The Effects Of Phase Durations On Defibrillation Efficacy Of Triphasic Waveforms In A Rabbit Model Of Cardiac Arrest

With an incidence ranging from 35 to 125 cases per 100,000 people,cardiac arrest(CA)remains a major public health problem all over the world.Despite the reported decreased incidence of ventricular fibrillation(VF)as presenting rhythm during the last decades,it remains the primary cause of cardiac arrest in the majority of instances.Electrical defibrillation,which consists of delivering of a therapeutic dose of electrical current to the fibrillating heart with the aid of a device known as defibrillator,is still the only effective way to treat this life-threatening arrhythmia.The waveform of a shock is the temporal pattern of its amplitude,measured by voltage(or current).It interacts with cardiac electrical activity via its electric field,which is the instantaneous spatial derivative of shock voltage.For decades,external defibrillators have used monophasic waveforms.Although monophasic defibrillation was recognized to be highly effective for the termination of VF,but risk of post-shock complications such as myocardial dysfunction and skin burns was high.Compared with monophasic waveforms,biphasic waveforms that consisting of two phases of opposite polarity significantly decreased the shock strength needed for defibrillation and caused less myocardial dysfunction.Even though biphasic defibrillation is more efficient than monophasic defibrillation,researches for more efficient low energy defibrillation are continuing.Triphasic waveforms,which composed of three pulses with the polarity of the second pulse reversed have been evaluated but with controversial results.The inconsistency might result from the difference in the waveform designsince earlier studies explored that defibrillation efficacy was influenced by the duration,tilt,amplitude,and polarity of each phase of biphasic shocks.The optimal parameters of triphasic waveforms,however,have not been identified.Thus we hypothesized that triphasic waveforms with optimal phase durations are more effective than biphasic waveform for ventricular defibrillation.In order to verify our hypothesis,we designed a phase duration adjustable arbitrary waveform system,which contain a self-made waveform generator and a pulse amplifier.The system was firstly tested in dummy load resistors.In order to select the potential triphasic waveforms that may superior to biphasic waveform,a preliminary experiment was performed in 17 New Zealand rabbits with 4 biphasic and 36 triphasic waveforms.After a lengthy investigation,we retrospectively selected 7 defibrillation waveforms,which included 6 triphasic rectangle waveforms of different durations and shapes and a biphasic rectangle waveform tested in another 20 rabbits.The triphasic waveforms had identical first duration but with different second and third phase durations.A 5 step up-and-down protocol was utilized for determining the defibrillation threshold(DFT).After a 5 min interval,the procedure was repeated.A total of 35 cardiac arrest events and defibrillations were investigated for each animal.The differences between measured and preset phase durations and voltage were 0.10±0.11% and 0.41±0.30% respectively.All animals survived until the end of the study.There were no significant differences in hemodynamics among the animals throughout the experimental procedures.Two triphasic waveforms with identical first and second phase duration but shorter third phase duration had significantly lower DFT energy than biphasic waveform(0.57 ± 0.18 J vs.0.80 ± 0.28 J,p = 0.001;0.60 ± 0.18 J vs.0.80 ± 0.28 J,p = 0.003).However,no statistical difference in DFT energy was observed between the two triaphsic waveforms that had identical phase duration but different voltages(0.57 ± 0.18 J vs.0.60 ± 0.18 J,p = 0.638).The present study demonstrated that certain triphasic waveforms were more effective than biphasic waveforms for external defibrillation.Phase durations played a main role on defibrillation success for triphasic rectangle waveforms.The optimal triphasic rectangle waveforms that composed of equal second and first phase durations but with shorter third phase duration were superior to biphasic rectangle waveform for ventricular defibrillation.