Figure 1. Diagram illustrating the concept that the site of flow limitation may be different from the site of airway collapse.

In a hypothetical patient, the nasal valve is the site of flow limitation, but the airway collapses at the soft palate and tongue. (A) In early inspiration most of the pressure drop occurs in the rigid nasal cavity due to a constriction at the nasal valve. (B) Once the local pressure falls below the buckling pressure of each anatomical structure, the airway collapses at that location, which further increases the pressure loss along the upper airway. Symbols: P_atm = atmospheric pressure; P_{b palate} = buckling pressure of the soft palate; P_{b tongue} = buckling pressure of the tongue.