CPAP Glossary Page
CPAP & Sleep Apnea Glossary
Plain-language definitions for the terms you'll come across in your diagnosis, your CPAP machine, and your sleep study or sleep report.
A
AHI Apnea-Hypopnea Index
The average number of apneas and hypopneas recorded per hour of sleep. AHI is one of the primary measurements used to identify and grade the severity of sleep-disordered breathing.
See also: AHI Severity Scale on our Buying GuideApnea
A complete pause in breathing lasting at least 10 seconds during sleep.
APAP Auto-Adjusting Positive Airway Pressure
A CPAP-style machine that automatically adjusts pressure throughout the night in response to changes in your breathing.
Adherence also called Compliance
How consistently someone uses their CPAP machine, usually measured using hours of use per night and nights of use. Most modern machines record this information automatically.
Aerophagia
Swallowing air during CPAP therapy, which can cause bloating, abdominal discomfort or a gassy feeling.
Airway
The passage through which air travels from the nose and mouth to the lungs. During obstructive sleep apnea, parts of the upper airway can narrow or collapse during sleep.
Anti-Asphyxia Valve AAV
A safety valve built into some CPAP masks that allows room air to enter if normal therapy airflow stops, helping prevent complete blockage of the mask's breathing pathway.
Arousal Index AI
The average number of arousals from sleep per hour of sleep. Arousals can be associated with breathing disturbances, limb movements, noise or other factors and may contribute to fragmented sleep.
Auto-Trak PAP technology
A technology used in some PAP devices to detect and respond to an individual's breathing pattern. Similar automatic breathing-response technologies may have different names depending on the manufacturer.
B
BiPAP Bilevel Positive Airway Pressure
A machine that delivers two pressure levels: a higher pressure during inhalation and a lower pressure during exhalation. It is commonly used when bilevel therapy is prescribed for specific breathing or ventilation needs.
Berlin Questionnaire
A screening questionnaire used to assess the likelihood of obstructive sleep apnea based on factors including snoring, daytime sleepiness, high blood pressure and body weight.
Bolus oxygen therapy
A measured burst of oxygen released by a pulse-dose oxygen concentrator when inhalation is detected.
C
CAI Central Apnea Index
The average number of central apnea events recorded per hour of sleep. Central apneas occur when breathing effort temporarily stops because the normal respiratory drive is reduced or absent.
Central Apnea
A pause in breathing caused by a temporary reduction or absence of the brain's signal to the muscles responsible for breathing, rather than by a physical blockage of the airway.
Central Sleep Apnea (CSA)
A type of sleep apnea in which breathing pauses occur because the normal respiratory drive is temporarily reduced or absent. Unlike obstructive sleep apnea, the airway may not be physically blocked during the event.
See also: Types of Sleep ApneaComplex Sleep Apnea also called Treatment-Emergent Central Sleep Apnea in some contexts
A term historically used when central breathing events become apparent during treatment of obstructive sleep apnea with PAP therapy. The terminology and diagnostic criteria used by clinicians may vary.
CPAP Continuous Positive Airway Pressure
A common treatment for obstructive sleep apnea in which a machine delivers pressurized air through a mask to help keep the upper airway open during sleep.
Chinstrap
A strap worn around the jaw and head to help reduce mouth opening during sleep, often used with nasal or nasal pillow masks.
Cushion
The soft part of a CPAP mask that contacts the face and creates the seal. Cushions are replaceable components and gradually wear with regular use.
COPD Chronic Obstructive Pulmonary Disease
A group of chronic lung diseases that restrict airflow and make breathing more difficult. COPD and sleep apnea can occur together.
Continuous Flow oxygen therapy
An oxygen delivery method that provides a continuous stream of oxygen at a prescribed flow rate, generally measured in liters per minute.
D
Desaturation
A drop in blood oxygen saturation. During a sleep study, repeated oxygen desaturations may occur alongside breathing disturbances.
Desaturation Index
Another term sometimes used to describe the Oxygen Desaturation Index (ODI), representing the frequency of oxygen desaturation events during a recording.
Diagnostic Sleep Study
A sleep study performed to investigate possible sleep disorders. Depending on the test, it may record breathing, oxygen levels, heart rate, sleep stages, body position, movements and other physiological signals.
Data Card also SD Card
The memory card used in many PAP machines to store therapy information such as usage, pressure, leak and respiratory-event data.
Diffuser
A component used with some CPAP mask vents or accessories to disperse exhaled airflow and potentially reduce perceived noise.
Disposable Filter also Ultra-Fine Filter
A replaceable filter designed to trap smaller airborne particles before air enters the CPAP machine. Replacement intervals vary by manufacturer and environmental conditions.
E
ECG / EKG Electrocardiogram
A recording of the heart's electrical activity. ECG or EKG monitoring during an in-lab sleep study can help track heart rate and rhythm throughout the night.
EEG Electroencephalogram
A recording of electrical brain activity using sensors placed on the scalp. EEG signals are an important part of polysomnography because they help determine whether a person is awake or asleep and identify sleep stages.
EOG Electrooculogram
A measurement of eye movements recorded during a sleep study. EOG signals help identify REM sleep and assist with sleep-stage scoring.
EPR Expiratory Pressure Relief
A comfort feature available on some PAP machines that reduces pressure slightly during exhalation to make breathing feel more natural.
Excessive Daytime Sleepiness (EDS)
Persistent sleepiness or an increased tendency to fall asleep during the daytime despite having an opportunity for sleep. It is a common symptom associated with several sleep disorders.
Elbow also Swivel
The connector that joins a CPAP mask to the therapy tubing. Many elbows include a swivel connection that allows the tubing to rotate more freely.
EPAP Expiratory Positive Airway Pressure
The pressure delivered during exhalation in bilevel PAP therapy. It is generally lower than IPAP.
Exhalation Port also Vent
The openings built into most CPAP masks that intentionally release exhaled air and help prevent carbon dioxide from being rebreathed.
F
Flow Limitation
A reduction or restriction in airflow caused by narrowing of the upper airway. Flow limitation may occur without meeting the criteria for a full apnea or hypopnea.
Forehead Support also Forehead Pad
A support or cushioned pad found on some CPAP masks that helps stabilize the mask and distribute contact pressure.
Frame
The structural part of a CPAP mask that supports the cushion and connects to components such as the headgear and elbow.
Full-Face Mask
A CPAP mask that covers both the nose and mouth. Full-face masks can be useful for people who breathe through their mouth during sleep or who cannot comfortably maintain nasal breathing.
See also: Choosing a MaskFlow Meter oxygen concentrators
The control and display used on many oxygen concentrators to set and show oxygen flow, usually measured in liters per minute.
H
Headgear
The strap system that holds a CPAP mask in position during sleep. Headgear can gradually stretch and wear with regular use.
Hypoallergenic Filter also Fine Filter
An additional fine filter designed to capture smaller airborne particles such as dust and allergens. The exact filtration performance depends on the specific filter and manufacturer.
Hypoxemia
A lower-than-normal level of oxygen in the blood. Sleep studies and overnight oximetry can help identify periods of reduced oxygen saturation.
Humidifier
A device or integrated water chamber that adds moisture to PAP airflow, helping reduce dryness of the nose, mouth and throat for many users.
See also: Climate & Humidity Settings for the UAEHypopnea
A reduction in airflow during sleep that meets defined scoring criteria for a partial breathing event. Hypopneas are included in the calculation of AHI.
Hypopnea Index HI
The average number of hypopneas recorded per hour of sleep. The hypopnea index contributes to the overall AHI.
HSAT Home Sleep Apnea Test
A simplified sleep apnea test performed outside a sleep laboratory, often at home, using portable equipment to record signals such as breathing, oxygen saturation and heart rate.
See also: WatchPat One Home Sleep TestI
IPAP Inspiratory Positive Airway Pressure
The pressure delivered during inhalation in bilevel PAP therapy. IPAP is generally higher than EPAP.
L
Leak Rate
The amount of air escaping from the mask system rather than being intentionally vented. PAP machines may report leak information to help assess mask fit and therapy performance.
LPM Liters Per Minute
The unit commonly used to describe continuous oxygen flow from an oxygen concentrator or other oxygen source.
LTOT Long-Term Oxygen Therapy
Supplemental oxygen therapy prescribed for ongoing use in people with chronic low blood oxygen levels caused by certain medical conditions.
Loops
A minimal headgear design used on some masks that uses straps or loops around the ears or sides of the head.
M
Micro-arousal
A brief shift from deeper sleep toward lighter sleep or wakefulness. Arousals can be associated with respiratory events, limb movements and other disturbances and may contribute to fragmented sleep.
Mixed Apnea
An apnea event that contains both central and obstructive characteristics, with a period of absent respiratory effort followed by continued effort against an obstructed airway.
Mask Clips also Headgear Clips
The clips or fasteners used to attach CPAP headgear straps to the mask frame. Some masks use magnetic clips while others use conventional clips.
Membrane valve membrane
A flexible component inside some mask safety valves that moves to allow room air through when the valve opens.
MSLT Multiple Sleep Latency Test
A daytime sleep test that measures how quickly a person falls asleep during a series of scheduled nap opportunities. It is commonly used when evaluating excessive daytime sleepiness and certain disorders such as narcolepsy.
MWT Maintenance of Wakefulness Test
A daytime test that evaluates a person's ability to remain awake in a quiet environment. It may be used to assess daytime alertness or treatment effectiveness.
N
N1 Sleep Stage N1
The lightest stage of non-REM sleep and the transition from wakefulness into sleep. N1 normally represents a relatively small portion of total sleep time.
N2 Sleep Stage N2
The second stage of non-REM sleep and usually the largest portion of normal adult sleep. It is deeper than N1 and has characteristic patterns such as sleep spindles and K-complexes.
N3 Sleep Slow-Wave Sleep / Deep Sleep
The deepest stage of non-REM sleep. N3 is associated with slow brain-wave activity and is commonly referred to as deep or slow-wave sleep.
Nasal Cannula oxygen therapy
A lightweight tube with two small prongs positioned near the nostrils to deliver supplemental oxygen from an oxygen source.
Nasal Mask
A CPAP mask that covers the nose and delivers therapy airflow through the nasal passages.
See also: Choosing a MaskNasal Pillow Mask
A minimal CPAP mask that uses small cushions positioned at or around the nostrils, providing less facial coverage than most nasal and full-face masks.
Nasal Pressure Signal
A breathing signal commonly recorded during a sleep study using a nasal cannula or pressure sensor. It helps detect changes in airflow and respiratory events.
NREM Sleep Non-Rapid Eye Movement Sleep
Sleep consisting of stages N1, N2 and N3. NREM sleep alternates with REM sleep throughout a normal night.
O
OAI Obstructive Apnea Index
The average number of obstructive apnea events recorded per hour of sleep. It is one of the measurements that can be reported separately from the overall AHI.
Obstructive Sleep Apnea (OSA)
The most common form of sleep apnea, caused by repeated narrowing or blockage of the upper airway during sleep, resulting in reduced or interrupted airflow.
See also: Sleep Apnea: What It Is and How Common It IsODI Oxygen Desaturation Index
The average number of oxygen desaturation events recorded per hour. Sleep reports may specify the oxygen drop used to define an event, such as ODI-3% or ODI-4%.
Oximetry
Measurement of blood oxygen saturation, usually using a pulse oximeter placed on a finger. Overnight oximetry can help identify patterns of oxygen desaturation.
Oral Appliance
A dental device worn during sleep that is designed to modify the position of the jaw or tongue and help maintain a more open upper airway. Oral appliances may be prescribed for selected people with obstructive sleep apnea.
Oxygen Concentrator
A device that takes in room air and concentrates oxygen for delivery to a user through a nasal cannula or other prescribed interface.
Oxygen Concentration also Purity
The percentage of oxygen contained in the gas produced by an oxygen concentrator. The exact output depends on the device, flow setting, operating conditions and manufacturer specifications.
Oxygen Conserver
A device or feature that delivers oxygen in response to inhalation rather than continuously, helping conserve the available oxygen supply.
Oxygen Nadir
The lowest blood oxygen saturation level recorded during a sleep study or overnight oximetry recording. It is commonly reported as the lowest SpO₂ percentage reached during the test.
P
Periodic Breathing
A repeated pattern of changing breathing intensity that may include periods of deeper breathing alternating with reduced or absent breathing. Some sleep reports and PAP devices record the amount of time spent in periodic breathing.
PLMI Periodic Limb Movement Index
The average number of periodic limb movements recorded per hour of sleep. PLMI is a separate measurement from AHI because it evaluates limb movements rather than breathing events.
PLMS Periodic Limb Movements of Sleep
Repetitive movements of the legs or, less commonly, the arms that occur during sleep. A sleep study can detect and count these movements and determine whether they are associated with arousals.
Polysomnography PSG / In-Lab Sleep Study
A comprehensive overnight sleep study that records multiple physiological signals, which may include brain activity, eye movements, muscle activity, heart rate, breathing, oxygen saturation and body position. It is commonly performed in a sleep laboratory.
Pressure Titration
The process of determining an appropriate PAP pressure or range of pressures to control breathing disturbances while maintaining comfortable therapy.
Pillows Nasal Pillows, mask component
The small soft cushions used on nasal pillow masks to create a seal around the nostrils. They are commonly available as replaceable mask components.
Pulse Dose also Pulse Flow, oxygen therapy
An oxygen delivery mode that releases a measured amount of oxygen when inhalation is detected rather than providing a continuous flow. Pulse-dose settings should not be confused with continuous-flow LPM settings.
Port Cap also Port Plug
A small cap used to close a pressure measurement port on certain CPAP masks. If the port is left open, it can create an unintended air leak.
R
RDI Respiratory Disturbance Index
The average number of respiratory disturbances recorded per hour of sleep. Depending on the scoring method, RDI may include apneas, hypopneas and respiratory effort-related arousals (RERAs), so it can be higher than AHI.
REM AHI
The apnea-hypopnea index calculated specifically during periods of REM sleep. Some people experience more breathing disturbances during REM sleep than during other sleep stages.
REM Latency
The amount of time between sleep onset and the first recorded period of REM sleep. It is one of the timing measurements that may appear on a detailed polysomnography report.
REM Sleep Rapid Eye Movement Sleep
A distinct stage of sleep associated with rapid eye movements, characteristic brain activity, vivid dreaming and changes in muscle activity and breathing.
RERA Respiratory Effort-Related Arousal
A breathing event involving increased respiratory effort and airflow limitation that ends with an arousal from sleep but does not meet the scoring criteria for an apnea or hypopnea.
REI Respiratory Event Index
The average number of respiratory events recorded per hour during a sleep apnea test. REI is commonly reported in home sleep apnea testing, where actual sleep time may not be directly measured.
Ramp Feature
A PAP setting that begins therapy at a lower pressure and gradually increases toward the prescribed treatment pressure, making it easier for some users to fall asleep.
Rainout
Condensation that forms inside CPAP tubing when warm humidified air cools. Heated tubing and appropriate humidity settings can help reduce rainout.
See also: Common Issues We See LocallyReusable Filter also Foam Filter
A washable filter used in some CPAP machines to capture larger airborne particles. Follow the manufacturer's instructions for cleaning and replacement.
S
Sleep Architecture
The structure and pattern of sleep across a night, including the amount of time spent in N1, N2, N3 and REM sleep and how those stages cycle throughout the night.
Sleep Efficiency
The percentage of the relevant sleep period that was actually spent asleep. It is generally calculated by comparing total sleep time with the amount of time spent in bed or the applicable sleep period.
Sleep Fragmentation
Repeated interruptions or disruptions of sleep that prevent sleep from remaining continuous. Breathing events, arousals, limb movements and environmental factors can contribute to fragmented sleep.
Sleep Latency
The amount of time it takes to fall asleep after the beginning of the sleep period. It is commonly reported in minutes on sleep-study reports.
Sleep Onset Latency SOL / Sleep Latency
The amount of time between the beginning of the sleep period and the first recorded period of sleep. It is one of the standard sleep-timing measurements used in polysomnography.
Sleep Stage Distribution
The percentage or amount of total sleep spent in each sleep stage, typically reported as N1, N2, N3 and REM.
Snoring Index
A measurement describing the frequency or amount of snoring detected during a sleep study. The exact calculation can vary between sleep laboratories and devices.
SpO₂ Peripheral Oxygen Saturation
A measurement of the percentage of hemoglobin carrying oxygen in the blood, usually measured by a pulse oximeter. Sleep studies commonly record SpO₂ continuously during the night.
Split-Night Study
A sleep study in which the first part of the night is used for diagnostic assessment and, when appropriate, the remaining portion is used for PAP titration.
STOP-BANG
A screening questionnaire used to assess the risk of obstructive sleep apnea. The name comes from Snoring, Tiredness, Observed apneas, high blood Pressure, BMI, Age, Neck circumference and Gender.
Supine AHI
The apnea-hypopnea index measured specifically while sleeping on the back. Some people experience more obstructive breathing events in the supine position.
Supine Sleep
Sleep while lying on the back. Body position is commonly recorded during an in-lab sleep study and may help identify positional patterns in sleep apnea.
Sieve Bed oxygen concentrators
An internal component of an oxygen concentrator containing a molecular sieve material that selectively removes nitrogen from incoming air to concentrate oxygen.
Short Tube
A shorter connector tube used on certain CPAP mask systems between the mask elbow and the main therapy tubing.
Swivel
A rotating connector, often incorporated into a CPAP mask elbow, that allows tubing to move more freely without pulling the mask out of position.
T
T90 Time Below 90% Oxygen Saturation
The amount or percentage of recording time spent with blood oxygen saturation below 90%. T90 is commonly reported when evaluating overnight oxygen levels.
Thermistor Airflow Sensor
A temperature-sensitive sensor placed near the nose and mouth during some sleep studies to detect airflow by measuring changes in temperature as air moves in and out.
TIB Time in Bed
The amount of time spent in bed during the sleep period being analyzed. TIB can include periods when a person is awake before falling asleep or after waking.
Titration Study
A sleep study focused on determining appropriate PAP pressure settings to control breathing disturbances while maintaining comfortable therapy.
TRT Total Recording Time
The total length of time a sleep-study device records its available physiological signals. In home sleep apnea testing, TRT may be used when actual sleep time cannot be measured directly.
TST Total Sleep Time
The total amount of time actually spent asleep during the recorded sleep period. TST differs from total recording time because it excludes periods when the person was awake.
U
UPPP Uvulopalatopharyngoplasty
A surgical procedure that removes or reshapes selected tissues in the throat to enlarge the upper airway. It may be considered for selected people with obstructive sleep apnea.
Upper Airway
The part of the respiratory system extending from the nose and mouth through the throat toward the lower airway. Obstruction of the upper airway during sleep is a key feature of obstructive sleep apnea.
W
WASO Wake After Sleep Onset
The total amount of time spent awake after initially falling asleep. WASO is an important measurement of sleep continuity and fragmentation on detailed sleep reports.
Wide Straps also Comfort Straps
A headgear design using wider straps to distribute pressure over a larger area and potentially reduce pressure points or skin marks.
Z
Zeolite
A mineral used in the molecular sieve beds of many oxygen concentrators. It selectively adsorbs nitrogen from room air, allowing the device to produce concentrated oxygen.
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