Response to Letter Regarding Article, “BIPHASIC Trial: A Randomized Comparison of Fixed Lower Versus Escalating Higher Energy Levels for Defibrillation in Out-of-Hospital Cardiac Arrest”

Abstract

We appreciate the interest of Tang and colleagues, who have made substantial contributions to advancing care for patients with cardiac arrest. We provide additional information here to clarify the issues they raise, beginning with their last point.The letter expresses concern about an imbalance between groups 1 in the incidence of asystole as an initial rhythm. However, the multishock groups, on which the primary end point is based, are nearly balanced: 2 of 55 versus 0 of 51 patients initially in asystole.The letter expresses concern about aggregating first and subsequent shocks in the primary analysis. An abstract of our study reported separate results for subsequent shocks in which energy levels differ most between groups, revealing a larger advantage for higher-energy shocks: ventricular fibrillation termination, 71% for 150 J versus 85% for 300 to 360 J (Pϭ0.01); conversion, 24% versus 43% (PϽ0.01). 2 Another stated concern is that the automated external defibrillators studied were modified. They were not; study sites used their existing standard LIFEPAK 500 automated external defibrillators, providing protocol choices starting as low as 150 J and increasing to as high as 360 J. Although the Philips automated external defibrillators discussed by Tang and colleagues are limited to a fixed 150-J protocol, many automated external defibrillators allow various protocol choices, including 150 J fixed. Because most devices support multiple protocols and evidence to recommend 1 protocol over another remained lacking, our study addressed an important question: When multiple protocols are available, which should be chosen?The letter suggests that the lower peak current of the 150-J shocks we studied compared with different 150-J shocks discussed in the letter reduces defibrillation effectiveness. This would be true only if the waveform shape were the same. Compared with the 100-F shocks discussed in the letter, the longer time-constant 200-F shocks we studied provide higher average current for any given peak current. Consequently, as established in the literature, highercapacitance waveforms defibrillate with less peak current. 3 Most defibrillators increase energy by increasing shock intensity (current and voltage) rather than changing waveform shape or duration. We evaluated the effect of shock intensity without changing waveform and found that a protocol using higher intensities for subsequent shocks produced better heart rhythm outcomes than one maintaining a lower intensity. The principle demonstrated by our results would apply to any other waveform unless either (1) the first energy level succeeded for 100% of all shocks or (2) increasing the intensity above that of the first shock caused the peak current to exceed the level at which clinically significant myocardial injury appears. Multiple clinical studies report relatively low biphasic ventricular fibrillation termination rates (Ͻ75%) and subsequent shock success lower than the first shock success, providing ample evidence to rule out condition 1 for any def...

Update on cardiac arrhythmias in the ICU

“…The use of biphasic defibrillation, when compared with monophasic waveforms, increases the first shock success rate but does not result in a higher return of spontaneous circulation or survival [47 ]. A recent study of out-of-hospital arrests showed that escalating higher energy (200-300-360 J) biphasic automatic defibrillation had a greater success rate than a fixed lower energy regimen (150-150-150 J) [48 ]. The actual energy delivered by defibrillators is proportional to the patient's transthoracic impedance so methods that provide the same energy equivalents independent of the impedance need to be developed [49].…”

Increased cycle length during long-duration ventricular fibrillation is caused by decreased upstroke velocity as well as prolonged refractoriness

“…Studies have demonstrated an inverse relationship between time to defibrillation and survival from sudden cardiac death, with a grim prognosis for patients who do not receive shocks within 10 min 2. While certain communities achieve quick response times,3 in many cases emergency medical services responders reach patients more than 10 min after a witnessed arrest 4. Yet, the majority of animal studies of ventricular fibrillation (VF) focus on the first minute of VF 5…”

Comparison of Low-Energy Versus High-Energy Biphasic Defibrillation Shocks Following Prolonged Ventricular Fibrillation

“…The BIPHASIC trial compared fixed-lower versus escalating-higher energy levels for prehospital defibrillation in humans 25. Patients were randomized to receive either 1) fixed-energy biphasic shocks (150 J) or 2) escalating-energy biphasic shocks (200–300–360 J).…”