Non-contact respiratory monitoring with a bioelectric impedance technique to detect abnormal respiration during bathing

Preliminary theoretical calculations and examinations were carried out to develop a method of non-contact respiratory monitoring during bathing to detect abnormal respiration using the bioelectric impedance (BEI) technique. First, theoretical calculations regarding the frequency-dependence of the impedance amplitude were made using a two-dimensional model consisting of a circular body and bath water surrounding the body. The body consisted of concentric layered circles representing the lung and the body fluid. The calculations showed that increases in chest size due to inspiration cause a decrease in the impedance amplitude in the frequency range from 1 kHz to 100 MHz. Next, BEI measurements were carried out from 4 to 400 kHz with the pair of electrodes placed in the vicinity of the chest at the maximumend-expiration and maximum-end-inspiration stages. These provided results consistent with those obtained from the theoretical calculations. Third, the time-dependence of the impedance amplitude was measured at 10, 100, and 400 kHz, respectively. At all of these frequencies, the impedance amplitude corresponded well with the respiratory waveform obtained from rubber strain gauge plethysmography, which was used as a reference.