Biphasic and Monophasic Waveforms for Defibrillation


The principle of defibrillation is to deliver a current to depolarise a critical amount of myocardium so that ventricular fibrillation is abolished. If the heart is viable, the natural pacemaker of the heart takes over after a variable pause. Recurrence of VF should not be considered as a failure of the shock. Shock success cannot be equated to restoration of a perfusing rhythm or survival.
Biphasic and monophasic waveforms have been used for defibrillation. Older defibrillators used to deliver monophasic waveforms while the newer ones deliver biphasic waveform. Monophasic as the name implies, delivers current in only one direction. Biphasic shocks of 200 Joules seem to have efficacy similar, if not better than 360 Joules of monophasic shocks. High efficacy of biphasic shocks would mean that final outcome will depend more on the interval between collapse and CPR or defibrillation.

Two types of waveforms for biphasic shock are biphasic truncated exponential waveform and rectilinear biphasic waveform. Selected and delivered energies differ for rectilinear biphasic waveform in the usual range of thoracic impedance, usually the delivered energy is higher than that selected. Hence lower energies are selected with this waveform initially. Automatic external defibrillators have been programmed to deliver fixed or escalating energy levels on repeated shock, though evidence if favour of either protocol is lacking (Circulation. 2005;112:IV-35 – IV-46).

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