Measurements of total-body electrical impedance in the frequency range between 200 Hz and 300 kHz were performed on 37 diabetic patients undergoing chronic haemodialysis. Special attention was paid to the instrument design, where a self-balancing differential current source was used, reducing considerably the common-mode voltage at the amplifier input. The patient-instrument interface includes screened leads, separately driven by unity-gain buffers. The measurement error was < 1% for the impedance within the range of 20 to 1000 omega and < 0.3 degree (mean) for the phase angle. Impedance/phase and ultrafiltration measurements were carried out throughout the entire procedure. Total and extracellular water were computed and compared with extracted fluid volumes. The trends of change of the extracellular and intracellular fluid volumes during and immediately after dialysis corresponded to the respective clinical condition of the patients and enabled us to divide them into four groups. This approach is a step toward continuous monitoring and adaptive treatment, tailored to the individual patient needs.

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.

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.

No abstract