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. Author manuscript; available in PMC: 2012 Nov 14.
Published in final edited form as: Eur Respir J. 2009 Dec 23;36(3):569–576. doi: 10.1183/09031936.00153809

FIGURE 2.

FIGURE 2

Method of calculation of upper airway critical closing pressure (Pcrit) during stage II sleep by linear regression in one volunteer. Peak air flow during flow-limited breathing is plotted as a function of mask pressure. Throughout the study night, 12 negative-pressure ramps were performed, while flow limitation was observed. Values derived from 31 flow-limited breaths were used for analysis and extrapolated to Pcrit (mask pressure at zero flow) by linear regression. Note that, at a given mask pressure, peak flow during flow-limited breathing varied throughout the overnight study, suggesting that the balance between the collapsing and dilating forces acting at the upper airway varied throughout the night.