Transthoracic electrical impedance during external defibrillation: comparison of measured and modelled waveforms

Abstract

The transthoracic electrical impedance is an important defibrillation parameter, affecting the defibrillating current amplitude and energy, and therefore the defibrillation efficiency. A close relationship between transthoracic impedance and defibrillation success rate was observed. Pre-shock measurements (using low amplitude high frequency current) of the impedance were considered a solution for selection of adequate shock voltages or for current-based defibrillation dosage. A recent approach, called 'impedance-compensating defibrillation' was implemented, where the pulse duration was controlled with respect to the impedance measured during the initial phase of the shock. These considerations raised our interest in reassessment of the transthoracic impedance characteristics and the corresponding measurement methods. The purpose of this work is to study the variations of the transthoracic impedance by a continuous measurement technique during the defibrillation shock and comparing the data with results obtained by modelling. Voltage and current impulse waveforms were acquired during cardioversion of patients with atrial fibrillation or flutter. The same type of defibrillation pulse was taken from dogs after induction of fibrillation. The electrodes were located in the anterior position, for both the patients and animals.

Possibilities for predictive measurement of the transthoracic impedance in defibrillation

“…This was previously shown and recently reported by Tacker and Geddes [13] and by Hatib et al [14]. Therefore, we called it 'apparent resistance' R p , which is similar but different from the 'apparent impedance' Z AP =V PEAK / I PEAK introduced by Geddes et al [7].…”

“…It can be speculated that the two highenergy waveforms have a different effect on the tissues during the passage of the current, which thus provokes the formation and the meeting of different resistances. The nonlinearities of the transthoracic resistance during the shock [14] might play a role in this process. However, excluding the initial rising part of the de brillation pulse, these changes were only a few percent with respect to the mean R p value obtained by the tting procedure, as shown in the Method section above.…”

Estimation of current density distribution under electrodes for external defibrillation

“…1a), simulating the electrode metal, the electrode-skin interface and part of a human body. All structures are modeled as purely resistive, as it has been shown that in defibrillation there was no phase difference between applied voltage and current passed through the thorax [19,20]. …”

Current is better than energy as predictor of success for biphasic defibrillatory shocks in a porcine model of ventricular fibrillation

“…The TTI of human adult ranges from 30 to 200 , 15,21,22 while the TTI of experimental pigs is in the range of 30 to 70 , that covers only a small part of human's range. 19,23 To simulate the full spectrum of human's TTI, a junction box, composed by a defibrillation source switch, a cascade adjustable load resistance , in steps of 10 ) and a rescue shock push button, was placed in series with the defibrillation pads, in a circuit previously described.…”