Continuous Positive Airway Pressure (CPAP) is the primary treatment for Obstructive Sleep Apnea (OSA), a condition where the upper airway repeatedly collapses during sleep. The machine delivers pressurized air through a mask to prevent these collapses, ensuring continuous breathing overnight. While CPAP’s direct action is on the upper airway, its effects on the respiratory system are far-reaching and lead to significant, though often indirect, improvements in how the lungs function. Understanding CPAP requires differentiating between its immediate mechanical action on the throat and its broader physiological consequences on gas exchange and breathing effort.
How CPAP Supports the Upper Airway
CPAP operates by delivering pressurized air into the patient’s airway, acting as a pneumatic splint to the soft tissues of the throat. The upper airway in people with OSA is prone to collapse when the muscles relax during sleep. The positive pressure counters the negative pressure generated by inhalation, which otherwise causes the pharyngeal walls to occlude.
This pressurized air keeps the airway open, eliminating the obstructions that define obstructive sleep apnea. By maintaining this patency, CPAP ensures that air flows freely from the mask to the lungs without interruption. This mechanism is purely mechanical, preventing the collapse of the extrathoracic airway.
CPAP does not chemically or structurally alter the lung tissue itself, such as the alveoli or bronchioles. Its direct therapeutic target is the obstruction point in the throat. This mechanical splinting sets the stage for the body’s entire respiratory system to function more efficiently by removing the primary roadblock to breathing.
Indirect Benefits to Breathing Mechanics
The resolution of nocturnal airway collapse leads to a stable exchange of oxygen and carbon dioxide throughout the night. Preventing apneas and hypopneas avoids the cyclical drops in blood oxygen saturation (hypoxia) that characterize untreated sleep apnea.
CPAP significantly reduces the muscular strain involved in breathing that occurs when a person attempts to inhale against a closed airway. During an apnea event, the chest and diaphragm muscles strain hard, creating strong negative pressure swings within the chest cavity. CPAP alleviates this intense respiratory effort, allowing the respiratory muscles to rest during sleep and normalizing the pressure within the chest.
For many individuals, the continuous positive pressure can also help stabilize and slightly increase the functional residual capacity (FRC). FRC is the volume of air remaining in the lungs after a normal exhalation. CPAP helps keep smaller airways and alveoli from collapsing at the end of a breath, particularly in those with obesity. Maintaining this higher lung volume can improve lung compliance, meaning the lungs are easier to inflate, which further reduces the work of breathing.
Addressing Respiratory Issues in Complex Conditions
While CPAP primarily treats the upper airway, its application in patients with coexisting lung or ventilation disorders provides more direct respiratory benefits.
Obesity Hypoventilation Syndrome (OHS)
For patients with OHS, CPAP can significantly improve gas exchange. OHS patients suffer from chronic daytime hypercapnia, often due to the mechanical burden of excess weight on the chest wall. In OHS patients who also have severe OSA, CPAP’s removal of nocturnal obstructions allows for better clearance of accumulated carbon dioxide. CPAP does not directly increase alveolar ventilation, but by stabilizing the upper airway and sometimes recruiting collapsed alveoli, it can facilitate an improvement in tidal volumes and help lower the chronic carbon dioxide levels.
Overlap Syndrome (O/S)
Another complex scenario is the Overlap Syndrome, which is the coexistence of OSA and Chronic Obstructive Pulmonary Disease (COPD). Patients with O/S experience profound and prolonged drops in oxygen saturation at night, far worse than those with either condition alone. Treating the OSA component with CPAP significantly mitigates the nocturnal hypoxemia, which helps reduce the risk of respiratory failure exacerbations.
Pulmonary Hypertension
The chronic nocturnal oxygen desaturation caused by sleep apnea can also lead to secondary pulmonary hypertension, a condition characterized by high blood pressure in the arteries of the lungs. By eliminating the desaturation events, CPAP therapy has been shown to reduce the blood pressure in the pulmonary arteries over time. This reversal of structural stress on the pulmonary vasculature represents a direct functional improvement to the circulatory component of the respiratory system.