The acoustic reflection technique for non-invasive assessment of upper airway area

Non-invasive assessment of upper airway area by acoustic reflections has been developed in the last 12 yrs. The technique is based on the analysis of sound waves reflected from the airways. Measurement of the amplitudes of the reflections and their times of arrival at the sensing microphone permits construction of a plot of airway area vs distance from the microphone. We describe the theoretical foundations of the method and review the underlying assumptions. This is followed by a summary of the results of in vitro and in vivo validation studies, with particular attention to the reproducibility, accuracy and variability of the technique. The description of clinical and physiological applications of this technique includes detection of tracheal stenosis, demonstration of structural and functional abnormalities of the pharynx and glottis in patients with sleep apnoea, dichotomous response of the airway area to exercise, and airway vs parenchymal hysteresis. Finally, we comment on the future directions that might be investigated using this technique.