Electrodeless Measurements of the Effective Resistivity of the Human Torso and Head by Magnetic Induction

A magnetically coupled impedance measuring instrument has been developed for determining an effective electrical resistivity of human subjects without electrodes. The instrument operates at 100 kHz. The value obtained represents an average of the resistivity of tissue near the coils. The human torso and the head were found to act, in regard to this measurement, as if they had homogeneous resistivities of about 425 and 350 Ωcm respectively. Thoracic resistivity fluctuations due to respiration (tidal volume) and pulsatile blood flow were about 4 percent (breathing) and 1 percent (blood pumping). These measurements were taken when the subjects were lying face down with their hearts directly over the coil system. The records resemble in shape typical ventricular stroke-volume curves provided that the subject holds his breath at maximum level of inspiration. Various tests were performed which indicate that the records reflect predominantly the changing volume of the heart. Peak-to-peak effective resistivity fluctuations of the head during the cardiac cycle were found to be less than 0.05 percent.