 Although other factors are important, the most important factor in OSA pathogenesis is upper airway collapsibility. The upper airway collapsibility can be determined during sleep when patients with OSA are on CPAP. The CPAP level is decreased until flow limitation and finally collapse is achieved. The pressure at which the airway collapses is called the pharyngeal critical closing pressure, or Pcrit. When measured in research laboratories, the Pcrit correlates with OSA severity. Subjects with a high Pcrit e.g. 5cm of water pressure typically have severe OSA, while those with a lower Pcrit e.g. 2cm of water pressure have mild OSA, and those with a negative Pcrit e.g. -4cm of water, the airway must be sucked closed to collapse typically have no disease. The Pcrit may also have important implications for treatment. For example, patients with OSA and a very high Pcrit may require CPAP, however, those with a lower or slightly negative Pcrit might be successfully treated by an oral appliance. Thus, knowledge of the Pcrit might be useful in clinical practice, but it is rarely measured outside of the research lab. 