Measurement of upper airway movement by acoustic reflection.
The acoustic reflection method provides a noninvasive way to determine the airway geometry. Based on a discrete upper airway model, an inverse scheme is developed to infer the upper airway area as a function of distance. We incorporate this scheme into a system that can generate multiple acoustic pulses to sample the upper airway geometry at a maximum frequency of 30 Hz, making possible determination of the airway area-distance relation as a function of time. Therefore, we can monitor the dynamic behavior of the upper airway during breathing. To validate the approach, we visualized vocal cord movements in three normal subjects via laryngoscopy; simultaneously acoustic measurements were made at 10 Hz. Video images of vocal cord movement were recorded and digitized. We compared the laryngeal area from analysis of the video images with the acoustic assessment at the level of the glottis. Linear regression analysis shows that the correlation coefficients are between 0.85 and 0.9 for all three subjects. We conclude that the acoustic reflection method is a useful tool for measuring vocal cord movement without the use of laryngoscopy, and the approach promises to be a useful one to measure the movement of the whole upper airway. This paper also discusses the limitations inherent in the algorithm and some useful procedures to ensure accurate and reliable area computation during implementation.