Monitoring temperature-induced changes in tissue during hyperthermia by impedance methods.

The electrical conduction in living tissue depends on temperature in two ways: (1) the temperature coefficients of conductivity of the intra- and extracellular electrolytes and (2) temperature-induced fluid volume shifts in the tissue. Measurements in rat skeletal muscle and tumors (DS sarcoma) during hyperthermic treatment reveal that the contribution of fluid volume shifts to changes in conductance is of the same order of magnitude as the change in fluid conductivity. In skeletal muscles, blood volume changes are caused by the temperature-dependent regulation of the vessel diameter (vasodilatation). In tumors, fluid content changes irregularly. These effects render temperature measurements by impedance methods, for example, electrical impedance tomography (EIT), questionable. However, monitoring fluid volume changes in tissue and the state of cell membranes is an interesting application of impedance (or admittance) spectroscopy and tomography as well.