Static and dynamic upper airway obstruction in sleep apnea -: Role of the breathing gas properties

increased upper airway collapsibility in the sleep apnea/hypopnea syndrome (SANS) 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 SANS (apnea-hypopnea index: 59 +/- 14 events/hour) when breathing air and helium-oxygen (He-O-2) during non-REM sleep. The continuous positive airway pressure that normalized flow (CPAP,P,) was measured. R-up and P-crit were determined from the linear relationship between maximal inspiratory flow V-Imax and nasal pressure (P-N):V-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-O-2 (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-O-2 (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-O-2 (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 SANS.