A glossary of sleep apnea terms

Getting diagnosed with sleep apnea can be daunting. You’ve undergone a sleep test, and now your sleep specialist or clinician sounds like she’s speaking in a foreign language as she explains your results and prescription. It’s easy to get overwhelmed!

If you’re just starting out, or need a quick refresher, here’s a helpful list of terms to help guide you from diagnosis to understanding your therapy equipment.

The basics

You’ve just been diagnosed with sleep apnea. What does that mean? Here are some terms you’ll soon be hearing a lot.


“Apnea” is Latin for no breath, which is exactly what’s happening to those with sleep apnea. For diagnosis reasons, an apnea is classified as stopping breathing for 10 seconds or longer during sleep.


Closely related to apneas, a hypopnea is a partial blockage of the airway. During a hypopnea, breathing is shallow. Your breathing is 30-50% less than your normal breathing for 10 seconds or longer.


This acronym stands for Apnea-Hypopnea Index. AHI is measured by taking the total number of apneas and hypopneas (“events”) you have during the night, then dividing this by the total number of hours slept. On a therapy device, this is measured the same way, but the number of events is divided by the number of hours you actually used the device that night.

Even people without sleep apnea may have some events during the night. As long as your score is 5 or below, you’re doing great! If you start getting scores above 5, speak with your clinician or sleep specialist. An increase in AHI may just be poor mask fit or leak, but it could indicate that your therapy needs some adjustment.


OSA stands for Obstructive Sleep Apnea. This is the most common type of sleep-disordered breathing. OSA is when an apnea or hypopnea occurs due to a complete or partial blockage or collapse of the upper airway.

CSA stands for Central Sleep Apnea. In this case, the airway is actually open, but the brain doesn’t send a signal to the body to breathe.

Some people can also have mixed apneas, or a mixture of OSA and CSA related events throughout the night.


Pressure measures the force of air needed to keep the airway open during therapy in centimetres of water. The prescription written by your sleep specialist might be a set pressure, like 12 cm H2O, or it may be a range, like 6-15 cm H2O. The pressure will be set by a sleep clinician and should not be adjusted without first consulting with your sleep specialist.

All about your device

You’ve taken your therapy device home and are getting to know it better. What are all these settings? What did the clinician say the ramp was, again? Here’s a breakdown of some of the options you may see on your therapy device.


These acronyms describe the basic types of therapy devices available to treat sleep apnea patients.

CPAP stands for Continuous Positive Airway Pressure. This type of therapy will deliver the same prescribed fixed pressure throughout the night to keep your airway open.

APAP is Automatic Positive Airway Pressure. As a patient’s pressure needs may change throughout the night, APAP therapy, unlike CPAP, will automatically change the air pressure being delivered. APAP responds to events, or lack thereof, by either increasing or decreasing the pressure. This helps deliver the optimum therapy pressure needed for each stage of your sleep, and may be more comfortable than CPAP therapy.

VPAP stands for Variable Positive Airway Pressure. This is the name of ResMed’s bi-level technology. Bi-level therapy devices deliver therapy at two different pressure levels: a higher pressure when you breathe in, and a lower level pressure when you breathe out.

VPAP devices are suitable for patients who require pressure settings above 20 cm H₂O.

Ramp/Ramp time

The Ramp feature is designed to help make therapy more comfortable by gradually easing you into your prescribed therapy pressure each night. When ramp is turned on, the air pressure will start at a lower level than prescribed and slowly ramp up to your full therapy pressure over a maximum of 45 minutes.

The AirSense 10 also comes with a new ramp feature called AutoRamp™, which gradually increases the pressure similar to the standard ramp option. However, once the device detects that you’ve fallen asleep it will automatically increase to your prescribed therapy level without waiting the full 45 minutes.

EPR™/Pressure Relief

EPR™, which stands for Expiratory Pressure Relief, is another comfort feature available on some ResMed therapy devices. It mildly lowers the pressure delivered when you breathe out, making it easier to exhale against the device.

On S9 and S8 device menus this is labelled “EPR,” on the AirSense 10 it is called “Pressure relief.”


This feature will begin your sleep apnea therapy as soon as you breathe into your mask, and stops it automatically a few seconds after the mask is removed. SmartStart™ works best with full-face and nasal masks.


Leak measures the amount of air escaping (or leaking out of) your mask. Leak can be caused by a number of issues—having the wrong mask size or type, the mask not being assembled or fitted correctly, or parts being worn out and needing replacement.

The acceptable leak rate is up to 24 litres per minute. Anything below this means you are still getting the full benefits of therapy. If your leak rate is regularly above 24L/min, you may want to check your mask for any wear and tear, go over the fitting instructions in the user guide, or speak with a clinician for further assistance.

Airplane Mode

This feature is available on AirSense 10 devices with wireless connectivity. Turning Airplane Mode on will stop the therapy device from transmitting data, as required by in-flight regulations. When this setting is turned off, the device will resume sending data.

Adding some humidity

Now that you’re more familiar with your therapy device settings, let’s talk a bit about humidifiers. Anyone who has spent a summer in far North Queensland will be familiar with the muggy feeling of humidity. This may be unpleasant when it comes to the weather, but in the world of sleep apnea turning up the humidity can actually be a good thing.


A humidifier adds moisture to the air by warming water added to a tub. Warm air holds moisture better than cool air, so heated tubing was made available with newer ResMed therapy devices to hold onto the moisture as the air travels to the mask. Many patients find breathing dry air to be uncomfortable, but adding moisture to the air with a humidifier helps alleviate this. (For more information on using a humidifier, please read the Humidifier settings FAQs.)

Humidifiers for older devices, such as the S9 and S8 models, come separately from the device itself. The AirSense 10’s humidifier is integrated into the therapy device.


“Rainout” is commonly used to describe condensation that collects within the therapy device tubing and mask. Condensation occurs when the warm, humidified air from your device hits the colder room temperature air and cools. Cooler air cannot hold as much moisture as warm air, causing the water in the humidified air to form water droplets.

Using a heated tube, such as the ClimateLineAir™, will help prevent rainout. If you use an older therapy device model that does not have heated tubing you can use a tubing wrap or run the tubing beneath your blankets to keep it warm. (For more information, please read the Preventing humidification condensation FAQs.)

A quick overview of masks

You likely trialled several masks before picking one to take home, so remembering your mask’s name and size months later might be tricky. Here’s a short primer to help when it’s time to order replacement parts.

Full face

You’re probably most familiar with full face masks, this style is frequently used to illustrate sleep-disordered breathing in the media. Full face masks cover both the nose and mouth. If you breathe through your mouth, this may be the best option for you. It’s also great to have one on hand if you suffer from allergies or nasal congestion—air can still flow in through your mouth when your nose is blocked.


Nasal masks cover just the nose. “Mouth breathers” may experience high leak with a nasal mask since air going into the nose will escape through the mouth. This can be remedied by using a chin strap which helps keep the mouth closed.


Pillows masks are quite popular due to their size. This is the smallest type of therapy mask, with two soft “pillows” that sit just under the nostrils. If you are claustrophobic this may be a good option for you. As with nasal masks, you may get high leak if you breathe through your mouth, so a chin strap may be necessary.


This is the portion of the mask that comes into contact with your face, either surrounding the nose and/or mouth or sitting underneath your nostrils. We recommend checking the cushion once a month for wear and tear. (For more information, please visit the How to care for your equipment page.)

On most ResMed masks, the cushion or pillow size is moulded or printed somewhere on this piece. Sizes generally run from x-small to large, with some masks sized as “wide.”


These are the straps that hold the mask on to your head. The headgear will stretch and lose elasticity over time, and will need to be replaced to provide good fit. (For more information, please visit the How to care for your equipment page.)

Some headgear will have the mask name and size printed somewhere on the headgear. However, if you can’t find this information on the headgear, the outlet where you purchased should be able to look up your purchase records to confirm.

We hope this list clears up some of the confusion you may be experiencing as you start out. Are there any terms we left off the list?

Sleep Well,

The sleepvantage Team