Nasal CPAP in obstructive sleep apnea: mechanisms of action.

Sixteen patients with the obstructive sleep apnea syndrome (OSAS) were studied for 1-2 h while receiving continuous positive airway pressure (CPAP) delivered via a nasal mask. Obstructive apneas were obliterated in all. Eight patients had studies of genioglossal muscle activity (GG EMG) and one patient had computed tomograms (CT) of the upper airway while on nasal CPAP. The GG EMG studies showed two patterns: suppression and augmentation of GG EMG while on CPAP. The CT scan showed the airway to be narrowed while the patient was awake off CPAP. It returned to a normal caliber when CPAP was applied, despite sleep. These results are interpreted to suggest three potential mechanisms of action for nasal CPAP in OSAS: 1) reduced upper airway resistance due to prevention of sleep-induced collapse of the airway; 2) reduced upper airway resistance due to dilatation of the airway by nasal CPAP beyond its dimension in the awake state; and 3) possible stimulation of mechanoreceptors leading to an increase in airway tone while CPAP is applied.