Multilead impedance pneumography and forced oscillation technique for assessing lung tissue mechanical properties

Forced oscillation technique measuring transfer impedance (Ztr) is a traditional method to assess mechanical properties of the respiratory system. Lung tissue and airways mechanical properties are of clinical relevance, but difficult to distinguish from the Ztr measurand. We propose a novel method that replaces the plestymographyc chamber in Ztr by a multilead impedance pneumography (MLIP) system. We hypothesise that mechanical properties of the lung tissue should be more evident in the new method that in Ztr . We applied pressure oscillations at the mouth from 2 to 27 Hz in one sitting healthy subject holding breath at residual volume. Thoracic electrical resistivity changes where simultaneously recorded at three different heights of the thorax by a MLIP system. Lung volume changes were extract from the MLIP signals by subtracting the cardiac component calculated by means of ensemble averaging. Consequently, mechanical impedances at the three thorax levels were calculated as the complex ratio of mouth pressure (Pao) and the time integral of the lung volume MLIP signals (Q̇IP ): ZIP = Pao/Q̇IP . We found a decrease in the real part of ZIP from the upper to the lower areas of the chest. This is coherence with the known pleural pressure gradient distribution. Moreover, we observed ZIP to share similarities with Ztr at low frequencies, (i.e. same resonance frequency). However, ZIP for the upper and lower regions of the thorax differed greatly with Ztr for frequencies over 10 Hz. Results need deeper investigation and larger clinical study samples. Nevertheless, this preliminary study shows the potential of ZIP for assessing lung tissue mechanical properties.