A Continuous Positive Airway Pressure (CPAP) machine is a common medical device used primarily to treat Obstructive Sleep Apnea (OSA). The question of whether CPAP provides Positive End-Expiratory Pressure (PEEP) is central to understanding how this therapy works. The straightforward answer is yes; CPAP inherently functions as a form of PEEP by maintaining a constant level of pressure in the airways, which keeps the upper airway from collapsing during sleep.
The Core Concept of PEEP
Positive End-Expiratory Pressure (PEEP) is defined as the pressure maintained in the lungs above atmospheric pressure at the end of the exhalation cycle. During normal, relaxed breathing, the pressure inside the lungs briefly returns to atmospheric pressure after a breath out. PEEP prevents this return to zero by intentionally keeping a positive pressure within the airways.
This mechanism is crucial for preventing the collapse of the small air sacs in the lungs, known as alveoli. If the alveoli completely collapse, a condition called atelectasis occurs, which impairs the body’s ability to transfer oxygen into the bloodstream. By splinting these structures open, PEEP increases the functional residual capacity—the volume of air remaining in the lungs after a normal exhalation. This increased volume improves the surface area available for gas exchange, resulting in better oxygenation and stabilizing the airways.
CPAP Delivers Continuous Positive Pressure
CPAP machines operate by delivering a single, fixed pressure level that remains constant throughout the entire breathing cycle, during both inhalation and exhalation. This continuous pressure is measured in centimeters of water (cm H₂O) and is set by a healthcare provider. The machine’s constant flow of pressurized air acts as a pneumatic splint, preventing the soft tissues of the throat from collapsing and causing an apnea event.
Because this pressure never drops to zero, the pressure remaining in the airways at the end of the patient’s breath out is the set CPAP pressure. This constant baseline pressure is functionally the definition of PEEP, as it is a positive pressure maintained at the end of expiration. In the context of CPAP therapy, the set pressure is essentially the Expiratory Positive Airway Pressure (EPAP), which is the pressure that keeps the airway open.
The CPAP pressure prevents the walls of the upper airway from collapsing, which is the primary cause of obstructive sleep apnea. This continuous application of pressure results in a stable airway, which in turn reduces the frequency of breathing interruptions. The constant positive pressure helps to maintain patency not just in the pharynx, but also in the smaller airways and alveoli, offering the physiological benefits associated with PEEP.
CPAP Versus BiPAP Pressure Variation
The concept of PEEP becomes clearer when CPAP is contrasted with Bilevel Positive Airway Pressure (BiPAP) therapy. BiPAP machines utilize two distinct pressure settings, offering a different level of support for breathing in and breathing out. This device is often prescribed when patients require higher pressure settings or have difficulty exhaling against the single, high pressure of a CPAP machine.
In a BiPAP system, the Inspiratory Positive Airway Pressure (IPAP) is the higher pressure delivered during inhalation to assist the breath. The Expiratory Positive Airway Pressure (EPAP) is the lower pressure delivered during exhalation, which functions as the therapeutic PEEP. The EPAP is the minimum pressure maintained in the lungs and is responsible for keeping the airway open at the end of the breath.
The difference between IPAP and EPAP is known as pressure support, which assists the work of breathing. CPAP, by delivering the same pressure for both inhalation and exhalation, has zero pressure support. This means the patient must initiate and complete their own breath against the fixed PEEP. BiPAP allows for a differential pressure, reinforcing that the EPAP setting in BiPAP, and the single set pressure in CPAP, are both the functional PEEP.