Monophasic vs Biphasic AED
Abstract & Commentary
by John P. DiMarco, MD, PhD, Professor of Medicine, Division of Cardiology, University of Virginia, Charlottesville. Dr. DiMarco is a consultant for Novartis, and does research for Medtronic and Guidant.
Synopsis: For defibrillation success there is no significant difference between monophasic and biphasic shocks.
Source: Kudenchuk PJ, et al, Transthoracic Incremental Monophasic Versus Biphasic Defibrillation by Emergency Responders (TIMBER): A Randomized Comparison of Monophasic with Biphasic Waveform Ascending Energy Defibrillation for the Resuscitation of Out-of-Hospital Cardiac Arrest Due to Ventricular Fibrillation. Circulation. 2006;114:2010-2018.
The paper by Kudenchuk and his colleagues from the Emergency Medical Service System in Seattle describes a randomized trial comparing use of automatic external defibrillator (AED) with a monophasic defibrillation waveform versus a biphasic waveform. The basic device used was the Medtronic Emergency Response Systems, Inc. model LIFEPAK 500. The monophasic defibrillator models delivered a damped sinusoidal waveform of 3100 to 4450 volts at 200- and 360-joule settings respectively. Biphasic defibrillators provided a truncated exponential waveform of 1400 to 2000 volts at similar energy settings. All other features of the AED were the same. Manual defibrillators, which could be used by emergency medical technicians (EMTs), were matched for waveform type to the AEDs used by the first response paramedics equipped with an AED. The authors collected prehospital data including a narrative report and a detailed sequence of resuscitation events. The primary endpoint consisted of admission to the hospital with a spontaneously perfusing rhythm in patients who had ventricular fibrillation (VF) as their first rhythm. Secondary events included conversion of VF, return of an organized rhythm and return of spontaneous circulation (ROSC).
During the study, 862 patients were treated for cardiac arrest from all causes. Of these, 694 were excluded either because they only received basic life support (105 patients), they arrested after EMS arrival (89 patients), the initial rhythm was not ventricular fibrillation (482 patients), or miscellaneous reasons (18 patients). All patients received 90 seconds of CPR before delivery of the first shock. This left 168 patients who were randomized to receive either monophasic- or biphasic-shock therapy. Because of a mismatch between the AED first used by the paramedic team and the manual defibrillator used by the EMTs, 20 patients who had mixed types of shocks were also excluded from the main analysis.
The mean age of the patients was about 65 years, and roughly three-quarters in both groups were men. Seventy percent of the cardiac arrests were witnessed, and approximately half of the patients received bystander CPR. The time from the 911 call to first responder arrival was quite short with a mean of 3.5 + 1.3 minutes. The time to paramedic arrival was also short at 6.9 + 3.1 minutes. The time to first shock was 7.5 + 2.1 minutes.
There was no significant difference in response to the first or later shocks, whether evaluated at 5, 10 or 20 seconds after shock delivery, between the 2 groups. VF was terminated by the first shock in 75 of 80 patients in the monophasic treatment group (82%) and in 65 of 68 patients in the biphasic group (88%). Sustained ROSC was seen in 18 patients in each group. VF recurrence was observed in 18 of 75 patients in the monophasic group (24%) and in 16 of 65 patients in the biphasic group (25%). Similarly, there was no difference between outcomes after subsequent shocks. Among patients treated with monophasic shocks, 58 of 80 were resuscitated (73%) and admitted alive to the hospital as compared to 52 of 68 patients (76%) who received biphasic shocks. Discharge from the hospital alive was 34% for those who received monophasic shocks and 41% for those who received biphasic shocks. No statistically significant differences in neurologic outcome at hospital discharge were seen between treatment groups among the survivors. As in previous studies, time to shock delivery was an important prognostic indicator. However, when patients were categorized as to whether they received their first shock within 10 minutes of EMS dispatch, there were no statistically significant differences between monophasic and biphasic shocks.
The authors conclude that this randomized trial comparing biphasic and monophasic shocks showed no statistically significant difference in outcome of one waveform over the other.
Commentary
This study by Kudenchuk points out some important issues in terms of out-of-hospital cardiac resuscitation. First, it must be noted that the resuscitation rate for these patients was very high and perhaps achievable only in a city with a very advanced EMS system such as Seattle. However, out of 862 cardiac arrests, there were only 168 patients who were eligible for entering into the study. The overall survival rate, although not reported here, would still be quite low. In other series, biphasic shocks had been shown to be more effective for cardioversion of atrial fibrillation and in smaller series of patients with ventricular arrhythmias and cardiac arrest. There does appear to be an advantage in the energy requirements with biphasic as opposed to monophasic devices. However, if one is operating on a flat part of the energy response curve, such as in this study where the initial shock was at 200 joules, this difference becomes smaller. The authors do mention that they saw some favorable trends in their data even though the trends did not reach statistical significance. Whether any of these would become significant in a much larger trial would be difficult to test since the changes were relatively small. However, in communities in which response times are longer and the resuscitation rate much lower, it might be possible to compare the 2 waveforms more effectively.
One important observation that comes out is that is not necessary at the present time to automatically replace all AEDs in the field to biphasic units. It seems reasonable to phase in the biphasic devices over time. This should help EMS systems throughout the country phase in new AED technology without serious budgetary constraints.
For defibrillation success there is no significant difference between monophasic and biphasic shocks.You have reached your article limit for the month. Subscribe now to access this article plus other member-only content.
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