Contactless measurement of thoracic conductivity changes by magnetic induction

A noninvasive technique of measuring thoracic conductivity changes caused by respiration and cardiac activity is described. Conductivity is measured with a hand-held sensor comprising a planar coil, a radio-frequency (RF) oscillator and a closed-loop control system to hold constant the amplitude of oscillations. This system automatically compensates for changes in the power dissipated by eddy-currents in the thorax by adjusting the RF current driven into the coil. This results in the control signal being proportional to the conductivity of the thorax. Simulations with a thorax model consisting of four eccentric spheres were used to determine the magnitude of changes for a range of tidal volumes and stroke volumes. The importance of tissue properties, such as lung conductivity and hematocrit was assessed. Finally, an example of signals recorded in-vivo with the sensor is presented. Use of this technique to monitor cardiac output would require a one-time measurement of geometrical data specific to the patient's thorax (e.g. the distance between the heart and the body-surface), in order to transform measured cardiosynchronous conductivity changes into stroke volume.