Figure 2. Sample record demonstrating the technique used to determine placement of the nasopharyngeal catheter, and to establish P_crit.

In the left panel, the tip of the nasal catheter was placed downstream, or caudal, to the flow-limiting segment (FLS) of the pharyngeal airway. Note that once flow limitation occurs (i.e. V̇_I,max), P_n and P_hp continue to drop in parallel. When the nasal catheter is moved upstream (i.e. rostral) to the FLS (right panel), nasal pressure (P_n) nadirs at V̇_I,max. This nadir in P_n associated with the onset of flow limitation is defined as P_crit. The nasopharyngeal catheter was placed upstream to the FLS during all experiments (see text for explanation).