Study objectives: Previous studies have shown that the level of flow through the upper airway in patients with obstructive sleep apnea (OSA) is determined by the critical closing pressure (Pcrit) and the upstream resistance (RN). We developed a standardized protocol for delineating quasisteady-state pressure-flow relationships for the upper airway from which these variables could be derived. In addition, we investigated the effect of body position and sleep stage on these variables by determining Pcrit and RN, and their confidence intervals (CIs), for each condition.
Design: Pressure-flow relationships were constructed in the supine and lateral recumbent positions (nonrapid eye movement [NREM] sleep, n = 10) and in the supine position (rapid eye movement [REM] sleep, n = 5).
Setting: University Hospital Antwerp, Belgium.
Patients: Ten obese patients (body mass index, 32.0+/-5.6 kg/m(2)) with severe OSA (respiratory disturbance index, 63.0+/-14.6 events/h) were studied.
Interventions: Pressure-flow relationships were constructed from breaths obtained during a series of step decreases in nasal pressure (34.1+/-6.5 runs over 3.6+/-1.2 h) in NREM sleep and during 7.8+/-2.2 runs over 0.8+/-0.6 h in REM sleep.
Results: Maximal inspiratory airflow reached a steady state in the third through fifth breaths following a decrease in nasal pressure. Analysis of pressure-flow relationships derived from these breaths showed that Pcrit fell from 1.8 (95% CI, -0.1 to 2.7) cm H(2)O in the supine position to -1.1 cm H(2)O (95% CI, -1.8 to 0.4 cm H(2)O; p = 0.009) in the lateral recumbent position, whereas RN did not change significantly. In contrast, no significant effect of sleep stage was found on either Pcrit or RN.
Conclusions: Our methods for delineating upper airway pressure-flow relationships during sleep allow for multiple determinations of Pcrit within a single night from which small yet significant differences can be discerned between study conditions.