Current and energy in external cardiac defibrillation.

External (Transthoracic) defibrillation is achieved by passing a large uniaxial current through the chest for a brief period of time. This current is determined by 1) the applied voltage and 2) the transthoracic impedance (TTI). In modern defibrillators the source of voltage is universally from a charged capacitor, the discharge waveform being modified in some cases by the inclusion of an inductor in the discharge circuit (Lown & Edmark waveforms, fig. 1), in others by direct discharge of a capacitor, such discharge being electrically truncated after a given period of time (Truncated Exponential, fig. 1). Although it is current that is responsible for successful defibrillation, defibrillator output is most commonly measured in units of energy (Joules) which is easily calculated knowing the voltage to which the defibrillation capacitor has been charged. Recent measurements of TTI show wide variations from 28 to 150 ohms3. Attempts at defibrillation (assuming the same energy setting) will thus result in a wide range of delivered currents. It is known that high defibrillation currents produce myocardial damage, conversely, currents which are too low will fail to achieve defibrillation. There is increasing evidence to suggest that defibrillators employing truncated exponential (trapezoidal) waveforms may be ineffective in subjects having high TTI. Additionally, there remains a need for a "smart" defibrillator which can calculate pre-discharge TTI and automatically adjust delivered current such that it is neither too low nor too high for the patient undergoing defibrillation.