Application of quartz tuning forks in multiple-breath-helium washout measurement

Multiple-breath-washout (MBW) measurements are regarded as a sensitive technique for lung function studies which can reflect the ventilation inhomogeneity of our respiratory airways [7,8,9]. Typically nitrogen is used as the tracer gas and is washed out by pure oxygen in multi-breath-nitrogen (MBNW) washout tests. In this work, instead of using nitrogen, 4He is used as the tracer gas. A commercial quartz tuning fork with a resonance frequency of f-32 kHz has been used for detecting the concentration of 4He gas at room temperature. This low-cost sensor has been used previously for detecting the gas density [11]. The resonance frequency of the tuning fork decreases linearly with increasing density of the surrounding gas and we use this effect to determine the helium concentration. The resulting washout curve from an asthmatic volunteer has smaller curve linearity compared to a normal healthy person which reflects the higher ventilation inhomogeneity of the respiratory airways.