Hyperaemia evaluation in clinical diathermy by four-electrode impedance measurements

Abstract

The four-electrode electrical impedance measurement technique is proposed for the evaluation of the hyperaemia variation in tissues treated by diathermic therapy. An impedance meter suitable for such measurements is described, and an electrical model of the heated tissues, concerning the impedance variation during diathermy and its relation with hyperaemia, is presented. The occurrence of the substantial contribution of blood to the overall transverse impedance is demonstrated by comparing the experimental results with those arising from a 2D electrical/thermal model of the treated tissues. A two-admittance model is proposed to explain the electrical behaviour of the tissues treated by diathermy. The model allows us to separate the impedance violation due to the temperature dependence of tissue conductivity from that due to the change of tissue blood content. The results of preliminary measurements of tissue impedance on healthy volunteers treated by electromagnetic diathermy are presented and discussed, showing the feasibility of impedance detection of hyperaemia variations inside tissues.

An electrical model of biological tissues undergoing hyperaemia

“…The model introduced by Olmi et al (1997) assumes that the overall tissue admittance consists of two contributions acting in parallel. For a power pulse beginning at t = 0 and lasting t = t dia min, indicating by Y t the contribution due to the temperature variation of the tissue conductivity and by Y b the contribution that depends on the blood content, a slight modification of the formulae given in the paper cited above gives…”

“…The above model has been tested on several tens of healthy volunteers treated by various modalities (electromagnetic diathermy, mild hyperthermia, infrared, laser). The impedance variation following the treatment was measured on every volunteer by a four-electrode impedance meter operating at 12 kHz; a detailed description of the instrument and of the experimental cases is reported in Olmi et al (1997). Apart from differences coming from the chosen anatomical site (upper arm, neck), from interpersonal variations and, above all, from the heating modality, the time behaviour of the admittance variation presents the above two contributions, as shown in the case reported in figure 1.…”

“…The overall variation of tissue conductivity during and after diathermy is due to two contributions, one dependent on the temperature and the other, analysed in the previous section, dependent on the variation of blood content. According to the model developed in Olmi et al (1997) and briefly outlined in section 2.1, the relative variation of tissue conductance and, consequently, that of tissue conductivity can be expressed as follows…”

“…A method recently proposed by the authors, based on four-electrode impedance measurements, allowed the qualitative estimation of the hyperaemic response of tissues subjected to diathermic therapy (Olmi et al 1997). The relevant physiological mechanisms involved in hyperaemia have also been investigated (Spiazzi et al 1996) and a model of the microcirculatory system has been developed.…”

Noninvasive measurement of transdermal drug delivery by impedance spectroscopy

“…Conversely, the measurement of bioimpedance is widely considered as a very-low invasive investigation method, extensively exploited both in biomedical research in general, and particularly also in dermatology: to limit reproducibility problems in the context of ex – vivo penetration studies for topical applied drugs10; to measure variations of the hydration state of the horny layer11, and, in this way, to predict its permeability12; to differentiate the interaction of diverse pharmacological principles with the skin tissue13; to allow early detection of pressure ulcers in vivo 14; to measure the blood flow caused by treatment heat transfer15; and, last but not least, for cancer diagnosis16. As a first approximation, a tissue can be considered as an electrolyte containing densely packed cells.…”