Flow-dependent positive airway pressure to maintain airway patency in sleep apnea-hypopnea syndrome

Airway obstruction in patients with sleep apnea-hypopnea syndrome (SAHS) is due to increased critical pressure (P-crit) of the upper airway. The ideal nasal pressure (P-n) to maintain airway patency should consist of the constant term to account for P-crit and a term (R-n.(V) over dot ) proportional to flow ((V) over dot ) to account for the dynamic pressure drop through nasal resistance (R-n). Continuous positive airway pressure (CPAP) applied to avoid flow limitation results in a P-n greater than required over most of the breathing cycle. The aim was to assess a flow-dependent positive airway pressure (FDPAP) based on adapting P-n to the instantaneous flow: P-n = P-0 + k.(V) over dot . FDPAP was tested on collapsible airway models and its applicability was assessed in nine patients with SAHS during sleep. In models, FDPAP prevented flow limitation with lower mean P-n and work of breathing than CPAP. In patients FDPAP allowed the patients to breathe normally with a mean P-n (6.6 +/- 1.2 cm H2O) systematically and significantly (p < 0.05, paired t test) lower than when applying CPAP (9.1 +/- 1.2 cm H2O). The results found in models and in patients suggest that adapting the applied nasal pressure to the instantaneous breathing flow may be of potential practical interest in SAHS.