The principle of defibrillation is to deliver a current to depolarise a critical amount of myocardium so that ventricular fibrillation (VF) is abolished. If the heart is viable, the natural pacemaker of the heart takes over after a variable pause. Since the process of depolarisation of the critical amount of myocardial cells occurs in the 300 to 500 seconds after delivery of shock, successful defibrillation is defined as termination of ventricular fibrillation for at least 5 seconds following a shock. 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 deliver monophasic waveforms while the newer ones deliver biphasic waveform. Monophasic as the name implies, delivers current in only one direction. In monophasic damped sinusoidal waveform, the current returns to baseline gradually while in the monophasic truncated exponential waveform, current abruptly returns to baseline of zero. Though no waveform is consistently associated with higher return of spontaneous circulation or survival, biphasic waveforms are found to be safe and effective in lower doses than monophasic waveforms. 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).