Improved efficacy of anodal biphasic defibrillation shocks following a failed defibrillation attempt

Although it is generally assumed that defibrillation becomes more difficult when the duration of VF is prolonged, after a failed defibrillation attemp t, there is little information on the defibrillation efficacy of multiple s hocks delivered at the same energy. The purpose of this study was to system atically examine the efficacy of a second shock delivered at the same or re versed polarity after a failed first shock. Defibrillation was attempted af ter I a seconds of VF in 12 pigs (30-56 kg) using biphasic waveforms and a nonthoracotomy lead system. Shock energy was held constant for the first an d second shocks art 50%-90% of the DFT. The second shock was delivered 10 s econds after a failed first shock. First and second shock polarity (first p hase) was randomized to (+, +), (+, -), (-, -), (-, +). The incidence of su ccessful defibrillation (for all polarities) was 12.3% for first and 49.1% for second shocks (P < 0.0001). Anodal first shocks had a 17.2% incidence o f success as opposed to a 7.4% incidence of success with cathodal first sho cks (P = 0.001). Anodal second shocks had a 55.5% incidence of success comp ared to a 42.7% incidence of success with cathodal second shocks (P = 0.008 ). There was no significant benefit from polarity reversal after a failed f irst shock (P = 0.29). In conclusion, less energy is required for successfu l defibrillation by a second shock after a failed first. The optimal config uration for first and second shocks is with the RV as anode. Polarity rever sal of a second shock after a failed first does not affect the probability of second shock success.