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Static and Dynamic Upper Airway Obstruction in Sleep Apnea

Role of the Breathing Gas Properties

Unitat de Biofísica i Bioenginyeria, Facultat de Medicina, Universitat de Barcelona; and Servei de Pneumologia i Al.lèrgia Respiratòria, Hospital Clinic Provincial, Institut d'Investigacions Biomèdiques August Pi Sunyer, Barcelona, Spain

Correspondence and requests for reprints should be addressed to Ramon Farré, Ph.D., Unitat de Biofisica i Bioenginyeria, Facultat de Medicina, Casanova 143, E-08036 Barcelona, Spain. E-mail: rfarre{at}ub.edu

Increased upper airway collapsibility in the sleep apnea/hypopnea syndrome (SAHS) is usually interpreted by a collapsible resistor model characterized by a critical pressure (P_crit) and an upstream resistance (R_up). To investigate the role played by the upstream segment of the upper airway, we tested the hypothesis that breathing different gases would modify R_up but not P_crit. The study was performed on 10 patients with severe SAHS (apnea–hypopnea index: 59 ± 14 events/hour) when breathing air and helium–oxygen (He–O₂) during non-REM sleep. The continuous positive airway pressure that normalized flow (CPAP_opt) was measured. R_up and P_crit were determined from the linear relationship between maximal inspiratory flow _Imax and nasal pressure (P_N):_Imax = (P_N - P_crit)/R_up. Changing the breathing gas selectively modified the severity of dynamic (CPAP_opt, R_up) and static (P_crit) obstructions. CPAP_opt was significantly (p = 0.0013) lower when breathing He–O₂ (8.44 ± 1.66 cm H₂O; mean ± SD) than air (10.18 ± 2.34 cm H₂O). R_up was markedly lower (p = 0.0001) when breathing He–O₂ (9.21 ± 3.93 cm H₂O·s/L) than air (15.92 ± 6.27 cm H₂O·s/L). P_crit was similar (p = 0.039) when breathing He–O₂ (4.89 ± 2.37 cm H₂O) and air (4.19 ± 2.93 cm H₂O). The data demonstrate the role played by the upstream segment of the upper airway and suggest that different mechanisms determine static (P_crit) and dynamic (R_up) upper airway obstructions in SAHS.

Key Words: obstructive sleep apnea • hypopnea • flow limitation • continuous positive airway pressure • breathing gas density

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