Possibilities for predictive measurement of the transthoracic impedance in defibrillation

Abstract

Transthoracic electrical defibrillation is administered by high voltages and currents applied through large size electrodes. Therefore, the defibrillator load impedance becomes an essential factorfor the efficacy of the procedure. Attempts at prediction of transthoracic impedance by pre-shock measurement with low-amplitude high-frequency current have yielded apparently promising results. A reassessment was undertaken of the comparison between transthoracic impedance measured over a wide frequency range (bioimpedance spectroscopy) and measured during the shock. An estimation of the possibilities for pre-shock 'prediction ' of the impedance was performed, to allow adequate selection of the defibrillation energy or current with the intention of increasing the possibility for positive results with the first shock. Data were obtained from experimental fibrillation/defibrillation cycles on dogs andfrom cardioversion of atrial fibrillation or flutter in patients. The final results suggest that high-frequency low-amplitude impedance measurements cannot predict the corresponding value during the shock with very high accuracy, as differences up to 15-17% were found using biphasic pulses in patients. However, the method can be used for approximate assessments.

Transthoracic impedance study with large self-adhesive electrodes in two conventional positions for defibrillation

“…This fact suggested that the antero-posterior position of the electrodes is favourable for defibrillation since the lower TTIs are associated with lower energy for successful defibrillation. The observed TTI measurements with self-adhesive electrodes (active area of about 92 cm 2 ) are slightly higher than the reported results for 65 cm 2 hand-held paddles, which were interfaced to the skin by standard saline soaked pads in the sub-clavicular/subaxillar position (Krasteva et al 2001). This TTI spectroscopy study with a high-frequency low-amplitude current indicated that at 30 kHz the TTI range is between 76 and 108 (mean of about 92 ).…”

“…Prediction equations were developed based on a statistical comparison between the pre-shock TTI measurements with low-amplitude high-frequency current (20-100 kHz) and the apparent impedance measured during the shock. These equations were obtained by dividing the peak applied voltage to the peak current (Geddes et al 1976, Geddes 1994, Krasteva et al 2001. Although the results show high correlation, the individual differences (up to 15-17%) suggest that this approach should be considered with caution for approximate assessments and pre-settings of the adequate shock energy or current.…”