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Synopsis: The use of an automatic external defibrillator with a biphasic waveform is more effective for defibrillating with out-of-hospital cardiac arrest.
Source: Schneider T, et al. Circulation 2000;102: 1780-1787.
Schneider and associates compared the efficacy of automatic external defibrillators (AEDs) using either biphasic or monophasic shock waveforms in patients with out-of-hospital cardiac arrest. The study involved the emergency medical services (EMS) in four cities: Mainz and Hamburg in Germany, Brugge in Belgium, and Helsinki, Finland. All EMS units involved in the trial carried either a biphasic AED that delivered a 150 joule, impedance compensated biphasic waveform shock or a monophasic AED that used either a monophasic truncated exponential or a monophasic damped sine defibrillation waveform. A daily schedule of randomly selected AED types was used in each city. After arrival at the scene, the AED appropriate for that day was placed on each victim of sudden, out-of-hospital cardiac arrest whenever defibrillator application was felt to be clinically indicated. Manual defibrillators were available for back-up in case the AED was unsuccessful. The sequence of shocks was 200, 200, and 360 joules for the monophasic units and three 150 joule shocks for the biphasic units. If three consecutive shocks failed to defibrillate or if the AED did not advise a shock be delivered, then local life support protocols were followed. The primary end point of the study was the percentage of patients with ventricular fibrillation (VF) as the initial monitored rhythm who were defibrillated in the first series of three shocks or less. Defibrillation with one or two shocks, return of spontaneous circulation, neurologic status at discharge, and survival to hospital admission and discharge were secondary end points.
The study enrolled 338 patients with out-of-hospital cardiac arrest and, of these, 246 were eligible for randomization. Ventricular fibrillation was the initial monitored rhythm in 115 individuals and these subjects formed the final study group. The groups were comparable in terms of age, gender, weight, cardiac diagnosis, and cause of cardiac arrest. Eighty-eight percent of the arrests were witnessed by bystanders. Forty-four percent of the patients received bystander cardiopulmonary resuscitation. The time from the emergency call to delivery of the first shock was 8.9 ± 3 minutes overall and this interval was not different in the two treatment arms.
Four patients in the monophasic shock group were not treated with an AED due to low amplitude VF that was not detected and identified as VF by the AED. The first shock efficacy was 59% for patients who received a monophasic shock and 96% for patients who received an initial biphasic shock. Three shocks or less defibrillated 69% of the patients in the monophasic AED group vs. 98% in the biphasic AED group. Return of spontaneous circulation was seen in 54% of the monophasic group and 76% of the biphasic group (P = 0.01). Fifty-one percent of the monophasic group vs. 61% of the biphasic group survived to hospital admission (P = 0.27). There was no difference in survival to hospital discharge with 31% of the monophasic group surviving vs. 28% of the biphasic group. At the time of discharge however, cerebral performance was rated as good in 87% of the patients who were treated with the biphasic AED vs. 53% of the monophasic AED. Schneider et al conclude that the use of an AED with a biphasic waveform is more effective for defibrillating patients with out-of-hospital cardiac arrest. Larger studies are indicated to see if survival rates to hospital admission and discharge will be improved with the use of these new units.
Comment by John P. DiMarco, MD, PhD
There have been numerous recent advances in external defibrillators. Biphasic waveforms were first shown to have advantages in terms of energy requirements, size, and weight in implantable defibrillators. More recently, studies using external defibrillators have also shown an advantage for the biphasic waveform in patients with both supraventricular and ventricular arrhythmias. This well-organized trial confirms the superiority of the biphasic waveform in automatic external defibrillators used by emergency medical services personnel in the field.
It is somewhat disappointing that the higher defibrillation efficacy did not translate into increased survival to hospital discharge. It is likely that delay from the time of collapse to deliver of the first shock using any waveform remains the most critical variable in survival to hospital discharge. The small amount of time required to deliver multiple shocks and the myocardial dysfunction from any additional shocks required may be much less important than the time to defibrillation. However, the increased defibrillation efficacy and the increased rate of return of spontaneous circulation are important parameters to assess in evaluating a complete resuscitation algorithm and the data presented here are quite convincing.
Although it will involve considerable expense, it is clearly time that hospitals and emergency medical systems begin a replacement cycle for their monophasic defibrillators. This study, as well as other studies, is unanimous in supporting the enhanced efficacy of biphasic units. Unfortunately, time to defibrillation of any type still remains the primary parameter influencing survival in out-of-hospital cardiac arrest.