Pcrit (critical closing pressure)
A measure of the pressure at which the throat closes during sleep. The higher the number, the more easily the throat closes.
A little more detail
Passive critical closing pressure (Pcrit) is the nasal pressure at which the upper airway closes when muscle activity is taken out of play, expressed in cmH2O; it is measured in the laboratory by briefly lowering the pressure given through the nose. In the 75-person study by Eckert and colleagues, Pcrit was 0.3 cmH2O in patients with OSA and −6.2 cmH2O in controls. Even so, in 19% of patients the airway was relatively hard to collapse, similar to controls (−2 to −5 cmH2O). In 56% of patients, non-anatomical traits such as muscle response, arousal threshold and ventilatory control also played an important role.
Source: Eckert DJ, White DP, Jordan AS et al. Defining phenotypic causes of obstructive sleep apnea. Identification of novel therapeutic targets. Am J Respir Crit Care Med. 2013;188(8):996-1004. doi:10.1164/rccm.201303-0448OC; Gold AR, Schwartz AR. The pharyngeal critical pressure. The whys and hows of using nasal continuous positive airway pressure diagnostically. Chest. 1996;110(4):1077-1088. doi:10.1378/chest.110.4.1077