The relationship between Continuous Positive Airway Pressure (CPAP) and Positive End-Expiratory Pressure (PEEP) often causes confusion. Both terms relate to applying pressurized air to assist breathing, but they describe different concepts—one a treatment mode and the other a physiological measurement. This article clarifies the technical distinction and explains why CPAP is fundamentally linked to PEEP. Understanding this connection is important for anyone using or researching positive airway pressure therapies.
What is Continuous Positive Airway Pressure (CPAP)?
Continuous Positive Airway Pressure (CPAP) describes a method of delivering air pressure that remains constant throughout the entire breathing cycle. A CPAP machine draws air from the room, pressurizes it to a prescribed level, and delivers this single, steady pressure via a mask. This consistent pressure is maintained during both the inhalation phase and the exhalation phase of breathing.
The primary medical application for CPAP is the treatment of Obstructive Sleep Apnea, a condition where the upper airway collapses during sleep. By providing a continuous pneumatic splint, the device keeps the airway open, eliminating disruptive breathing pauses. Prescribed pressures typically range between 6 and 14 centimeters of water (cmH2O). The user must breathe spontaneously, as the machine maintains the pressure but does not initiate the breath.
Defining Positive End-Expiratory Pressure (PEEP)
Positive End-Expiratory Pressure (PEEP) is a physiological concept referring to the pressure remaining in the lungs at the end of the exhalation phase. In a clinical setting, PEEP is the deliberate application of pressure above atmospheric pressure to the airway. Its main purpose is to prevent the tiny air sacs in the lungs, known as alveoli, from collapsing completely at the end of a breath.
This pressure helps to stabilize the alveoli, keeping them partially inflated and ready for the next inhalation. Even in healthy individuals, a small amount of physiological PEEP, typically 3 to 5 cmH2O, is naturally present. In mechanically ventilated patients, clinicians set an extrinsic PEEP level to improve oxygenation and increase the functional residual capacity of the lungs. The value of PEEP is therefore a measurement or a setting, not an entire mode of therapy itself.
The Direct Answer: CPAP as a Constant PEEP
The answer to whether CPAP has PEEP is yes. CPAP delivers a constant pressure that never drops to zero, meaning it always maintains a positive end-expiratory pressure. Since CPAP maintains the same pressure during both inhalation and exhalation, that constant value is inherently the pressure present at the end of exhalation. The single pressure setting on a CPAP device functions identically to a continuously applied PEEP.
The distinction is primarily one of terminology and application. CPAP is classified as a mode of ventilation used for spontaneously breathing patients, particularly in non-invasive settings like sleep therapy. PEEP, conversely, is a parameter incorporated into various modes of ventilation, including CPAP. When a doctor prescribes a CPAP pressure of 8 cmH2O, that pressure serves as both the inspiratory pressure and the PEEP.
Beyond Continuous Flow: Pressure Dynamics in Other Devices
Other positive airway pressure devices use varying pressure dynamics, contrasting with the constant pressure of standard CPAP. Bi-level Positive Airway Pressure (BiPAP) machines, for instance, deliver two distinct pressure levels. They use a higher pressure during inhalation, called Inspiratory Positive Airway Pressure (IPAP), and a lower pressure during exhalation, known as Expiratory Positive Airway Pressure (EPAP).
In BiPAP, the EPAP setting is the functional PEEP, as it is the pressure maintained at the end of exhalation to stabilize the airway. This two-level system can be more comfortable for patients who struggle to exhale against high, constant CPAP pressure.
Automatic Positive Airway Pressure (APAP) machines automatically adjust the pressure within a set range based on the user’s real-time breathing needs. APAP devices deliver a continuous but variable pressure, making the lowest pressure delivered the functional equivalent of a variable PEEP.