Mechanism of CPAP
CPAP works primarily as a pneumatic splint that mechanically maintains upper airway patency by delivering continuous positive pressure throughout the respiratory cycle, preventing pharyngeal collapse during sleep. 1
How CPAP Functions
Physical Mechanism
CPAP devices are essentially air pumps (fan-driven or turbine systems) that draw in external, filtered air and deliver pressurized airflow through a nasal, oral, or oronasal interface. 1 The device delivers a single, fixed pressure continuously to the patient during the night, distinguishing it from other PAP modalities. 1
Physiological Effects
The primary mechanism is passive mechanical splinting of the upper airway, NOT reflex activation of upper airway muscles. This is a critical distinction supported by research showing that:
- When CPAP eliminates obstructive apneas, upper airway muscle EMG activity (genioglossal and alae nasi muscles) actually decreases or is eliminated rather than increases 2
- The reduction in muscle activity correlates with improved oxygen saturation, indicating CPAP acts as a pneumatic splint rather than triggering neuromuscular reflexes 2
- Mean inspiratory airflow increases for a given esophageal pressure despite decreased upper airway muscle activity, confirming mechanical airway splinting 3
Secondary Mechanisms
Beyond mechanical splinting, CPAP produces several additional effects:
- Increases functional residual capacity (FRC) and mean airway pressure 4, 3
- Recruits underventilated lung regions, similar to PEEP in mechanically ventilated patients 4
- Offsets intrinsic PEEP in COPD patients, reducing ventilatory work and potentially lowering PaCO2 4
- Unloads inspiratory muscles, reducing inspiratory work (though may worsen hyperinflation in some airflow obstruction cases) 4
Clinical Application
The mechanism translates to clinical practice through titration protocols where CPAP pressure is increased until obstructive respiratory events are eliminated: apneas, hypopneas, respiratory effort-related arousals (RERAs), and snoring. 1 The recommended starting pressure is 4 cm H2O, with typical therapeutic pressures ranging up to 15-20 cm H2O depending on patient age. 1
Important Caveat
While CPAP mechanically prevents airway collapse, it does not address the underlying pathophysiological abnormalities in OSA including impaired muscle responsiveness, arousability, or respiratory drive—it only suppresses respiratory disturbances while in use. 5, 6 This explains why adherence is critical and why some patients seek alternative therapies.