Biphasic Overview

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.