Abstract
Electrical impedance method has been used for measurements of various biological parameters in a noninvasive manner. In the present study, the relationship between the thoracic impedance and the thoracic air volume was quantitatively analyzed in human using four-electrode system. A constant current excited from an oscillator (500 or 50kHz) was injected between a pair of electrodes on the bilateral thorax walls. The impedance between another pair of electrodes was determined assuming that the voltage drop between those two electrodes is linear to the variations of the corresponding impedance change. The impedance was stably recorded without high-pass filtering when the postural changes of the examinees in a sitting position were minimum. The thoracic impedance thus measured contained components attributable to the variation of air volume whose frequency was normally far below 1Hz and blood volume whose frequency was around 1.3Hz. The ration between these two components largely depended on the positions of the electrodes. Basal thoracic air volume was changed intensively to various given levels. Then lung volume was further changed within a given range by inspiring or expiring air from a thin polyethylene bag with a given volume (500~4000 ml). The impedance change thus recorded varied linearly to the thoracic air volume when the four electrodes were set within restricted areas. The advantage and disadvantage in the assessment of thoracic air volume using electrical impedance method will be discussed. [Jpn J Physiol 54 Suppl:S203 (2004)]
