Comparing the Transthoracic Biphasic Waveforms
Now that we have highlighted the key elements of effective biphasic waveform design, we turn to a brief overview of other external biphasic waveform technologies on the market. The SMART Biphasic waveform was introduced in 1996 with substantial patent protections. There are also patent restrictions on various other technologies. Consequently, the biphasic technologies are all different as are the associated energy protocols.
Because of these design differences, the energy protocol for each manufacturer’s defibrillator should be individualized. The need for product-specific energy protocols is confirmed by ECRI, a non-profit organization whose mandate it is to objectively evaluate biomedical equipment: "…a waveform designed for low-energy defibrillation may result in an overdose if applied at high energies, while another waveform designed for high-energy may not defibrillate at lower energies." 31
Most importantly, compared to the Philips SMART Biphasic waveform, other manufacturers have relatively few published, peer-reviewed studies to demonstrate the performance of their waveforms. Some manufacturers have no data at all, and others rely heavily upon small sample abstract and animal data to demonstrate waveform performance. Collectively, the amount and breadth of published SMART Biphasic clinical research exceeds that of all other
The Low-Energy Rectilinear Biphasic Waveform Alternative
The Rectilinear Biphasic Waveform (ZOLL Medical Corporation) shares with SMART Biphasic a low-energy, low-capacitance design, but there are significant differences. Most importantly, the Rectilinear waveform offers only limited peer-reviewed evidence to support its performance. As of this writing, we are aware of no published, peer-reviewed data reflecting performance with the challenging long down-time patient population most difficult to treat effectively.
The Rectilinear waveform does little to adjust current in response to the problem of shunt current pathways within the chest. The waveform utilizes what company literature describes as a "constant current" approach in the first phase of the waveform. In contrast to SMART Biphasic, which modifies peak current, waveform shape and duration based on patient impedance, the Rectilinear approach is to hold the overall waveform duration and ratio between the two phases constant regardless of patient impedance (see Figure 9).
The published adult energy protocol for the Rectilinear Biphasic waveform device starts at 120J and escalates to 200J. As noted earlier, escalating energy was employed historically to increase peak current with inherently inefficient monophasic waveforms, but is not necessary with biphasic when the first shock is adequatley dosed, as in the case with SMART Biphasic.
For any selected energy setting, the actual delivered Rectilinear waveform energy varies widely across the range of patient impedance. Further, the Rectilinear Biphasic waveform loses the constant current profile, essentially becoming a BTE waveform very similar to SMART Biphasic (Figure 10), when patient impedance values exceed 100 ohms and 200J of energy is selected. 32
In summary, the Rectilinear waveform, marketed as a "constant current" waveform, does little to adjust current in response to current shunting in the patient’s chest. The manufacturer abandons the hallmark "constant current" approach for some high impedance patients and, perhaps most importantly, has only limited published data on which to measure its waveform’s performance, none of which reflects performance with the ischemic SCA patient.
Figure 9. SMART Biphasic vs. Rectilinear Biphasic
Figure 10. "Constant" Current Not Always Constant
The High-Energy Biphasic Waveform Alternatives
There are several escalating high-energy biphasic waveforms currently on the market. It is beyond the scope of this paper to describe in detail each of the designs. Instead, we offer a high level summary of technology issues to consider with highenergy waveforms as a class.
The specific methods of impedance compensation vary with the manufacturer. Figure 11 illustrates the SMART Biphasic waveform compared to one of the common high-energy biphasic waveforms on the market. It is evident that the two waveforms modify peak current, waveform shape and duration similarly in response to patient impedance. The big difference is that the high-energy waveforms require high energy to deliver adequate current to the patient because of their large capacitors, while the Philips low-energy BTE waveform delivers adequate current on the first shock without the need to escalate.
Figure 11. SMART Biphasic vs. High-Energy Biphasic
Most importantly, as of this writing there is only one published, peer-reviewed study reflecting waveform performance of one high-energy biphasic waveform with the ischemic, long downtime SCA patient population. This study compared the highenergy ADAPTIV biphasic waveform (Metronic Physio-Control) with a conventional monophasic damped sine (MDS) waveform.33 While first shock efficacy for the biphasic waveform was high, there was an atypical trend towards better MDS waveform performance compared to the ADAPTIV biphasic waveform for all other outcome variables.
Compare these results to a similar study7 in which the SMART Biphasic waveform was associated with improved return to spontaneous circulation, improved survival to hospital admission and discharge, and superior neurological outcome for survivors compared to monophasic treatment.
There is no peer-reviewed research in humans to compare the performance of one biphasic waveform to another. A swine study by Walker, et al. is often cited as evidence of high-energy superiority with moderate to high impedance patients. However, the study utilized a disputed animal model 34,35 that is inconsistent with the way in which impedance actually occurs in either animal or human populations.
The Walker, et al. study also yields results inconsistent with numerous peer-reviewed studies demonstrating superior efficacy of the low-energy SMART Biphasic waveform defibrillator to high-energy therapies across a diverse human population, including patients with high impedance.
In summary, high-energy biphasic waveforms offer little or no published, peer-reviewed data in comparison to that published for the Philips SMART Biphasic waveform, and only one study reflecting performance with the long down-time SCA patient population.
As pointed out, high biphasic energy has been associated with increased cardiac dysfunction.15
Finally, the AHA has issued no science-based recommendations regarding biphasic defibrillation > 200 Joules.
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