Eddy Current Based Flexible Sensor for Contactless Measurement of Breathing

Changes in the lung air volume change the conductivity distribution in the thorax. Based on this effect, we describe a non-contact instrument to measure lung volume which is suitable for applications such as monitoring of breathing in sleeping subjects for sleep apnea. The instrument is based on a magnetic coil and is positioned in a flexible mattress underneath the subject. AM frequency current is applied to the coil to generate eddy currents in nearby conductive tissues, which in turn generate a magnetic field opposed to the transmitted field. This effect makes changes in the thoracic conductivity distribution due to breathing and blood flow change the effective inductance of the coil. We describe a new design to measure this effect using a single transmit and receive coil. The coil forms part of a Colpitts oscillator tuned to be at its stability margin, in order to give large frequency changes for given changes in conductivity. The oscillator output is then demodulated using a frequency counter. This device was tested in healthy volunteers and showed a good correlation with pneumotachograph measurements