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Sleep Apnea & CPAP Glossary: Key Terms Patients Need to Know

What Does CPAP Mean

Your sleep test results are back, and you’ve learned that you do, in fact, have sleep apnea and need CPAP. Congratulations are in order! You’re taking the right steps to treat your sleep and get back to a healthier and happier you. 

But it might not feel like a time to celebrate. You’re probably feeling stressed and overwhelmed–not only with the idea of using a CPAP machine and trying to understand which CPAP equipment is right for you, but even just trying to comprehend all of the medical terms your sleep specialist is throwing out there!  

Don’t worry! cpapRX is here to help. 

If you’re just getting started with CPAP, or need a quick refresher, we’ve put together a list of the key terms that will help you understand your diagnosis and therapy equipment. (And double don’t worry–we’ve stripped out the jargon, lingo, and other medical speak so it’s easier for non-medical folks to understand.)

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Adaptive Servo-Ventilation (ASV)

ASV is a type of positive airway pressure (PAP) therapy created specifically to treat a range of central breathing disorders, including central sleep apnea and/or complex sleep apnea (although it is sometimes used for obstructive sleep apnea). 

It’s different from CPAP because air pressure is adjusted to trigger breathing when you need it, versus the continuous pressure found with CPAP. This is important for central breathing disorders, which occur when your body’s automatic act of breathing has stopped even though your airway is open. 


The literal meaning of “apnea” is “no breath.” In the industry, we count an apnea when you stop breathing for ten seconds or longer during sleep. 

Stop right now and hold your breath for ten seconds, counting in mississippis. We’ll wait.

See! That’s a long time to not breathe! Now think about doing that multiple times during the night and for longer than 10 seconds. Do you see now how serious sleep apnea is?! 

Apnea–hypopnea index (AHI)

AHI is the number of apneas and hypopneas you have per hour while sleeping. It’s measured during a sleep test, and helps to indicate the severity of sleep apnea and determine the therapy needed.

Automatic positive airway pressure (APAP)

APAP is one of the three main types of positive airway pressure therapies that helps to open the airway during sleep. APAP air pressure is designed to deliver automatically throughout the night based on the lowest pressure you need at any given time. Compare this to continuous positive airway pressure (CPAP) therapy, which delivers air at one fixed pressure throughout the night. 

TL;DR: APAP = automatic and customized. CPAP = continuous and unchanging.  

Shop Auto CPAP Machines.


Basic Sleep Cycle

This refers to the natural progression through different sleep stages during sleep. Sleep cycles typically repeat every 90 to 110 minutes, and are made up of different phases, the two main phases being rapid eye movement (REM) and non-REM sleep. 

BiLevel/Variable Level Therapy

BiLevel therapy, also called variable level therapy (VPAP), BiPAP and/or BPAP, delivers two different levels of air pressure. When you breathe in, a higher level is used. When you breathe out, a lower level is used. Some people with sleep apnea prefer this BiLevel therapy because they find it to be more comfortable during sleep. 

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Body Position

The different positions a person assumes during sleep. The four key positions are left side, right side, prone (on your stomach) and supine (on your back). Body position can impact the severity of sleep apnea, specifically when supine, and thus body position is evaluated during a sleep study to get a full picture of your sleep situation. 


Central Sleep Apnea (CSA)

CSA is a type of sleep apnea where your breathing stops but your airways are not blocked. If there’s nothing physically blocking the airway, then why do you stop breathing?! Well, something gets lost in translation. Or lost in communication, rather. With CSA, there’s an interruption in the signal from your brain telling your body to breathe. 

CO2 Wash-Out

You inhale oxygen and exhale carbon dioxide (CO2) when breathing normally. But when you’re wearing a therapy mask, some CO2 can get trapped in the mask. To prevent you from re-breathing in too much of that lingering CO2, every CPAP mask must have a way to push out that CO2. This is called CO2 wash-out. 

Complex Sleep Apnea (CompSA)

CompSA is, well, complicated. It occurs when someone with obstructive sleep apnea (read: blocked airway) develops central sleep apnea (read: non-blocked airway) after using CPAP or BiPAP. Or perhaps CSA was there all along, and treating the OSA made it more obvious. 

Treating CompSA is not so complicated, however. Often it’s just a matter of time–getting used to the therapy, maintaining compliance, getting the pressure, mask and other equipment right. In some cases, you might need to switch to BiLevel or ASV therapy. 


Compliance is when a patient adheres to their CPAP therapy. Typically this is an average of four hours per night for at least 70% of nights, although it may differ depending on your therapy. Repeat after us: A healthy sleep apnea patient is a compliant one. 

Conducted Noise

This is the noise created by your CPAP machine and equipment. With today’s machines, conducted and radiated noise is usually minimal and shouldn’t disrupt your or your partner’s sleep. If it does, you may have a leak or something else is wrong. Contact us and we’ll do our best to help you sort it out.

Continuous Positive Airway Pressure (CPAP)

CPAP therapy is a common treatment for obstructive sleep apnea. CPAP is often used as a general term for the therapy, and for the CPAP machines and technology itself. CPAP machines deliver air to a mask worn over the nose and/or mouth to help maintain consistent breathing. With CPAP, air pressure remains fixed throughout the night for inhaling and exhaling, rather than changing as with BiPAP and APAP.  

Shop CPAP Machines.

CPAP Pillow

CPAP Pillows are specially designed bed pillows that accommodate the mask and hose through dimples and/or cutouts, and offer better head and neck support. Not to be confused with CPAP Nasal Pillows, which is a type of CPAP mask that rests under the nostril and is held in place with headgear. 

CPAP Prescription

CPAP devices are considered a Class II Medical Device, and as such require a prescription. There are a couple of reasons why a prescription is necessary: 1. Insurance requires a prescription to cover payments; 2. A pressure setting is required for a CPAP machine to function adequately, and that pressure setting should only come from a board-certified sleep specialist’s interpretation of sleep study results; and 3. Incorrect pressure settings can be ineffective at best and harmful at worst. 

If you need a prescription or renewal, we can save you the trip and the copay. To get started, head over to our cpapRX Prescription Service to place your order and complete a short questionnaire about your sleep health history. 

CPAP Pressure

CPAP pressure refers to the air pressure that is asserted by your CPAP machine. It’s measured in centimeters of water (cmH20) and typically falls between 6 and 14 cmH20. As a general rule, it should be just strong enough to hold your airway open, but not so strong that it disrupts sleep or causes nasal congestion. Your pressure requirement may change throughout your therapy, and should only be adjusted by a medical professional.

CPAP Titration

CPAP titration is a sleep test that is used to determine your optimal positive airways pressure (PAP) settings. It is performed after being diagnosed with sleep apnea and prescribed CPAP.


CPAP mask cushions are what come into contact with your face. They are found in a variety of styles–covering your entire nose and/or mouth area or sitting underneath your nostrils–and a variety of materials, including silicone, gel, foam and memory foam. Cushions should be cleaned every day to remove oils that can cause leaks, and checked at least once a month for wear and tear. 




ENT Physician

A doctor specializing in Ear, Nose and Throat (ENT) and normally performs surgical repair or reconstruction in these areas, and works with sleep apnea patients for CPAP therapy.


An event in sleep terminology is when an apnea or hypopnea occurs. During a sleep test, the number of events that happen per hour is measured to calculate your apnea–hypopnea index or AHI.

Excessive Daytime Sleepiness (EDS)

EDS is one of the most common sleep-related patient symptoms. Common causes include obstructive sleep apnea, sleep deprivation, and sedating medications. Other potential causes of EDS include certain medical and psychiatric conditions and sleep disorders, such as narcolepsy. 

Exhalation Relief

Exhalation relief is the ability for a CPAP or BiPAP machine to reduce pressure during exhalation, making it easier for you to breathe out against the pressure. The different CPAP manufacturers all use different names for their exhalation relief features, but for the most part, they all work the same. Respironics calls theirs C-Flex, A-Flex, or C-Flex+. ResMed named theirs EPR (expiratory pressure relief, get it?!).

Expiratory Positive Airway Pressure (EPAP)

Instead of using artificial air pressure, EPAP uses the natural pressure created by your own breathing to keep your airway open during exhalation. 


Fisher & Paykel

Fisher & Paykel is a top manufacturer of sleep apnea and CPAP machine supplies. We offer a full line of Fisher & Paykel CPAP masks, machines and other CPAP supplies. Shop F&P CPAP supplies


Whether breathing normally or with the help of a CPAP therapy device, flow refers to the stream of air entering your lungs when you breathe.

Flow Limitation

When the flow of air into your body is obstructed or blocked in your upper airway. Can be an Inspiratory Flow Limitation (IFL) on inhale or Expiratory Flow Limitation (EFL) on exhale. 

Full Face Masks

Full face CPAP masks are a type of CPAP mask used during positive airway pressure (PAP) therapy. Somewhat contrary to their name, full face masks don’t actually cover the entire face, just the nose and mouth. 




Habitual Snoring

Loud snoring that occurs at least three nights per week is considered habitual snoring. It is often one of the most recognizable tells of obstructive sleep apnea, however, habitual snoring does not always indicate OSA. If you think you’re a habitual snorer, talk to your doctor to discuss your sleep (you knew we were going to say that, didn’t you?!). 


CPAP headgear is what holds your CPAP mask in place on your face. It usually consists of straps, clips, cushions, and the mask frame. CPAP headgear can stretch and lose elasticity over time, causing discomfort and/or reducing the effectiveness of treatment. It’s important that you replace headgear parts when they wear out to ensure you’re getting the most out of your CPAP therapy. 

If you’re unsure which replacement parts will fit, check your headgear and mask. Oftentimes the mask name and size will be printed on it somewhere. If not, check your original order, contact the outlet where you purchased, or get in touch with our team. Even if you didn’t purchase it with us, we may be able to help you locate it so you can order the right replacement parts. 

Home Sleep Apnea Test (HST)

A diagnostic test that is self-administered by the patient in their own home to diagnose or rule out sleep apnea. HSTs can be a convenient and cost-effective method for testing sleep apnea instances.

cpapRX offers the ONLY disposable home sleep study on the market. Learn more about the WatchPAT One Disposable Sleep Apnea Test and order yours today! 


A humidifier adds moisture to your CPAP therapy air. This can reduce nasal congestion, prevent dry nose and mouth, and help you wake up feeling more refreshed, instead of parched and cottonmouthed. 

Some CPAP humidifiers are heated, delivering more moisture and allowing you to set the right amount of humidity. Some CPAP humidifiers are not heated. These “cold” or “passover humidifiers” simply pick up moisture as air flows over the humidifier chamber. Some humidifiers are integrated into the therapy device, and some come separate. 


Sleep condition where you experience significant episodes of sleepiness, even after getting a (seemingly) quality seven hours or more of sleep at night. It’s also called hypersomnia and it’s the fancy term for Excessive Daytime Sleepiness.


Extreme fear or anxiety of falling asleep. It can be seen with sleep apnea patients who wake up gasping for breath, and then are fearful of going back to sleep again. Also called clinophobia, somniphobia, sleep dread, or sleep anxiety. 


Hypopnea means abnormally slow or shallow breathing caused by a partial blockage of your airway. Along with apneas, hypopneas are the events measured during a sleep study to calculate AHI, and determine the appropriate sleep therapy. To be considered a hypopnea, breathing must be reduced by 50% for 10 seconds or more. 



A sleep condition where you have trouble falling or staying asleep. Acute insomnia lasts from one night to a few weeks. Chronic Insomnia is when it happens at least three nights a week for three months or more.

Inspiration/Inspiratory Phase

Breathe in. Inhale. Fill up your lungs. Inspiration is the part of the breathing cycle when you take air into your body. We use this to refer to the spontaneous inhales you take automatically and to refer to the mechanical inhales you take with the help of a CPAP therapy device. 

Inspiration Pressure

Refers to the pressure delivered to you by a CPAP machine during your inhalation or inspiration.

Inspiratory Positive Airway Pressure (IPAP)

The pressure that is delivered upon inhalation or inspiration during BiLevel or BiPAP therapy.







When air escapes out of your mask due to incorrect fit, incorrect assembly, or normal wear and tear that requires placement. If leak rate is excessive, you may not be getting the full benefits of your therapy.



CPAP masks are used with a CPAP machine to deliver continuous air pressure. There are numerous different types of masks, including nasal pillows, nasal masks, and full-face masks. Masks are often a personal preference, and you may need to try a few different kinds before picking which one is the most comfortable and right for you. 

Mixed Sleep Apnea

Mixed sleep apnea is when you have a combination of both obstructive sleep apnea (OSA) and central sleep apnea (CSA). OSA for when there is a blockage or obstruction in the upper airway and CSA for when the brain’s signal to breath is not received by the body. Also called Complex Sleep Apnea. 

Mouth Leak

This CPAP leak refers specifically to when you breathe through your mouth rather than your nose, and your mouth is not covered by a CPAP therapy mask. The main solution to mouth leak is to use a full face mask, rather than a nasal mask. 

Multiple Sleep Latency Test (MSLT)

A sleep laboratory test that measures how quickly you fall asleep in a quiet environment, and is most often used to diagnose narcolepsy. Consists of five 20-minute nap opportunities spread across the day. Also called a Daytime Nap Test. 


Nasal Masks

Nasal CPAP masks are a type of therapy mask that cover only the nose. 

Nasal Pillows Masks

Nasal pillows masks are a type of CPAP mask that rest at the entrance to the nose, gently sealing in the nostrils. These masks are minimalistic and typically less obtrusive than nasal or full face masks.


Obstructive Sleep Apnea (OSA)

OSA is one of the three main types of Sleep Apnea (the other two being Central Sleep Apnea and Complex/Mixed Sleep Apnea). In OSA, the airway is partially or totally blocked (or obstructed), which interrupts breathing patterns.


Usually used during a Home Sleep Test, an oximeter measures your pulse rate and the amount of oxygen in your blood. 

Oxygen Saturation (SaO2)

Oxygen saturation measures the oxygen levels in your body, and is typically recorded during a sleep study. Because sleep apnea patients stop breathing repeatedly throughout the night, they’re not getting the amount of oxygen into their bloodstream that they need. The level of oxygen desaturation you experience will help to inform your CPAP therapy needs. 



Pap is the name of our sheepish mascot! When Pap’s not busy helping you find the sleep therapy products you need, you can find Pap jumping over fences, dozing off for a nap, or munching on some grass. 


Pap’s Pathway

Pap’s Pathway is our Exclusive Sleep Apnea Treatment Journey. It’s a truly a simplified and accommodating process that can take just 6-12 days from diagnosis to prescription to supplies (compare that to the 6-12 months it can take if you go the traditional route!). Learn more about Pap’s Pathway.


Periodic Breathing

Periodic breathing refers to unstable breathing patterns during sleep. It involves intermittent periods of deep and shallow breaths, often leading to periods of no breath (i.e., Apneas) even though your airway is not blocked or obstructed. 

Polysomnography (PSG)

A PSG is a test performed at a sleep center to assess quality of sleep, and diagnose or rule out sleep disorders such as obstructive and central sleep apnea, restless leg syndrome (RLS) and more in adults and children.

Prescribed CPAP Pressure

See CPAP Pressure. 

Pressure Port

The pressure port is the area in the mask where oxygen tubes and/or pressure lines can be directly connected, allowing monitoring of oxygen and pressure. 




Radiated Noise

See Conducted Noise. 


Also known as condensation, CPAP rainout refers to moisture or water droplets in your tubing or mask. Rainout can cause issues with your CPAP therapy. Increasing the temperature in your bedroom and/or using a humidification solution can help minimize rainout. 


Most modern CPAP machines have a feature called “ramp,” which starts the machine at lower pressure and gradually ramps up over a set period of time (usually around 45 minutes). This can help you ease into your CPAP therapy by giving you time to fall asleep each night or get used to breathing with your equipment before you reach your prescribed setting. 

Rapid Eye Movement (REM)

REM is one of the main sleep types. It’s when you have the deepest sleep and when you dream. The name refers to the quick back-and-forth eye movements that can be observed while the eyelids remain closed. 


ResMed is a premier manufacturer of sleep apnea and CPAP supplies. We offer a full line of ResMed CPAP masks, machines, and other CPAP supplies. Shop ResMed CPAP Supplies.

Respiratory Disturbance Index (RDI)

Respiratory disturbance index (sometimes called respiratory distress index) is taken during a sleep test and indicates the number of respiratory events that occur during sleep. It’s similar to AHI in that it measures the number of apneas and hypopneas, but it goes one step further and measures a third category–respiratory effort related arousals (RERAs)

RERAs essentially measure any sleep disruptions not considered an apnea or hypopnea, such as snoring, allergies, asthma, deviated septum, inflammation or swelling caused by illness, etc. Combined with AHI, RDI can provide a clearer picture of problematic sleep, and thus help to define the right therapy.


Philips Respironics provides high-quality sleep apnea care and CPAP machine supplies. We will soon offer a full line of Respironics CPAP masks, machines, and other CPAP supplies. Shop Respironics CPAP supplies.


Sleep Apnea

Sleep apnea is a serious condition that causes you to stop breathing during sleep. There are three main types of sleep apnea: 1. Obstructive sleep apnea (OSA) where the airway is blocked; 2. Central sleep apnea (CSA) where the airway is not blocked but the body does not receive the proper signal to breathe; and 3. Complex/Mixed sleep apnea, which is a combination of the first two. Proper diagnosis and treatment are necessary to ensure healthy oxygen levels and adequate sleep.

Learn why CPAP is always our first choice for a sleep apnea treatment.

Sleep Apnea Test

A type of sleep study where health professionals and sleep specialists assess you for symptoms of Sleep Apnea. Diagnostic equipment is used to evaluate breathing-related measurements, blood oxygen levels, and other factors. Sleep apnea tests can be conducted at a sleep center or at home with a WatchPAT One Disposable Home Sleep Test

Sleep-Disordered Breathing (SDB)

Often interchangeable with “sleep apnea,” SDB is a general term used to describe conditions that prevent you from breathing properly while sleeping. 

Sleep Log or Sleep Diary

A detailed document of sleep patterns, feelings and emotions that can help to evaluate your perceived sleep quality. A sleep specialist may ask you to keep a sleep log for two weeks or so, and compare this information with your sleep study results. 


Getting a good seal is imperative for you to get the full benefit of CPAP therapy. A good seal ensures that you have no leaks, and that the air pressure is being delivered evenly and comfortably. 

If you hear or feel air leaking, you may have a CPAP mask that is too big or too small, may not be the right mask type for your face, or might not be put together properly. If you have questions about mask fit, don’t hesitate to reach out to our team

Snore Index

When you snore, you create waves of pressure. The strength of these waves are measured and calculated into an index during a sleep study, helping your doctor better understand your sleep situation and recommend the correct CPAP therapy. 


SoClean offers fast and easy CPAP equipment maintenance products. Shop our selection of SoClean products on


Therapy Pressure

See CPAP pressure. 

Tidal Volume

Volume of air that moves into and out of the lungs in an ordinary, normal breath.

Travel CPAP Machine

Travel CPAP machines allow you to maintain your CPAP treatment no matter where you go. Portable travel CPAPs work the same as home machines, but use a lightweight and compact design to make it easier to pack and use in unfamiliar environments. 













If you feel we’ve missed any key terms (or that we’ve accidentally slipped into medical speak and it’s still confusing), please drop us a note here. We want to ensure that this list is useful to all patients at every stage of their CPAP journey. 

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