Measuring upper and lower airway resistance during sleep with the forced oscillation technique

The forced oscillation technique (FOT) is a non-invasive technique to monitor airway obstruction in those with asthma. The aim of this study was to design and validate a system to use FOT during sleep, both with and without bi-level positive airway pressure (BPAP), and to separate upper airway resistance from lower. 8 Hz pressure oscillations were supplied, over which the subject breathed, pressure and flow measurements were then used to calculate impedance. A phase-shift induced by the pressure transducer tubing was characterized, and FOT resistance was compared to steady flow resistance both with and without BPAP. A Millar catheter was used to measure pressure at the epiglottis, allowing the separation of upper from lower airway resistance. A phase shift of -0.010 s was calculated for the pressure transducer tubing, and the average error between FOT and steady flow resistance was -0.2 ± 0.2 cmH₂O/L/s without BPAP and 0.4 ± 0.2 cmH₂O/L/s with BPAP. The system was tested on three subjects, one healthy, one with obstructive sleep apnea, and one with asthma. The FOT was well tolerated and resistance was separated into upper and lower airway components. This setup is suitable for monitoring both upper and lower airway obstruction during sleep in those with and without asthma.