Determining transthoracic impedance, delivered energy, and peak current during defibrillation episodes

A simplified method has been developed to determine peak current, transthoracic impedance, and delivered energy during a damped sinusoidal defibrillation pulse. The discharge waveform information is generated from sampling the peak discharge current through a current transformer and measuring the voltage stored on the energy storage capacitor. For a given defibrillator circuit a unique relationship exists between the peak discharge current IM and the unknown external impedance Rext presented to the defibrillator by the patient; hence, measurement of IM allows calculation of Rext. A microprocessor-controlled algorithm provides delivered energy information using known internal resistance, capacitance, and inductance parameters. The benefit of this method of delivered energy calculation is that the current and voltage waveforms need not be digitized and then integrated to provide the desired information. This method also keeps defibrillation circuitry ground isolated and simplifies operation through the high electromagnetic fields generated during the discharge. The delivered energy information, along with time, date, and other patient parameters, is documented automatically with an annotation strip-chart recorder.