Letter by Tang et al Regarding Article, “BIPHASIC Trial: A Randomized Comparison of Fixed Lower Versus Escalating Higher Energy Levels for Defibrillation in Out-of-Hospital Cardiac Arrest”
To the Editor:
We read with interest the article by Stiell and colleagues1 in which the efficacy of defibrillation with fixed low energy was compared with escalating high-energy biphasic waveforms in out-of-hospital cardiac arrest settings. When patients received multiple shocks, the likelihood of converting ventricular fibrillation was reportedly greater with high-energy escalating shocks.
The study addresses important issues. However, the devices that were compared did not take into consideration differences in waveforms and, most especially, the time duration of the currents delivered. The comparisons were made with the commercially available biphasic waveform device that delivers escalating shocks marketed by the sponsor. The fixed 150-J shocks also were delivered with the Medtronic LIFEPAK 500 modified to deliver 150-J shocks using the same capacitor, substantially reducing the current delivered to below the manufacturer’s recommended minimal dose of 200 J. Because it is the current delivered over a defined time rather than the energy itself that is the predominant determinant of successful defibrillation, the comparisons are not valid. More specifically, the Medtronic LIFEPAK 500, modified to deliver 150-J shocks, used a 200-μF capacitor. The only commercially available devices that use a fixed 150-J energy protocol are those manufactured by Philips Medical Systems, which uses a 100-μF capacitor.
Consequently, we have computed that the current was reduced substantially (>25%) and the likelihood of defibrillation accordingly. Moreover, higher delivered energies with waveforms that minimize currents produce greater postresuscitation myocardial dysfunction and worse outcomes.2 Comparison of “commonly used [automatic external defibrillator] energy dosing regimens” would therefore require a comparison of the Philips design at 150 J with the Medtronic design at 200 to 300 to 360 J to take into account the overriding role of waveforms and especially peak currents.
A second concern relates to the separate implications of the first and subsequent shocks in instances of recurrent ventricular fibrillation and shock-refractory ventricular fibrillation, although it is evident that the majority of the ventricular fibrillation events were in fact recurrent ventricular fibrillation. It is difficult to explain why recurrent ventricular fibrillation would not succumb to a shock of the same energy as the initial episode. Certainly, this is inconsistent with the earlier results using the 150-J waveform and a 100-μF capacitor. In addition, by aggregating first and subsequent shocks among patients who received multiple shocks, the authors do not provide certain evidence that escalation is advantageous, even when considering only shocks from a 200-μF capacitor.
Finally, only 1 outcome was significant, and that finding would have required further validation because 6 patients in the fixed group and none in the escalating group presented with asystole. Appropriate cofactors analysis would help to resolve this issue.
Because of these limitations, it is our opinion that further study is needed before the conclusions reached by Stiell and colleagues can be translated into clinical practices.
Stiell IG, Walker RG, Nesbitt LP, Chapman FW, Cousineau D, Christenson J, Bradford P, Sookram S, Berringer R, Lank P, Wells GA. BIPHASIC trial: a randomized comparison of fixed lower versus escalating higher energy levels for defibrillation in out-of-hospital cardiac arrest. Circulation. 2007; 115: 1511–1517.