To establish a noninvasive method for measuring upper airway critical closing pressure (Pcrit), so as to evaluate collapsibility of the upper airway during sleep.

Pcrit was determined through the use of a noninvasive positive/negative pressure (CPAP/CPNP) ventilator(with independent intellectual property rights) during stageⅡ of non-rapid eye movement sleep. For the direct measurement, Pcrit was the pressure below which the upper airway occluded. For the indirect measurement, nasal pressure was plotted against maximum inspiratory flow (V_imax), and linear regression was used to interpolate the pressure (i.e., Pcrit) at which zero flow occurred. Pcrit was attained from 19 subjects without obstructive sleep apnea syndrome(OSAS), and the correlation between direct and indirect measurement methods was analyzed.

Directly measured and indirectly measured Pcrit showed no significant difference [(-7.02±2.74 vs (-7.26±2.96) cmH₂O, 1 cmH₂O=0.098 kPa; t=1.667, P>0.05] and had a highly significant correlation (r=0.986, P=0.000). Bland-Altman analysis revealed that the mean between-method difference was (0.24±0.53) cmH₂O, and 95% limits of agreement ranged from -0.80 to 1.27 cmH₂O, and all points except one were within limits of agreement.

Pcrit derived from the direct and indirect measurement methods does not differ, and both methods could be used for evaluating the upper airway collapsibility.

Critical closing pressure; Obstructive sleep apnea syndrome; Direct measurement method; Indirect measurement method