The air pressure in a CPAP machine is measured in cm/H2O or Centimeters of Water- alternatively also called as CWP or Centimeters of Water Pressure. One way to understand the cm/h2O term is through the following example: if a person drinks water using a 3.9 inch (10 cm) straw, the suction generated would be 10 cm of H2O (water).


Speech is also measured in similar terms. It is calculated at around 7 cm/ H2O pressure once it crosses the vocal cord.

The Mean Pressure considered for Sleep Apnea treatment is 10 cm/H2O, with the normal range for OSA (Obstructive Sleep Apnea) being 6 cm to 15 cm/ H2O. However, this is not fixed and can vary for some individuals.

Air pressure from the machine performs the role of a pneumatic splint that aids in keeping the throat open. Of note, the pressure is much lesser than one generates by sneezing and very rarely can result in ears popping. (Source: TS Johnson, MD et al., Sleep Apnea – The Phantom of the Night, p. 92.)


A scientific term, Titration, means slowly increasing something (pressure, medicine dosage, etc.) till a desired effect is achieved. In the context of Sleep Study, the sleep technician gradually increases the pressure in the CPAP machine (1 cm/water) until the patient stops having Apnea/Hypopnea episodes. This, along with specific other measurements, helps the technician to reach a level that is comfortable for the user.

Knowing the Machine and the Prescription

One has to learn about the Titrated pressure from the sleep study conducted and also has to be aware of the pressure prescribed by one’s doctor. If one does not have both of these, then one must ask for a copy of the same from the sleep study center as well as doctor’s script.

Starting at a specific pressure setting and then Titrating the pressure works for most people and they continue with the same setting with no further issues. But, in case one feels tired despite increased pressure, one must check the accuracy of the machine and get it resolved and verify the machine settings and the pressure prescription by the Respiratory Therapist (RT).

Side Effects due to Wrong Settings?

No, there are no such side effects unless the user suffers from a serious medical illness. If the user gives up CPAP therapy, then the long-term effects alter. However, it is highly essential for the user to know their Titrated pressure arrived at through the sleep study, the pressure prescriptions, as well as understanding whether the machine’s set correctly by the RT (Respiratory Therapist).

Low Pressure Settings

It is vital to have the prescribed pressure settings on the user’s PAP machine. If the pressure is very low for the user, then the machine would not remove all Apneas/Hypopneas and could also face issues with clearing out the exhaled CO2 from the mask. Very low pressure is indicated by Insomnia, Snoring, Gasping Insomnia, subtle suffocated and claustrophobic feelings while on the PAP machine.

We could also detect lower-than-required settings if the patient has an Apnea-Hypopnea Index (AHI) of more than five events per hour, or feels sleepy, or tired during the day even after using the machine.

About lowest APAP setting, some would find the setting of 4 cm to 5 cm/H2O not good enough to pump out the exhaled CO2 and would require a setting between the range of 6 cm to 8 cm/H2O and sometimes even more.

High Pressure Settings

A very high pressure can result in mask leaks, leading to mouth breathing, dry throat and mouth despite having increased humidification, uncomfortable therapy, episodes of air swallowing (Aerophagia), feeling very sleepy and tired during daytime as well as having AHI of more than 5 events per hour.

More On Settings

People also tend to equate higher pressure settings with an increase in central Sleep Apneas that are pressure-induced. While for some users pressure settings of greater than 15 cm/H2O are considered very high, for others pressure ranging from 18 cm to 20 cm/H2O is very high. If one’s Auto PAP machine is left unchanged from the default factory setting of a wide range of 4 cm- 20 cm/H2O, one might start experiencing issues with both high and low-pressure settings.

Additionally, APAP machine takes more time responding because of a wide range setting. Not changing the default wide range setting or having other wrong pressure settings (APAP/CPAP mode, ramp, exhalation relief) may lead to AHI not being low enough as required, ineffectiveness of the therapy process or user discomfort leading into decisions of discontinuation of the therapy.

A normal AHI is when a sleeper has around five or less events every hour; many sleepers need AHI at the level of 2.5 events or less to feel refreshed. To know one’s AHI, one would require a machine that has a smart display and the required software.

SDB (Sleep Disordered Breathing) Types


A Greek word, Apnea, connotes ‘without breath’. A sleeper is said to have Apnea when he stops breathing for 10 or more than 10 seconds during sleep.


Hypopnea is an atypical shallow breathing condition where the air flows at a reduced speed through a sleeper’s throat, leading the sleeper into not getting the required amounts of oxygen. It usually lasts for nearly ten seconds.

  • Apnea-Hypopnea Index: AHI is an index used for Sleep Apnea, where 5 Apnea/ Hypopnea events per hour are typical. If a sleeper has 5 or more episodes per hour, then he/she has mild Sleep Apnea, having 15 or more than 15 is considered as moderate and having 30 or more events is deemed to be severe (T. S. Johnson, MD, Sleep Apnea - The Phantom of the Night, page 211).
  • UARS (Upper Airway Resistance Syndrome) or Flow Limitation: It is another subtle form of SBD, wherein the user does not have a smooth airflow in the nose and mouth, resulting in the brain sending impulses to the sleeper to wake up. AASM (The American Academy of Sleep Medicine) encourages counting of Apneas, UARS, and Hypopneas for determining Respiratory Disturbance Index.
  • RDI (Respiratory Disturbance Index): Used for determining SDB. A person who has Sleep Apnea in the range of 5 events to 20 events every hour is considered to have a mild SDB. If he has 20 events to 40 events per hour, then he has a moderate SDB. A person who greater than 40 events per hour is considered to have a severe SDB (Barry Krakow, MD, Sound Sleep, Sound Mind, page 245).

Importance of Using the Prescribed Pressure Setting

It is essential to verify whether the machine’s pressure setting is as per the prescription since the settings keyed into the user’s PAP machine by the RT (Respiratory Therapist) at an HME (Home Medical Equipment) / DME (Durable Medical Equipment) company can sometimes be wrong.

The company might not have had enough safety checks to determine the correct settings, so if keyed in wrong, the user might not even know, unless and until the user has alternate means of checking the settings.

The user has a right to ask as well as observe whether the settings have been checked by the Respiratory Therapist or by a lead RT after the settings have been keyed in by another or the same RT. If the same RT has keyed in the settings, then the user is entitled to have a demonstration by him in clear and understandable terms. Incase these checks are not done at the time of getting the machine one scan always get them performed a few days after getting the machine.

If the RT is not forthcoming with facilitating checks on the pressure settings, it could be because either they are unable to or unwilling to verify the machine settings.

If the RT is unable to confirm settings on account of lack of adequate knowledge of setting up the PAP machine, the chances of the company making errors are very high, and it becomes essential to have it checked again.

If the RT is unwilling, it might be due to an inhibiting company policy of not allowing the users to learn how to set the pressure settings, lest it gets tampered with. Nonetheless, the user has every right to verify if the setting is as per the doctor’s prescription much in the same way as one is allowed to see if the prescribed medicine and the dosage are correct.

If the user encounters resistance, then he/she can talk with either a lead RT or even the branch manager. If the DME insists on requiring a script for observing correct settings, then the user should get the script to check the machine on their own.

Learning Patient-Controlled Settings

The user should know the pressure settings and read the manuals that accompany the machine as well as the heated humidifier to understand how to set and control the various functions and features such as reading the display screens, using the magnifying glass and the flashlight. If the user has issues with pushing the buttons, then it is advisable to get somebody’s help.

Checking PAP Machine’s Pressure Setting Accuracy Using the Clinician’s Manual

If the user can handle basic technology, then using the clinician’s manual would be easy. In fact, checking the set-up menu would be a lot similar to changing the time on a digital clock and would not require more than a minute once the user has an idea of what to do.

If one is technologically challenged and finds it difficult to program the machine or any other modern device, then getting help would be advisable. One could rely on either the RT of the local DME Company for checking the prescribed settings or seek help from the prescribing MD or someone else who is better versed.

Acquiring a Clinician’s Manual

The manufacturers do send a Clinician’s Manual on how to set the PAP machine’s pressure settings along with the new machine’s User’s Manual. The local HME/DME Company; however, fearing liability issues, may give only the User’s Manual and not the Clinician’s Manual without a script from the doctor.

While asking for a script for getting the manual, the user should inform the MD that absence of the prescription would result in their disability to check the accuracy of the pressure settings keyed in by the HME/DME. One should also discuss the settings in depth along with the prescription. . An understanding of the User’s Manual helps in understanding the Clinician’s Manual. So, one should clarify one’s queries about the user’s manual with the doctor. This will help in approaching the CM more easily.

Buying a PAP machine online ensures that all the manuals are available to the users. Though some users purchase Clinician’s Manual online, they have to ensure that they receive the manual for their machine and not any other machine. Many people seek the help of experienced users to understand simple directions such as using the buttons for accessing the set-up menu and may not need a hard copy of the manual.

One does not need to have the machine software for using the manuals to verify whether the RT of local DME/HME has set up the machine correctly. One can note down the guidelines for later use as one scrolls down through the settings. If the user finds any discrepancy with regards to setting up the machine or prescription not being followed, then it should be immediately reported to the doctor.

The user should take the faulty machine to the DME for correction or correct the machine on his own. The discrepancy should be reported to the supervisor and /or lead RT of the local HME/DME or branch/regional manager and user should also file a complaint regarding the discrepancy with The Joint Commission on Accreditation of Healthcare Organizations using the link in order to ensure that the error does not result in harm to other patients.

This could be helpful since DME/HME requires feedbacks from the public to improve their service. Using the clinician’s manual would help users to alter pressure settings to reflect their current needs, but one should seek medical advice and go through software readings for having feedback on the changes made.

Making Changes to PAP Machine’s Pressure Settings

To get the pressure settings on the PAP machine changed by the DME or the doctor, one has to either call the doctor or get an appointment and explain why a change should be made to the pressure settings. The user should ensure that the DME/HME receives the prescription and processes it internally so that the user’s local office can make the requisite changes.

The user has to get an appointment with the local DME/HME office and reach there with the machine to ensure that the RT makes the adequate changes. It is advisable to cross-check the settings once again, preferably by a different RT, and get a detailed demonstration of the user’s PAP machine that shows the correct settings.

Alternatively, the users can make the necessary changes on their own if they have the adequate knowledge about the same. After the user has attained mastery over using mask, hose, mask leaks, humidity, comfort, exhalation relief, and ramp/settling and has been stable enough with the usage of the present PAP machine pressure settings, he/she could get a Pulse Oximeter for obtaining overnight recordings (either rent it or buy online) for checking saturation levels of the blood oxygen during the night time and ensure that the PAP machine is working correctly. The user can also get a machine with the adequate software that aids users in knowing the efficacy of the PAP machines.