Philips HeartStart Defibrillators enable system-wide standardization and consistency of care. All HeartStart Defibrillators feature Philips' patented low-energy SMART Biphasic waveform, and use a 3-shock, 150 Joule protocol for adults in ventricular fibrillation (VF). This technology is proven effective in emergency resuscitation, exhibiting superior performance at terminating VF when compared with monophasic waveforms. It also minimizes post-defibrillation heart dysfunction, and is associated with improved return of spontaneous circulation (ROSC) and neurological outcomes.1 Data also demonstrates the SMART Biphasic waveform's superior cardioversion performance, (relative to monophasic), in treating atrial fibrillation, a rhythm for which use of energies up to 200 Joules is appropriate.2,3
The 1990's heralded a new era of transthoracic defibrillation in which the rules of conventional practice no longer apply. Seeking waveform designs more efficient than the traditional monophasic defibrillation waveforms, most external defibrillator manufacturers followed the lead of the implantable cardioverter-defibrillator (ICD) industry, which established clear research evidence of superior clinical and engineering performance of low-energy biphasic defibrillation. By 1998, virtually all ICD's employed biphasic defibrillation waveforms, offering manufacturers the ability to design defibrillators that were smaller, more reliable, and provided superior clinical performance using lower energies.
As with ICDs, modern day transthoracic biphasic waveform technologies also allow smaller, more reliable devices. However external waveforms must deal with the effects of varying patient chest impedance. Philips pioneered the first external biphasic defibrillation waveform in an automated external defibrillator.
Philips offers the low-energy, impedance-compensating SMART Biphasic truncated exponential (BTE) waveform across its defibrillator product line, and is unique in the defibrillator industry for its leadership in evidence-based design.
Each manufacturer have each taken different approach to defibrillation and impedance compensation. As a result, the notion of one, standardized energy protocol for all is no longer warranted or appropriate, and each defibrillation waveform design must be evaluated based on available research.
How does one differentiate the various biphasic designs? Which biphasic is better? The answer is no one knows. While peer-reviewed human research comparing each of the biphasic technologies within one study design is recognized as ideal, the likelihood of establishing performance differences that reach statistical significance using feasible sample sizes is remote. Thus, no manufacturer has undertaken a well designed, prospective study in humans to answer the question of superiority among biphasic technologies.
The American Heart Association (AHA) has, however, established a clear evidence-based process for evaluating technologies. In 1997, the AHA established a set of recommendations for manufacturers seeking to design "alternative waveforms".1 These guidelines were followed in 1998 by the first application of the new "evidence-based review" process,2 in which the AHA evaluated the research available for defibrillation waveforms and provided recommendations for clinical practice. The process resulted in a Class IIb recommendation ("safe, acceptable, and clinically effective") for nonprogressive 150J biphasic shocks, of which the Philips SMART Biphasic waveform was the first and only example.
Continuing the theme of evidence-based practice in the 2000 Guidelines document,3 the AHA issued no classification for high-energy defibrillation and a clear recommendation for low-energy biphasic. The following statement appears following a list of studies reflecting performance of the Philips SMART Biphasic waveform:
"Early clinical experience with the 150J, impedance-compensated BTE waveform for treatment of out-of-hospital long-duration VF was also positive. . . The growing body of evidence is now considered sufficient to support a Class IIa recommendation for this low-energy, BTE waveform." Page I-63. (Class IIa is defined as having "good to very good evidence", a "standard of care", "intervention of choice".)
In addition, the following generic recommendation for low energy biphasic defibrillation is provided:
The data indicates that biphasic waveform shocks of relatively low energy (< 200J) are safe and have equivalent or higher efficacy for termination of VF compared with higher-energy escalating monophasic waveform shocks (Class IIa)" Page I-63.
Finally, the need for comprehensive waveform-specific data is emphasized:
"The safety and efficacy data related to specific biphasic waveforms must be evaluated on an individual basis in both in-hospital. . . and out-of-hospital settings."
As noted earlier, manufacturers of modern day defibrillation waveforms employ different strategies for waveform design. Following the lead of the AHA, it is critical to rigorously review the published waveform performance data before making a product decision. Properly evaluating the differences in waveform designs also requires an understanding of some basic electrical concepts.
When it is time to defibrillate, switching circuitry within the defibrillator connects the charged capacitor to the patient's chest via paddles or electrode pads. Once connected, the voltage on the capacitor causes current to begin flowing through the patient. Just as the height of water in a tank creates pressure, forcing water through an open pipe, voltage is the driving force for electron flow (current) through a defibrillator circuit. It is current that delivers energy to the patient. Current, however, is resisted by the patient's impedance (measured in "ohms", or Ω) - an effect similar to a restricted water pipe. Contrary to common perceptions, patient impedance is not closely linked to patient size or weight.
Patient size or weight is often believed to be associated with both impedance and energy requirements. It is often considered a factor in defibrillation success. in This common misperception exists despite a lack of peer-reviewed evidence, and a statement to the contrary in the American Heart Association 2000 Guidelines recommendation . "There is no definite relationship between body size and energy requirements for defibrillation in adults."4
The influences of body weight5 and patient impedance6 were assessed in recent studies utilizing the fixed, low-energy SMART Biphasic waveform (Philips Medical Systems). Results demonstrated no association between either body weight or patient impedance and defibrillation efficacy, return to spontaneous circulation (ROSC) or survival outcomes. These findings are consistent with other studies showing high efficacy of the SMART Biphasic waveform in rigorous out-of-hospital patient populations, including patients with high impedance. Thus, the fixed-energy, impedance-compensating SMART Biphasic waveform has been designed to be effective across a wide range of patient impedances, and with no influence of body weight on shock success.
"Early clinical experience with the 150J, impedance-compensated BTE waveform for treatment of out-of-hospital long-duration VF was also positive. . . The growing body of evidence is now considered sufficient to support a Class IIa recommendation for this low-energy, BTE waveform." Page I-63. (Class IIa is defined as having "good to very good evidence", a "standard of care", "intervention of choice".)
The current through the patient's chest must vary during delivery in a specific manner in order to effectively defibrillate. The current delivered to a patient therefore changes during the course of a defibrillation shock. The pattern, or time course, of this current variation is called a waveform. As shown in the image to the left, the defibrillator current time course flows in one direction with traditional monophasic waveforms whereas current in a biphasic waveform circuit flows in both a positive and negative direction. This biphasic two-directional flow of current within the defibrillator is reflected by current going from pad-to-pad in one direction, then reversing to flow in the opposite direction.
Traditional monophasic waveform technology, while it saved many lives, had serious design limitations. High patient impedance degraded the waveform, resulting in relatively poor performance. So, an empirical strategy of escalating energy was developed, without supporting science, in an effort to compensate for monophasic design limitations.
With the advent of modern biphasic waveform technology, however, impedance compensation and other design improvements have led to generally superior clinical and engineering performance characteristics. It is now possible to produce a highly effective safe waveform without the need to escalate energy. The variety of external biphasic waveforms in the marketplace has prompted complexity and confusion around clinical practice guidelines. Since traditional, empirical rules-of-thumb are no longer sufficient to guide practice, the clinician is best advised to rely on the manufacturer's recommendations for their specific waveform and on the associated body of supporting peer-reviewed science.
The AHA has recommended an evidenced-based process for evaluating defibrillation waveforms, reflecting published research in both in- and out-of-hospital settings. To date, Philips has offered more published data reflecting performance in both in- and out-of-hospital patients than any other manufacturer, establishing a clear leadership position in evidence-based waveform design. Based on published research, the AHA has provided a IIa recommendation for low-energy biphasic defibrillation (< 200J), and no recommendation for higher energy.
1. Kerber RE, et al. Automatic external defibrillators for public access defibrillation: recommendations for specifying and reporting arrhythmia analysis algorithm performance, incorporating new waveforms, and enhancing safety: a statement for health professions from the AHA Task Force on Automatic External Defibrillation, Subcommittee on AED Safety and Efficacy. Circulation 1997;95:1677-1682.
2. Cummins RO, et al. Low-energy biphasic waveform defibrillation: evidence-based review applied to emergency cardiovascular care guidelines: a statement for healthcare professionals from the American Heart Association Committee on Emergency Cardiovascular Care and the Subcommittees on Basic Life Support, Advanced Cardiac Life Support, and Pediatric Resuscitation. Circulation 1998;97:1654-1667.
3. Cummins RO, et al. Guidelines 2000 for cardiopulmonary resuscitation and emergency cardiovascular care. Supplement to Circulation 2000;102(8):I-5,I-63,I-91.
4. Albert CM et al. “Triggering of Sudden Death from Cardiac Causes by Vigorous Exertion.” N Engl J Med. 1999; 343:1355-1361.
5. White RD, et al. Body weight does not affect defibrillation, resuscitation or survival in patients with out-of-hospital cardiac arrest treated with a non-escalating biphasicwaveform defibrillator.
6. White RD, et al. Transthoracic impedance does not affect defibrillation, resuscitation, or survival in patients with out-of-hospital cardiac arrest treated with a non-escalatingbiphasic waveform defibrillator.
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