Ventricular defibrillation by electric counter shock is the definitive treatment for cardiac arrest due to ventricular fibrillation (VF). Ventricular defibrillation is the fundamental cornerstone to a successful outcome. As most deaths from VF occur out of hospital, often in patients without pre-existent recognised cardiac disease, the major public health issue remains the widespread application of external defibrillation. This article therefore concentrates exclusively on transthoracic counter shock, while recognising clinically relevant recent developments in internal defibrillation that are applicable to a small proportion of the total population at risk.

Electrical defibrillation occurs when a sufficient mass of excitable cells is simultaneously depolarised by an adequate intracardiac current,1 thereby extinguishing activation fronts within a critical myocardial mass. A threshold current density must be attained within the myocardium to achieve defibrillation. The electrical circuit formed during defibrillation includes impedance of two electrode–electrolyte–tissue interfaces and of the intervening tissues. The main determinants of intracardiac current flow during electric counter shock are the energy selected and transthoracic impedance (TTI) of the patient. The development of methods to predict TTI in advance of a defibrillatory counter shock2 has been a significant aid, enabling novel techniques for defibrillation to emerge and closer examination of factors involved in determining the success or failure of counter shocks.