Phase tracking of the breathing cycle in sleeping subjects by frequency analysis of acoustic data

We tested the hypothesis that the inspiratory and expiratory phases of breathing could be identified from breath sound recordings during sleep. Breath sounds were digitally recorded from 10 subjects during sleep. Frequency spectra of inspiration and expiration were determined. The ratio of frequency magnitude bins between 400-1000 Hz to frequency bins between 10-400 Hz was calculated for inspiration (Ri) and expiration (Re) for each breath. The Ri/Re ratio was significantly greater than the thresholds of 1.5 (p < 0.001) and 2-fold (p < 0.001). Breathing phases were correctly identified in 90% and 73% of cases using the 1.5 and 2.0 thresholds, respectively.