Continuous positive airway pressure—commonly abbreviated as CPAP—is a medical term that describes both a therapy and a mechanism. Understanding this concept helps patients make sense of their treatment and enables more informed discussions with healthcare providers. Here’s what the term means clinically and how it applies to sleep apnea therapy.
The Clinical Definition
Continuous positive airway pressure is a form of noninvasive ventilatory support that delivers a constant level of positive pressure to the airways throughout the entire respiratory cycle. The pressure is measured in centimeters of water (cm H₂O) and remains the same whether the patient is breathing in or breathing out.
The key elements of this definition:
Noninvasive means the therapy is delivered through an external interface (a mask) rather than through a tube inserted into the airway. Unlike mechanical ventilation requiring intubation, CPAP can be applied and removed without medical procedures.
Constant level distinguishes CPAP from other modes that vary pressure during breathing. Bilevel devices provide different pressures for inhalation and exhalation; CPAP provides one unchanging pressure throughout.
Positive pressure means the pressure inside the airway is higher than atmospheric pressure. This creates an outward force that keeps airway tissues from collapsing inward.
Throughout the respiratory cycle emphasizes the continuous nature. The pressure doesn’t cycle off during any part of breathing—it maintains constant support from breath to breath, all night long.
How CPAP Differs From Other Ventilation Modes
Understanding CPAP requires distinguishing it from related but different respiratory therapies:
Bilevel Positive Airway Pressure (BiPAP) delivers two pressure levels—a higher inspiratory positive airway pressure (IPAP) during inhalation and a lower expiratory positive airway pressure (EPAP) during exhalation. This makes breathing out easier than with CPAP and is often prescribed for patients who need higher pressures or have difficulty exhaling against constant pressure.
Mechanical Ventilation actively pushes air into the lungs at set intervals, doing some or all of the work of breathing for the patient. CPAP, by contrast, doesn’t push air in—it simply maintains pressure while the patient breathes spontaneously. The patient initiates and controls each breath.
Positive End-Expiratory Pressure (PEEP) is related but not identical. PEEP refers specifically to the pressure remaining in the airways at the end of exhalation. CPAP maintains this positive pressure throughout the entire breath, not just at exhalation’s end, though the effect is similar.
Supplemental Oxygen adds extra oxygen to the air a patient breathes but doesn’t provide pressure support. CPAP primarily provides pressure rather than oxygen concentration changes, though oxygen can be added to CPAP systems when needed.
The Physiological Mechanism
CPAP treats obstructive sleep apnea through a pneumatic splinting effect. Here’s what that means:
During sleep, muscle tone decreases throughout the body, including in the muscles that help keep the upper airway open. In people with obstructive sleep apnea, this relaxation allows soft tissues—the tongue, soft palate, and pharyngeal walls—to collapse inward, narrowing or completely blocking the airway.
The positive pressure from CPAP pushes outward against these tissues, preventing them from collapsing into the airway. It’s analogous to inflating a balloon: the internal pressure keeps the walls expanded. Similarly, CPAP’s pressure keeps the airway walls apart.
This effect requires only modest pressure. Most patients achieve airway patency with pressures between 4 and 20 cm H₂O—far below the pressures used in other medical applications. These low pressures are gentle enough for all-night use but sufficient to prevent tissue collapse.
CPAP in Sleep Medicine
In the context of sleep apnea treatment, CPAP is typically delivered through a bedside machine that draws in room air through a filter, pressurizes the air to the prescribed level using a motor and blower, humidifies the air (in most modern devices) for comfort, and delivers the pressurized air through tubing to a mask interface.
The pressure setting is determined through a titration study—either in a sleep lab or through home auto-titration—that identifies the minimum pressure needed to eliminate apnea events for that particular patient.
CPAP Beyond Sleep Apnea
While sleep apnea is the most common indication for CPAP therapy, the same principle applies in other clinical settings:
Acute Respiratory Failure: In emergency departments and intensive care units, CPAP helps patients with pulmonary edema, pneumonia, and other conditions that cause respiratory distress. The positive pressure helps keep fluid-filled or collapsed lung units open.
Neonatal Care: Premature infants with underdeveloped lungs often receive CPAP (typically delivered through nasal prongs) to help maintain lung expansion and reduce the work of breathing.
Prehospital Care: Emergency medical services use portable CPAP devices to treat patients with acute respiratory distress before reaching the hospital.
In all these applications, the fundamental mechanism is the same: continuous positive pressure supports airway or alveolar patency, improves oxygenation, and reduces respiratory effort.
Why Understanding the Definition Matters
For patients, understanding what continuous positive airway pressure actually means provides several benefits:
It demystifies the therapy, transforming CPAP from an intimidating medical device into a logical tool that addresses a specific mechanical problem.
It explains why consistent use matters—the splinting effect only works while pressure is being delivered.
It clarifies why settings are individualized—different anatomies require different pressure levels to achieve the same effect.
It helps patients understand why alternative therapies like BiPAP might be recommended if CPAP proves uncomfortable or insufficient.
The technical definition of CPAP describes a simple concept: constant air pressure that keeps the airway open. But that simple concept, applied every night, can transform the sleep and health of people with obstructive sleep apnea.