Historically defibrillators have used large voltages given in different "shapes" to treat (terminate) ventricular fibrillation. The standard wave forms are: Edmark (Critically damped capacitor discharge) and Lown (Underdamped capacitor discharge). There are other wave forms that are in use for both internal and external defibrillation. Examples of these are monophasic rectangular, monophasic truncated exponential, biphasic truncated exponential, triphasic truncated exponential. It is emerging that biphasic defibrillation waves are highly effective and easier for circuits to produce. This equates to smaller devices and better battery life. In addition to delivering a biphasic wave form to the patient, the "Heartstream" device measures the patients transthoracic impedance and delivers an exact dose. Contrary to what the sales / marketing people tell you, at this time it is not known conclusively which waveform is the most effective in treating Vf. Do not buy a defibrillator solely on the biphasic hype!
Updated: March 19, 1999