Changes in electrical impedance of skeletal muscle measured during hyperthermia.

The electrical impedance of rat skeletal muscle was measured from 100 Hz to 40 MHz during the application of typical hyperthermia heating regimens. Trials were performed employing freshly excised tissue heated to target temperatures from 39.5 to 50 degrees C. Abrupt and rapid decreases in the low-frequency beta-dispersion occurred very shortly after reaching hyperthermia temperatures. These rapid decreases continued at a rate and to an extent dependent upon the target temperature, and then, as heating continued, abruptly changed to a much slower rate which continued indefinitely. The initial rapid changes were associated with microscopically observed muscle fibre rounding and radial shrinkage, with accompanying increasing interstitial oedema. The subsequent slow changes were associated with a slow histolysis. The time- and temperature-dependence of the rapid resistivity changes evidenced similarities to typical hyperthermia endpoint responses. An Arrhenius analysis of the rate of the resistivity changes yielded a break at 43 degrees C, with activation energies of 36.1 and 58.3 kcal/mol above and below this break. Preliminary in vivo impedance data displayed qualitative similarities to the excised tissue findings.