smaller and lighter than the MTE waveform originally used in ICDs. The desire to produce a smaller and lighter automated external defibrillator (AED) led designers to pursue this waveform for external applications in the mid-1990s.

The BTE waveform also has an advantage related to the shape of the defibrillation response curve. Figure 2, based on Snyder, et al., ECR 2002 (abstract), demonstrates the difference between the defibrillation response curves for the BTE and the MDS waveform. With the gradual slope of the MDS waveform, it is apparent that as current increases, the defibrillation efficacy also increases. This characteristic of the MDS response curve explains why escalating energy is needed with the MDS waveform since the probability of defibrillation increases with an increase in peak current, which is directly related to increasing the energy. The steeper slope of the BTE waveform, however, results in a response curve where the efficacy changes very little with an increase in current, past a certain current level. This means that if the energy (current) level is chosen appropriately, escalating energy is not required to increase the efficacy. This fact, combined with the lower energy requirements of BTE waveforms,2 means that it is possible to choose one fixed energy that allows any patient to be effectively and safely defibrillated.

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References
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