Mimicking a flow-limited human upper airway using a collapsible tube: relationships between flow patterns and pressures in a respiratory model.

The upper airway (UA) in humans is commonly modeled as a Starling resistor. However, negative effort dependence (NED) observed in some patients with obstructive sleep apnea (OSA) contradicts predictions based on the Starling resistor model in which inspiratory flow is independent of inspiratory driving pressure when flow is limited. In a respiratory bench model consisting of a collapsible tube and an active lung model (ASL5000), inspiratory flow characteristics were investigated in relation to upstream, downstream, and extra-luminal pressures (denoted as P, P, and P, respectively) by varying inspiratory effort (muscle pressure) from -1 to -20 cmHO in the active lung. P was provided by a constant airway pressure device and varied from 4 to 20 cmHO, and P was set at 10 and 15 cmHO. Upstream resistance at onset of flow limitation and critical transmural pressure (P) corresponding to opening of the UA were found to be independent of P, P, and P. With fixed P, when P fell below a specific value (P'), inspiratory peak flow became constant and independent of P. NED plateau flow patterns at mid-inspiration (V̇) were produced within the current bench setting when P fell below P'. V̇ was proportional to P, and the slope (ΔV̇/ΔP) increased linearly with P. P and P were the two final independent determinants of inspiratory flow. Our bench model closely mimics a flow-limited human UA, and the findings have implications for OSA treatment and research, especially for bench-testing auto-titrating devices in a more physiological way. NEW & NOTEWORTHY A respiratory model consisting of a collapsible tube was used to mimic a flow-limited human upper airway. Flow-limited breathing patterns including negative effort dependence were produced. Transmural and downstream pressures acting on the tube are the two independent determinants of the resulting inspiratory flow during flow limitation. The findings have implications for obstructive sleep apnea treatment and research, especially for bench-testing auto-titrating devices in a more physiological way.