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Sleep Apnea 101: Definitions

Sleep Apnea Definitions


CPAP therapy comes with its own lingo. This is the place to visit if you are wondering what "AHI" means, what "rainout" is or why some machines are APAPs and others are CPAPs. Please check the FAQ section below for an alphabetized listing of common sleep terms.

Definitions FAQs


The word "phage" in Greek means "to eat." Aerophagia is literally to eat air. In the modern context, it means swallowing too much air, a common cause of gas in the stomach and belching.

This is sometimes a result of CPAP Therapy. Unfortunately, the best solutions for this problem are not a catch all but instead an attempt to find the middle ground between needed CPAP pressures.

Here's what you can do:

  1. Tell your CPAP prescribing physician. It may be better to decrease the CPAP level a bit and put up with some minor airway problems if we can significantly decrease the bloating. Sometimes this works great, sometimes not at all. An auto adjusting CPAP should also be considered as these machines reduce the average pressure the user receives. Get help settings the maximum pressure on the auto as the highest pressures can make things much worse.
  2. As the chin of a CPAP user nears the chest, air has a greater change of entering the digestive tract. Where possible, keep chin up in relation to torso.
  3. Sleeping in a position different than normal helps keep the air out of the stomach. Try different sleeping positions. Left side, right side, inclined with pillows, or flat.
If these fail, It may be that you should consider a surgical alternative to CPAP. Many have tried all sorts of other solutions to this bloating problem with little success.

What is A-Flex?

A-Flex is a form of exhalation relief which helps the air pressure from an APAP machine mirror a person's breathing. Like C-Flex, A-Flex provides flow-based pressure relief at the beginning of exhalation. Like Bi-Flex, A-Flex softens the pressure transition from inhalation to exhalation to provide additional comfort in an auto-CPAP mode.

A-Flex Easy Breathing Technology Visual Graph A-Flex Easy Breathing Technology Visual Graph

AHI (Apnea Hypopnea Index)

Apnea Hypopnea Index is the number of apneas and hypopneas per hour. This index, as well as oxygen saturation levels (O2 Sat), is used to determine how severe a case of Sleep Apnea a patient has.

To determine AHI add the total number of apnea events, plus hypopnea events and divide by the total number of minutes of actual sleep time, then multiply by 60.


Apnea + Hypopnea divided by actual sleep time, then multiply by 60

200 Apneas, 200 Hypopnea (400 Total Events)

420 Minutes Actual Sleep time (7 hours * 60)

Divide 400 by 420 =.95 x 60 = 57 AHI (Severe OSA)

The Severity of OSAHS as defined by the American Academy of Sleep Medicine Task Force (1999) is:

AHI < 5 Normal
5 =< AHI <15 Mild
15 =< AHI <30 Moderate
30 =< AHI Severe

What is Ambient Tracking Technology?

Ambient Tracking technology is a rainout reduction feature created by Fisher & Paykel. Ambient Tracking Technology monitors the ambient temperature and adjusts heat to maintain the most effective humidification, given the ambient conditions. The Ambient Tracking Technology uses an auto-adjusting heater plate to maximize humidity delivered while adjusting for room temperature. The level of humidity delivered is dependent on room temperature.

Effects of Ambient Temperature on Humidification Ambient Temperature

What is an APAP Machine?

An APAP (Automatic Positive Airway Pressure) machine automatically adjusts on a breath by breath basis to blow the minimum pressure needed to keep your airway open during sleep. This allows your machine to provide you with your ideal pressure nightly.

APAPs tend to be more advanced than CPAPs in that they normally offer more therapy tracking information and more comfort features. You can learn more about therapy tracking information in our Software section of the Learning Center.

Every APAP machine can be placed in a CPAP mode to blow one straight pressure like a CPAP. However, no CPAP can automatically adjust pressure like an APAP. An example of an APAP machine is the DeVilbiss IntelliPAP AutoAdjust that comes with built in Smart Code therapy tracking software.
IntelliPAP AutoAdjust CPAP Machine


Apnea is the cessation of airflow for 10 seconds or more. Apnea is Greek for "without breath."


An Arousal is an abrupt change from a "deeper" stage of non-REM sleep to a "lighter" stage (Normally to stage 1).

This is distinct from an awakening, which is an arousal that lasts for more than 15 seconds.

Automatic Altitude Adjustment

Auto Altitude Adjustment is the CPAP machine's ability to compensate for changes in altitude automatically.

The feature allows the patient to travel to different altitudes without having the pressure setting adjusted. Because atmospheric pressure changes with altitude, most machines need adjustment in order to provide the correct pressure at a different altitudes.


An awakening is an arousal that lasts for more than 15 seconds.

An arousal is an abrupt change from a "deeper" stage of non-REM sleep to a "lighter" stage (Normally to stage 1).

What is Bi-Flex?

Bi-Flex is a feature that makes breathing back against CPAP pressure easier to do. Bi-Flex pressure relief technology offers pressure relief at the transition from exhalation to inhalation, at the transition from inhalation to exhalation, and during exhalation to make BiPAP therapy more like natural breathing.

Bi-Flex Easy Breathing Technology Visual Graph Bi-Flex Easy Breathing Technology Visual Graph

What is a BiPAP or BiLevel machine?

A BiPAP machine (also referred to as BiLevel or VPAP) delivers two separate and distinct pressures. A higher pressure when the patient inhales and a lower pressure when the patient exhales. The machine alternates between the two pressures which are set per the doctor's order or prescription. A prescription for a CPAP machine may not be used to purchase a BiPAP machine.

An example of an BiLevel machine is the Philips Respironics PR System One BiPAP Auto with Bi-Flex.
PR System One BiPAP Auto

What is the difference BiPAP and BiPAP ST?

A BiPAP, also referred to as a Bi-Level or VPAP, alternates blowing two set pressures, a higher pressure for inhalation and a lower pressure for exhalation.

A BiPAP ST Machine is a non-invasive ventilator. ST stands for Spontaneous Timed.

A BiPAP has two set pressures but some patients require a back up timed response in which the BiPAP will initiate a breath if a breath is not taken within the set timed parameters.

For example, a BiPAP ST might be set with an inhale pressure of 10 and an exhale pressure of 5 with a BPM (BPM=Breath Per Minute) of 12. A BiPAP ST will not breathe for you but will initiate a breath if you do not inhale 12 times a minute.

BR Arousal index

BR Arousal index is the number of breathing related arousals (apnea, hypopnea, snoring & RERAs)multiplied by the # hours of sleep.

Bruxism (Teeth Grinding)

This is a habit observed in a large number of people occasionally, and in a smaller number, habitually. The mechanism of causation is tension and spasm of the muscles used for mastication. The term also refers to clenching of the teeth, which causes similar problems.

Often, it occurs during sleep; even a short nap may induce it. In a typical case, the canines and incisors are commonly moved against each other laterally, i.e. a side to side action, which erodes tooth enamel, removing the sharp biting surfaces and flattening the edges of the teeth. Sometimes, there is a tendency to grind the molars together, which can be loud enough to wake a sleeping partner.

Over time, bruxing shortens and blunts the teeth being ground, and may lead to pain in the joint of the jaw, the temporomandibular joint, or headache. Teeth hollowed by previous decay (caries) may collapse; the pressure exerted by bruxism on the teeth is extraordinarily high.

The cause, or causes, of bruxism remains unclear. Some dentists believe it is due to a lack of symmetry in the teeth; others, that it reflects anxiety, digestive disturbances or a disturbed sleep pattern.

The effects of the condition may be quite advanced before sufferers are aware they brux. Eroded teeth may be brought to the sufferer's attention during a routine dental examination. If enough enamel is eroded, the softer dentine will be exposed, and erosion will accelerate. This opens the possibility of dental decay and tooth fracture, so early intervention by a dentist makes sense.

What is a built in humidifier?

A "built in" humidifier is designed as a part of the CPAP machine and cannot be removed or separated from the CPAP machine. As with an integrted humidifier, it eliminates the need for a second hose or power cord. The water chamber can be removed for cleaning or replacement purposes.

The advantage of a "built in" humidifier is it makes for a more compact unit with fewer parts. This design also helps to ensure therapy effectiveness by including the heated humidifier with the CPAP machine thereby providing comfort to the CPAP user.

A disadvantage is that a built in humidifier may produce less moisture if used as passover because the surface area is typically smaller. Also, since the humidifier is "built in" to the machine, detaching it is not an option. This can be an issue for frequent travelers. Lastly, if the machine or humidifier stops working, the entire unit must be sent in for repair or replacement, not just the component that failed.

An example of a machine with a built in humidifier is the ICON Auto CPAP Machine with Built In Heated Humidifier and SensAwake as seen below. ICON Auto CPAP Machine with Built In Heated Humidifier and SensAwake

What is Central Sleep Apnea (CSA)?

In Central Sleep Apnea (CSA), a person stops breathing throughout the night, not due to a blocked air passage, but because the brain temporarily stops sending signals to the muscles that control breathing.

BiPAP therapy can be used to assist persons with central sleep apnea. The BiPAP machine provides air pressure in an inhale/exhale pattern helping a person to breathe.

What is C-Flex?

C-Flex is a pressure relief feature that makes breathing back against CPAP pressure easier to do by reducing the pressure at the beginning of exhalation and returning to therapeutic pressure just before inhalation. Independent studies conducted by leading universities indicate that patient comfort, and therefore, patient CPAP compliance, is higher with machines that use C-Flex. C-Flex is a technology invented by Respironics, is proprietary, and is not available on other brands of CPAP Machines.

C-Flex Easy Breathing Technology Visual Graph C-Flex Easy Breathing Technology Visual Graph

C-Flex vs. Traditional CPAP: Brown University Study Interview, C-Flex in the Remstar Pro and Remstar Auto

What is C-Flex+?

C-Flex+ is an improved pressure relief technology over the previous C-Flex technology. Like C-Flex, C-Flex+ reduces the pressure at the beginning of exhalation. Like A-Flex, C-Flex+ softens the pressure transition from inhalation to exhalation to provide additional comfort in fixed-CPAP mode.

C-Flex+ Easy Breathing Technology Visual Graph C-Flex+ Easy Breathing Technology Visual Graph

What is Climate Control Technology?

Climate Control technology is a rainout reduction feature created by RedMed. The Climate Control system is made up of the combination of the S9 machines, H5i heated humidifier, and the ClimateLine heated tubing. Air is delivered at the temperature requested while five sensors, including one close to the mask, monitor multiple conditions to provide optimal humidification to avoid condensation.

Climate Control Technology Fact Sheet

What are compliance tracking machines?

Compliance tracking machines only track the numbers of hours the machine is used. These machines do not track any advanced information such as AHI or leak rate.

Compliance machines are basic machines which the manufacturers made to be able to track usage hours. Compliance tracking machines are a manufacturer response to Medicare requirements that mandate DME (Durable Medical Equipment) providers show the Medicare patient used the equipment for the required minimum number of hours set by Medicare. DME providers cannot continue to be paid by Medicare without this information.

There is one billing code for all CPAP and APAP machines. Therefore a DME will be paid the same from Medicare or insurance whether they provide a patient with a more expensive machine that is data capable with more comfort features or a basic compliance tracking machine. Therefore, compliance tracking machines are the most common type of machine that is provided to patients through local DMEs.

There is software created to show the compliance information from the machine, however does not recommend purchasing this software. This data will only tell you how many hours you actually used the machine. It will not tell you how effective your therapy is.

What is CPAP therapy?

CPAP stands for Continuous Positive Airway Pressure. CPAP therapy is the most commonly recommended, and the most effective, treatment for obstructive sleep apnea.

A CPAP machine provides air at a constant prescribed pressure. The air is delivered to the person through a tube and a CPAP mask. CPAP therapy provides a constant airflow which keeps the airway open so uninterrupted breathing is maintained during sleep. The air stream eliminates Sleep Apnea events and allows the person to get a restful sleep.

The level of air pressure required to maintain your airway is determined during your sleep study. Your doctor will write a prescription for a CPAP machine set at that pressure.

CPAP therapy is delivered through a nasal mask that seals around the nose or nasal pillows which seal at the nasal opening. Full face masks are available for a person who breathes through both the nose and mouth. Many innovative and comfortable options are available and advances in the delivery of CPAP therapy are continually occurring.

For instance: Some machines offer a comfort feature such as FLEX or EPR technology. It provides exhalation relief, a slight drop in pressure at the time of exhalation to help make therapy more natural. APAP, BiPAP and BiLevel machines offer various levels of pressure throughout the night.

What is the purpose of a chinstrap?

Nasal delivery devices are the preferred mode of delivering CPAP air. They are smaller, lighter and usually more comfortable, with many styles and sizes to choose from.

When using a nasal device with a CPAP, it is imperative that the mouth remain closed for the pressure to be maintained and the therapy to be effective. Most people will automatically keep their mouth closed while on CPAP, but some are unable to do so, at least in the beginning of the therapy.

Use of a chinstrap is the first response to opening the mouth during sleep. It is worn in addition to the CPAP mask to hold the jaw up and help keep the mouth closed.

Even with the use of a chinstrap, some people are still able to breathe through their teeth and lips. If so, a full face mask is an option worth considering.

What are data capable machines?

Data capable machines track your therapy progress and performance over time. Therapy data can be used to spot issues and used to see how changes to your CPAP setup effects your overall treatment. You and your doctor can use the therapy information to make changes in your setup. Some changes might be altering the mask or fit or increasing or decreasing your pressure to see if your therapy performance improves to ensure you are getting effective treatment.

The data capable machines not only let you know how many hours the machine was used, like pure compliance tracking machines, but also tracks advanced information such as AHI, Leak Rate, and Average Pressure (if using an APAP).

Depending on the machine, the information can be reviewed or accessed

  • On Machine Screen Display. Machines that display advanced tracking using the on machine display allow for fast review of therapy metrics. Many people like to use the on screen display to check every morning. Information such as AHI and leak rate for the previous night are displayed allowing the user to make adjustments such as mask fit. The information provided on the on machine screen is usually average values over different time frames, such as 1 day (last night), 7 night, 30 nights, etc.
  • SmartCode. SmartCode is a code that is accessed through the machine's display. This code is then inputted into the machine's product page on and a summary report is generated. You can see a Sample Report here. An example of an on machine display with the SmartCode is shown below. The only machine with advanced data capable SmartCode is the IntelliPAP AutoAdjust. The SmartCode provides therapy data averages for the following time frames: 1 day (last night), 7 day, 30 day, 90 day, and cumulative. This is like the information provided from the on screen display, however the information is displayed in chart format instead of a single number at a time. SmartCode Display
  • Optional External Software. Some machines have external software that can be used to download the data from the machine for detailed analysis. External software, unlike the on machine display or the SmartCode, allows you and your doctor to look into any night of use. Further, the information for any given night can be broken down hour by hour. This allows you and your doctor to look for patterns in your data through the night; for instance, do you tend to have more apneas in the early morning? Or do you begin to breathe with your mouth open in the middle of the night? Some external software is available for sale to patients, other software is only sold by the manufacturer to sleep professionals. External software may require other components to be able to get the information from the machine onto you computer with the software installed.

DME (Durable Medical Equipment Provider)

The purest definition of DME is a provider who accepts payment in exchange for CPAP Equipment and Service. Traditional DMEs and Online Providers fall into this category.

As referenced in culture, DME usually refers exclusively to traditional, local providers that accept insurance in exchange for CPAP Equipment and Services. Many who recount their experience with these local providers complain of poor service and high prices. Thus the use of "DME" or "evil DME" is prominent on the sleep apnea forum

What is EPR with Easy-Breathe?

EPR (Expiratory Pressure Relief) is a pressure relief feature developed by ResMed. EPR reduces pressure during exhalation in order to make breathing more comfortable. Easy-Breathe technology applies a smooth waveform that helps make breathing feel more natural.

EPR with Easy Breath Manufacturer Fact Sheet

What is exhalation relief?

Exhalation relief is a comfort feature offered by two manufactures which reduces the air flow at the time of exhalation making the breathing process more natural.

  • Respironics = C-Flex, C-Flex+, A-Flex and Bi-Flex
  • Resmed = EPR (Expiratory Pressure Relief)

What are the different types of filters?

There are five types of filters used with CPAP machines: reusable foam filters, disposable white fine filters, standard disposable filters for ResMed machines, hypoallergenic disposable filters for ResMed machines and bacteria filters.

Reusable Foam Filters. Reusable foam filters are placed at the air intake of the machine and are designed to catch larger particles. These filters can be lightly washed with a mild detergent to remove any particles that have collected. It is suggested to replaced a foam filter every 3 months or when the filter starts to break down. If a foam and disposable filter are used in a machine, the foam filter should be on the outside. M Series Filters

Disposable Filters. These filters cannot be washed. There are three types of disposable filters:

  • White Disposable Fine Filters. Ultra fine filters collect much smaller particles and are useful for those with allergies or sinus issues. These filters are not meant to be cleaned, but rather discarded when dirty. This filter is placed at the air intake for your machine and is used with a reusable foam filter. It is placed inside the machine first with the foam filter on the outside. This filter should not be washed and is suggested to be replaced every month, or more often with a more dusty environment. M Series Fine Filter
  • Standard Disposable Filters for ResMed machines. Disposable standard filters clean large particulate matter from the air as it enters the CPAP or BiLevel machine. This filter is placed at the air intake for your machine and is used by itself for ResMed machines. For the S8 machines, one side is colored, usually yellow or blue, and the other is white. It is placed with the color side facing out. For the S9 machines, they are all white and it does not matter which side faces into the machine. These filters should not be washed and it is suggested to be replaced every month, or more often with a more dusty environment. These come in different shapes depending on the machine. Below are examples of the standard disposable filters for the S8 and S9 machines.
S8 Standard Filter S9 Hypoallergenic Filter
  • Hypoallergenic Disposable Filters for ResMed machines. Hypoallergenic filters are made of a combination of two different density filter materials in order to block both the fine and large particulate matter. This filter is placed at the air intake for your machine and is used by itself. Hypoallergenic Disposable filters are available for the S8 and S9 ResMed machines if you want more filtration than the standard filter. This filter should not be washed and is suggested to be replaced every month, or more often with a more dusty environment. These come in different shapes depending on the machine. Below are the hypoallergenic filters for the S8 and the S9 machines. S8 Hypoallergenic Filter S9 Hypoallergenic Filter
    Bacteria Filters. Bacteria filters are fine enough to remove some bacteria and are clinically indicated when a CPAP machine is used by more than one person. However, many CPAP users find them helpful in reducing the occurrence of sinus infections and other nasal issues. Bacteria filters can be used with an CPAP/APAP/ or BiPAP machine.

    They may be used with humidifiers, but should be removed and allowed to air dry during the day. This filter is placed at the air outtake for your machine if you are not using a humidifier. If you are using a humidifier, it is placed at the air outake for your humidifier. The bacteria filter is placed in the air outtake and the CPAP hose is placed on the other end.

    The entire housing is disposable and should be discarded as soon as matter becomes visible. A clogged bacteria filter may affect the amount of air delivered by the CPAP.
    Bacteria Filter Bacteria Filter Air Flow

    What is a heated CPAP hose?

    A heated CPAP hose contains copper coils embedded in, or wrapped around, the hose. These coils are gently heated to conduct a constant temperature throughout the length of the hose. This enhances the comfort of the therapy and reduces or eliminates rainout caused by the water condensating as it travels through the hose to the mask.

    Heated hoses are more expensive alternatives to cloth tubing insulation, but they prevent rainout in nearly all cases. The ComfortLine Heated Tubing Kit is a stand alone option that can be used with any machine to prevent rainout and increase delivered humidification.
    Hybernite Rainout Control System
    Some manufacturers have developed heated hoses to work specifically with select machines in their product lines. The ClimateLine Tubing is used with S9 and H5i Climate Control System machines.
    ClimateLine Tubing
    The following video shows how to install a ClimateLine hose on a S9 Series Machine with H5i Heated Humidifier:

    The PR System One Heated Tube is used with PR System One 60 Series CPAP Machines.
    PR System One Heated Tube
    The ThermoSmart Heated Hose is used with Fisher & Paykel 600 Series machines.
    ThermoSmart Heated Hose

    What is a heated humidifier?

    A heated humidifier uses heat to warm the water to add moisture to the airstream. The heat is adjustable for more or less moisture. The water chamber for a heated humidifier is much smaller than a passover humidifier.

    In our considerable experience of specializing in CPAP equipment we have found that patients who use a heated humidifier have a much higher rate of effectiveness to their CPAP therapy.

    There are three types of heated humidifiers:

    • Stand Alone Humidifiers
    • Integrated Humidifiers
    • Built In Humidifiers

  • What is a hose cover?

    A hose cover is placed over the hose in order to insulate the hose and to make the hose more comfortable. The simplest and most cost effective way to insulate a CPAP hose is to wrap it in an insulating fabric. This enables the hose to remain flexible and adds little weight.

    Companies such as Snugglehose provide a cost effective tubing insulation available in several colors and styles. Not only will the covers reduce or eliminate rainout, they also provide a more personal and less institutionalized appearance. An example of a Snugglehose is shown below.


    What is humidification?

    Humidification adds moisture to the CPAP air, reducing irritation to the nasal passages caused by the increased airflow.

    CPAP air is an irritant - to one degree or another - to everyone. In some cases the irritation dries out the nasal passages and may cause bleeding. It may also cause swelling, excess mucous, congestion or sneezing. The irritation may create a very fertile ground for infections to begin. The irritation may be cumulative, building up over time. The only way to reduce the irritation is to add moisture.

    Humidification is therefore a critical part of CPAP Therapy.


    A hypopnea is a decrease in airflow by at least 50% for ten seconds or more, with a 3% desaturation of blood oxygen level.

    What is an integrated humidifier?

    An integrated humidifier is used with a specific CPAP and fits directly onto the machine. This eliminates the need for a second hose, and with most heated humidifiers the need for a second power cord.

    The advantages of an integrated humidifier are that it works very closely with the CPAP machine and is a compact unit that uses fewer parts.

    A disadvantage is that if it is being used as a passover humidifier it will produce less moisture because the surface area is typically smaller. An integrated humidifier will function only with the CPAP for which it was designed.

    You can see if your machine has an integrated humidifier option by reviewing our Compare Charts.

    An example of an integrated humidifier is the PR System One Heated Humidifier as seen below attached to a machine in the PR System One line. PR System One Heated Humidifier

    Manual Altitude Adjustment

    The feature allows the patient to travel to different altitudes without having the pressure setting adjusted. Because atmospheric pressure changes with altitude, most machines need adjustment in order to provide the correct pressure at a different altitude. When this feature is manual it means you must adjust the altitude setting on your CPAP machine yourself. Most CPAP machines have automatic altitude adjustment, meaning if you travel above sea level, the machine will adjust automatically to compensate for atmospheric pressure loss.

    The maximum altitude on a CPAP at this time is 11,000 feet. Therefore if you live above 11,000 feet, you will need to increase your CPAP pressure setting.

    What is Mixed Sleep Apnea (MSA)?

    In Mixed Sleep Apnea, the patient has a combination of both Central Sleep Apnea and Obstructive Sleep Apnea. Treatment is usually a combination of medication and CPAP use. There are now some advances in technology with some recent BiPap or BiLevel machines on the market made specifically to treat the special needs of those afflicted with MSA.

    What is Obstructive Sleep Apnea (OSA)?

    Apnea is Greek for "without breath." Obstructive Sleep Apnea (OSA) is a condition in which a person's muscles and tissues in the throat and air passage relax while sleeping. The tissues impede the flow of air into the lungs due to the blockage of the airway. This can occur many times per night in the sleep cycle, especially during the REM sleep stages.

    The scientific standard for an apneic event is when you stop breathing for at least 10 seconds. There must be an overall blood oxygen desaturation level of 3 to 4 percent along with certain potential changes in EEG frequencies and data. If a person has 5 or more of any type of events per hour of sleep then they are clinically diagnosed with sleep apnea.

    The body responds to the lack of oxygen by arousing, or waking, from sleep. This cycle leads to the person being unable to get the needed quantity of restful sleep and may result in:

    • Constant Fatigue
    • Difficulty Focusing
    • Irritability

    Untreated Sleep Apnea is potentially fatal and frequently results in the following serious health problems:

    • Heart Attack
    • Stroke
    • High Blood Pressure
    • Diabetes

    What is a passover humidifier?

    A passover humidifier uses a chamber of room temperature water with the CPAP to add moisture to the airflow and reduce irritation to the nasal passages.

    Air flows from the CPAP through a short hose to the humidifier chamber where the air "passes over" the water to pick up whatever moisture it can. The air then flows into the six foot hose to the mask.

    Passover humidifiers may provide sufficient moisture with CPAPs set at lower-end pressures. However, if the moisture is not sufficient there is no way to increase the amount of moisture added. Furthermore, because the water is room temperature, in cold climates the water temperature will drop causing the CPAP air to become very cold and the amount of moisture will be reduced

    PLM arousal index

    PLM arousal index: The number of periodic limb movements that cause arousals multiplied by the number of hours of sleep.

    What is rainout?

    Rainout is the accumulation of water in a CPAP tube due to warm moist air cooling and condensating on its way from your CPAP machine to your CPAP mask.

    The image below from Fisher & Paykel explains how rainout is formed. Warm moist air leaves the heated humidifier and travels down the CPAP tube. As the air travels down the tube the room temperature cools the tube and thereby cools the air traveling down the tube. As the air cools, it releases its moisture and condensation occurs, otherwise known as rainout.

    Solutions to rainout include:

    • Raising the temperature of your bedroom.
    • Keep your CPAP machine at the same level as your bed.
    • Insulating your tubing with a hose cover like Snugglehose.
    • Reduce the tempurature setting on your heated humidifier.
    • Purchasing a CPAP machine with a rainout reduction comfort feature.
    • Some machine systems offer a compatible heated CPAP hose which maintains the temperature from the machine all the way to mask reducing the occurrence of rainout. The list of available heated hoses can be found here: Hoses With Heating Coils.
    • If your machines uses a standard hose, consider the ComfortLine Heated Tubing Kit.

    Conventional Humidification

    What is ramp?

    The ramp button on a machine is a feature that allows for a gradual pressure build-up to your prescribed pressure. The feature is enjoyed by those with high pressures because they ease up to their pressure.


    Respiratory event-related arousals. Episodes that are not apneas or hypopneas, often related to loud snoring, that generally do not cause a decrease in oxygen saturation.

    What are reusable foam filters?

    Foam filters are designed to collect larger dirt particles and animal fur. If two filters are used in a machine, the foam filter should be on the outside.

    An example of a reusable foam filter is shown below.

    M Series Filters

    RDI (Respiratory Disturbance Index)

    Includes apneas and hypopneas, and may also include other respiratory disturbances such as snoring arousals, hypoventilation episodes, desaturation events, etc. They are often identical, but depending upon what is scored, the RDI may be larger than the AHI.

    What is a Sleep Study (Polysomnography)?

    The Sleep Study (Polysomnography) is the gold standard for the diagnosis or several sleep disorders including, sleep apnea, restless leg syndrome, and narcolepsy. The results of the test are referenced as a polysomnogram which is frequently abbreviates as PSG.

    Polysomnography is conducted in a sleep center, hospital or in a person's home. The test is usually performed at night to record the person's normal sleep pattern. Electrodes are placed on the scalp, the outer edge of the eyelids, and to the skin on the chin in preparation for the test.

    Characteristic patterns from the electrodes are recorded while you are awake with your eyes closed and during sleep. The time taken to fall asleep, time to enter REM sleep, movement, breathing pattern, and sleep architecture are all recorded using a computer.

    The person gathering and scoring the data is called a Polysomnographic Technician.

    What are SmartCode Report Definitions?

    This is a list of report topics generated when using SmartCode to track therapy effectiveness.

    Usage Threshold: Determines the minimum amount of time that the patient must use the device to be logged as a compliant day. Device settings allow for 4 or 5 hours, with 4 hours being the default setting. This setting can be changed in the device clinical set-up menu.

    Adherence Score: Shows the maximum adherence achieved for any period of thirty consecutive days of use within a ninety-day time frame. The score represents the percentage of days, within thirty consecutive days, where usage met or exceeded the 'Usage Threshold.' The Adherence Score reports the 'best' consecutive thirty days of usage. For example, if the 'best' 30 out of the last 90 days show 22 days used above the usage threshold and 8 days below the threshold, then the Adherence Score would show 73% (22/30). For this calculation, the usage must be continuous hours and longer than the Usage Threshold.

    Days at Least X Hours: Shows the number of days in the timeframe where the total usage within each day is at least X hours. "X" represents the 'Usage Threshold' setting in the patient's device. For this calculation, all usage within a 24-hour period is recorded.

    % Days at Least X Hours: Shows the percentage of days in the timeframe where the total usage within each day is at least X hours. "X" represents the 'Usage Threshold' setting in the patient's device. For this calculation, all usage within a 24-hour period is recorded.

    Day Count: Shows a count of days for each timeframe. If, for example, the 90-day code is collected at day 45 of therapy, then the "Day Count" for this code will be 45 days. In most cases, however, the day count is equal to the days in the code's timeframe(s).

    While Breathing Hours: While breathing hours are only available within the last-day timeframe and with the cumulative code. This measurement shows the total hours of use.

    95th Percentile Pressure: AutoAdjust pressure was at or below this pressure 95-percent of the time it is in use.

    90th Percentile Pressure: AutoAdjust pressure was at or below this pressure 90-percent of the time it is in use.

    AHI: Apnea/Hypopnea Index (AHI) represents the average number of apneas and hypopneas per hour for the timeframe.

    Pressure Plateau Time: Represents the percentage of time the AutoAdjust spent at the upper pressure setting. A value of 10% or more indicates a possible need to raise the upper pressure setting to allow higher pressures.

    High Leak Flow Time: Shows the percentage of time where leak was above 95 liters per minute. A value of 10% or more indicates that the mask fit should be checked.

    NRI: Non-Responding Event Index (NRI) represents the average number of non-responding events per hour. A non-responding event is a respiratory event that is detected but by design does not result in a pressure change during AutoAdjust therapy.

    EPI: The Exhale Puff Index (EPI) is the number of exhale puff events detected per hour. Exhale puffing is caused by inhaling through the patient circuit and exhaling through the mouth.

    Spontaneous arousal index

    Spontaneous arousal index: The number of spontaneous arousals (e.g. arousals not related to respiratory events, limb movements, snoring, etc) multiplied by the number of hours of sleep.

    Stages of Sleep

    • Stage 1: The lightest stage of sleep. Transitional stage from wake. Stage 1 shifts: The number of times the sleep stage changed to stage 1.
    • Stage 2: The first true stage of sleep.
    • Stages 3-4: The deepest, most restorative sleep.
    • Stage REM: The dreaming stage; Normally occurs every 60-90 minutes.

    What is a stand alone humidifier?

    A stand alone humidifier is a component that will work with any CPAP and does not attach directly to the machine. It has its own power cord and a short hose is used to connect it to the CPAP. Generally it will sit next to the CPAP machine and is slightly larger than an integrated humidifier.

    The advantage of a stand alone humidifier is the fact that it may be used with any CPAP machine.

    The main disadvantages are that the size is often larger than either a "built in" or integrated humidifier and that a short hose is needed to connect it to the CPAP machine.

    An example of a stand alone humidifier is the Fisher & Paykel HC150 Heated Humidifier as seen below. HC150 Heated Humidifier


    Supine is a sleep position: Sleeping on back. Non-supine means to sleep in a position not on your back.

    What is System One Humidity Control?

    System One Humidity Control technology is a rainout reduction feature created by Philips Respironics. System One Humidity Control technology analyzes ambient room temperature, relative humidity and patient flow in order to adjust heat to deliver an optimal level of humidification that avoids rainout conditions. The heated tube maintains the humidification all the way from the machine to the mask.

    System One Humidity Control Technology System One Humidity Control Technology

    What Surgery or Surgical Treatments are available for Sleep Apnea?

    There are several different types of surgeries available for Sleep Apnea, depending on the patient, but any surgery comes with inherent risks or possible complications, and can be painful. CPAP machines are still the most effective treatment for Sleep Apnea with very minimal risk to the end-user. Often the surgery success rate is relatively low and many people report being back on a CPAP machine, or the like, within a year, albeit often at a lower pressure.

    Here are some of the more common surgeries available, although doctors are always innovating and coming up with new procedures. Not all are covered here. Talk to a sleep doctor to find out more about what the newest and best options are in your own case.

    Tonsillectomy or Adenoidectomy

    This is performed to increase the size or opening of the air passage in the case of Sleep Apnea treatment.


    UPPP is performed to reduce or remove or reshape parts of the soft palate and the uvula. It may also involve removal of part of the soft tissue from the pharyngeal areas.

    Laser-assisted Uvulopalatoplasty

    LAUP is similar to the above description of Uvulopalatopharyngoplasty however it uses lasers or radiofrequency waves to remove and shape the tissue.

    Maxillomandibular Advancement

    MMA, also known as (MMO) Maxillomandibular Osteotomy or (Bi-Max) Bimaxillary Advancement, is usually reserved for severe cases which have not responded to other surgeries or an CPAP machine well or where craniofacial syndromes or structures are suspected as the cause of the problem. It is more invasive and involves removing the top and bottom jaw and moving them forward or restructuring the chin, maxilla or mandible. Often it is performed in conjunction with Genioglossus Advancement.

    Genioglossus Advancement

    This procedure, commonly known as tongue advancement, is designed to move or pull the tongue forward to increase the size of the airway. It is sometimes performed along with Maxillomandibular Advancement (see description above).

    Hyoid Suspension

    This procedure involves pulling forward the hyoid bone in the neck to place it in front of the larynx. The hyoid bone is one of the attachment points for the tongue.

    TAP Implant (Pillar Procedure)

    The Pillar system is different from other treatments. It is specifically designed to produce a lasting reduction in snoring after a single office procedure. Unlike other procedures, the Pillar procedure does not require heating or removing tissue, or the use of any sclerosing agents, which means the procedure causes minimal discomfort and recovery time is short. In fact, most patients resume a normal diet and activities the same day.

    During the Pillar procedure, no tissue is removed. Instead, three tiny inserts are placed in your soft palate during a brief office visit using local anesthetic. The inserts support and stiffen the palate, which has been shown to reduce snoring in individuals with very moderate sleep apnea. In addition, the inserts are made of a polyester material that causes a stiffening response in the palate tissue itself. This supporting and stiffening of the soft palate will last as long as the inserts are in place.

    Unfortunately, this is not a solution for those with more than mild sleep apnea. If the obstruction occurs in a place other than the site of the implants, this procedure will be ineffective.

    What is ThermoSmart Technology?

    ThermoSmart technology is a rainout reduction feature created by Fisher & Paykel. ThermoSmart technology is a unique heated hose which maintains the warm environment for the airflow to prevent condensation.

    With conventional humidification, as air flows from the heated humidifier through the CPAP hose, the surrounding air cools the air inside the CPAP hose. As the air cools, it releases moisture in the tube and mask. The heated hose creates a continuous heated environment to help the air hold its moisture and be delivered effectively to the user.

    Conventional Humidification Conventional Humidification ThermoSmart Technology ThermoSmart Technology

    What is titration, as it relates to sleep studies?

    If a sleep study determines you have obstructive sleep apnea (OSA), you will need a "titration study" to determine the pressure level required to maintain your airway during sleep. A titration study may take place during the same night as your diagnosing sleep study or it may be scheduled for a later date.

    Titration is a scientific lab term, meaning to slowly add a little bit more of something until you reach a desired effect. During your titration study, a skilled technician will monitor your sleep and slowly increase the air pressure on a CPAP machine until you are able to sleep without or with few sleep events (apneas and hypopneas). Your pressure needs vary throughout the night, so an overnight sleep study is required to prescribe you the correct pressure.

    Your prescribed CPAP single pressure setting will be set to the highest level you needed during the night. By prescribing the highest pressure, your doctor hopes to prevent as many sleep events as possible. The downside of this approach is that you will have the highest pressure at all times even when it's only necessary for part of the night.

    APAPs offer technologies which allow the pressure to be adjusted on a breath by breath basis, which you may find helps your therapy.

    Total # of PLMs

    Total # of PLMs: The number of leg movements in sleep that last greater than 0.5 seconds.

    Total arousal index

    Total arousal index: Total number of all arousals multiplied by the number of hours of sleep.

    TRT (Total Recording Time)

    TRT is the amount of time that is recorded during a Polysomnogram. This statistic can be used to compare how long it takes a patient to fall asleep, and Totals Sleep Time (TST).

    TST (Total Sleep Time)

    TST is the actual time a patient spends asleep during a Polysomnogram. This statistic can be used to determine sleep architecture.

    What is CPAP tubing / CPAP hose?

    CPAP tubing and CPAP hose are interchangeable terms. CPAP hoses / tubing are used for two main purposes: to connect a CPAP or BiLevel machine or a humidifier to a mask, or to connect the machine to a humidifier.

    Machine to Mask Hose. A hose is connected to the air outlet of either the machine or humidifier and it connects to the CPAP mask to deliver the airflow to the mask. In the past, the hose used to connect a machine to mask was a standard size. Today, there are a number of different hose options. There are three types of machine to mask hoses:

    • Standard / Performance Hose: A long hose or tubing is included with each CPAP / BiLevel machine purchase. The connections on all CPAPs, humidifiers and masks are a standard size, so a standard CPAP hose will fit on all. Standard hoses are also available in 8 foot and 10 foot sizes. There are lighter hoses available that were designed to be more flexible and put less pull on the CPAP mask.CPAP Performance Tubing
    • Heated Hose: A heated CPAP hose contains copper coils embedded in or wrapped around the length of the hose. These coils conduct a constant temperature through the hose. The majority of heated hoses are specific to the machine. Heated Hose
    • Hose with Sensor Line: A few auto-titrating CPAPs and Bilevels require a pressure line sensor fitted into the hose to detect breathing patterns and pressure needs but the majority of those machines are no longer in production. (Machines which use the tubing pictured below have been discontinued by the manufacturer.) Sensor Line Tubing

    Humidifier Hose. Humidifier hoses connect a CPAP or BiLevel machine to a stand alone humidifier. Humidifier hoses are shorter than standard hoses. They come in either 18 or 24 inches.
    18 in Humidifier Hose