Pressure Definition. Machine air pressure is measured in centimeters of water, cm/H2O or cwp, centimeters of water pressure. A pressure of 10 cm/H2O means that if you were drinking water from a 10 centimeter (3.9 inch) straw positioned at the top of the water to your mouth, the suction you create would be 10 centimeters of water pressure. Speech is measured at about 7 cm/H2O pressure past the vocal cords. The average pressure for treating sleep apnea is 10 cm/H2O. Typical pressures for treating obstructive sleep apnea are 6 to 15 cm/H2O. Some people require higher pressures. The air pressure acts as a pneumatic splint to keep the throat open. The pressure is less than a sneeze and rarely causes the ears to pop. Source: TS Johnson MD et al, Sleep Apnea – The Phantom of the Night, p. 92.

Titration is a scientific lab term, meaning to slowly add a little bit more of something until you reach a desired effect. During the sleep study, the sleep technician slowly increases the CPAP machine pressure one centimeter/water at a time until you stop having apneic events (apneas and hypopneas). That and some more calculation lead to a titrated pressure setting. See Reasons Why Your Titrated Pressure May Be Wrong in the peer coaching article CPAP Machine Choices. Research article on the inaccuracies of a one-night titration: http://www.ncbi.nlm.nih.gov/sites/entrez?cmd=Retrieve&db=PubMed&list_uids=14971878&dopt=Citation

Know your titrated pressure from the sleep study. Know the pressure your doctor prescribed. If you don’t know them, ask your doctor’s office for a copy of the sleep study report and doctor’s prescription. Some people start and continue CPAP treatment with the titrated pressure from their sleep study and have continuing good results with no problems. Others still feel tired, or feel tired again after a few months, and need to explore the correctness of their machine pressure setting and pressure prescription, as well as first solving other equipment problems.

Don’t worry; unless you have other serious medical problems, your immediate life is probably not in danger! Long term is another matter if you give up on CPAP therapy. It’s important to know your titrated pressure from a sleep study (if you have been titrated) and pressure prescription, and know that your machine has been set up correctly by the Respiratory Therapist (RT). The prescribed pressure setting on your PAP machine is very important.

If the pressure is too low for you, it won’t eliminate all the apneas or hypopneas and may not clear out all the exhaled carbon dioxide from your mask. Pressure too low may be indicated by snoring, insomnia, feeling starved for air, subtle feelings of suffocation or claustrophobia while on PAP, an AHI (apnea-hypopnea index) above the normal 5 or below events per hour, or still feeling tired or sleepy during the day. Some people find that a setting of 4 or 5 cm/H2O is not high enough to clear the exhaled carbon dioxide, and need a setting of 6, 7, or 8 cm/H2O or more as their lowest setting on an APAP.

Pressure that is too high for you may be indicated by uncomfortable therapy, large mask leaks, mouth breathing, dry mouth and throat even with heated humidification, aerophagia (swallowing air), an AHI above the normal 5 or below events per hour, and still feeling tired or sleepy during the day. Some people have concerns that too high a pressure setting may lead to pressure-induced central apneas (the brain not telling the body to breathe) unless the PAP machine algorithm (operating rules) prevent runaways as in Respironics machines. Pressure settings above 15 cm/H2O are considered high for some people; for others, 18, 19, 20 cm/H2O is high.

If your autopap machine is left at the factory default setting of 4 to 20 cm/H2O, you may experience the problems of both too high and too low. In addition, the APAP machine may have difficulty responding quickly enough with this large range. With pressure too low, too high, or other wrong setting (CPAP or APAP mode, exhalation relief, ramp or settling), your AHI may not be as low as it should be, or you may be more uncomfortable, and you may think the therapy doesn’t work very well and be tempted to give it up. To know your AHI, you need a machine with a smart display or better yet, software. Your AHI should be 5 events per hour or less to be considered in the range of normal sleepers. Many people require an AHI of 2.5 or less to be truly rested and invigorated.

Apnea. The Greek word “apnea” means “without breath.” You stop breathing during sleep for ten seconds or longer. Hypopnea. There is airflow through your throat but at a much reduced level, which leads to not getting enough oxygen. It’s abnormally shallow breathing lasting at least ten seconds.

1. AHI, Apnea-Hypopnea Index for sleep apnea: Less than 5 events (apnea or hypopnea) per hour is considered normal. 5 or more events per hour is considered Mild sleep apnea 15+ considered Moderate 30+ considered Severe (from T. S. Johnson MD, Sleep Apnea - The Phantom of the Night, page 211)

Flow limitation or Upper Airway Resistance Syndrome (UARS) is another important, subtle form of SBD. The airflow meets resistance in the nose or mouth, causing the brain to waken the sleeper. The American Academy of Sleep Medicine advocates counting apneas, hypopneas, and flow limitations/UARS for the Respiratory Disturbance Index, RDI.

2. RDI, Respiratory Disturbance Index for sleep apnea: 5 to 20 events per hour is Mild SDB 20 to 40 events is Moderate SDB More than 40 events is Severe SDB (from Barry Krakow, MD, Sound Sleep, Sound Mind, page 245)

Check the accuracy of your machine’s prescribed pressure setting. This is important. Your doctor’s prescription was entered into the PAP machine by a Respiratory Therapist at a DME/home medical equipment company who could make a mistake. The DME company may not have procedures to detect setup errors, so if they made an error, you would never know, unless you had other means to check the settings. (Their follow-up phone call asking “how are you doing?” isn’t precise enough to detect set-up errors.) You are entitled to ask and observe to have the settings checked by a Respiratory Therapist other than the one who set it up, or by a lead respiratory therapist; or minimally, by a slow and careful walk-through demonstration by the same RT who programmed the machine originally. See the end of this article for a sample walk-through of machine set-up. The check could be done when you get the machine, or a few days later if you were overwhelmed by your diagnosis and too much information when you first picked up the machine.

If you run into resistance from the RT about checking your machine setting, is it because they are unwilling or unable? If unable because they are not competent in setting up the machine, there is a greater chance they made an error, and all the more reason to have it checked. If they are unwilling, it may be that, following company policy, they are unwilling to risk your learning how the machine is set so you don’t “tamper” with it. Nevertheless, you are entitled to see if it’s a correct prescription, just as you are entitled to see if the label on a bottled drug prescription or the pill has the right dosage. If you run into resistance, talk with the lead RT or branch manager, or if the DME insists, get a doctor’s prescription to let you observe the correct settings, or get a doctor’s prescription for the clinician’s manual from the DME and check it yourself.

Read the user’s manuals that come with your machine and heated humidifier to learn how to set the features you can control. For example, in a older, classic tank Respironics REMstar autopap, that would be heat/humidity level, C-Flex level, ramp pressure and duration of ramp (if in CPAP mode rather than APAP on an older machine), mask-off alert/auto-off, and button lights. Learn to read the display screens. Use a flashlight and magnifying glass if necessary. If you have trouble pushing buttons, you may want to get someone to help you. Discussion thread on a ramp (settling) pressure setting that was too low: http://www.cpaptalk.com/viewtopic.php?p=134380#134380

Using a clinician’s set-up manual to check the accuracy of your prescribed pressure setting

The clinician’s set-up manual is boxed with the new machine, along with the user’s manual. The local DME/HME company should give you the patient/user’s manual but, fearing liability, will probably withhold the clinician’s manual unless you have a doctor’s prescription for it. When you ask your doctor for a prescription for the manual (before or after you get your machine), tell him/her that without it you can’t check the correctness of your machine settings made by the DME. Discuss your ability to scroll through a menu with your doctor, show him/her the sample dialog at the end of this article if he/she isn’t familiar with a machine’s setup, and show him/her the user’s manual if you already have one. The clinician’s manual isn’t much different; if you can follow the user’s manual, you can follow the clinician’s manual.

If you buy your machine online at https://www.cpap.com, all manuals will be included. Some people buy clinician’s manuals online, but be sure that you get the one for your machine, not another model. Some people ask other experienced users for the simple directions on how to use the buttons to access the set-up menu in the display; you really don’t need the hardcopy set-up manual. 

You do not need machine software to use a set-up manual or instructions to check whether the DME RT set your machine correctly. As you scroll through the settings, write them down for later reference. If you find that the DME set up the machine wrong, not following your doctor’s prescription, inform your doctor and take the machine into your DME for correction by the lead RT or correct it yourself. Report the error to the local DME supervisor/lead respiratory therapist or branch or regional manager, and consider reporting it to the DME branch or regional manager and filing a complaint with The Joint Commission of Accreditation of Healthcare Organizations so other patients won’t be harmed. The DME needs feedback to improve their quality of service to other patients. See the article on Preventing and Reporting Errors in Your Care.

With a clinician’s manual, you will be able to change the pressure settings beyond the original prescription, but it is inadvisable to do that unless you have supervision from your doctor and software to give you feedback on the results of changes.

To get the machine pressure changed by your doctor and DME, you need to get an appointment or phone your doctor, explain why you think a change is needed, make sure that a prescription was sent to the DME and received, make sure that the DME processed the prescription internally so your local office can make the change, make an appointment at the DME, drive to the local office, wait, have the respiratory therapist make the change, which literally takes about 30 seconds, and drive home. Before you leave, request that another RT check the accuracy of the setting, or get a careful walk-though demonstration from your machine showing correct settings.

An alternative is to team with your doctor to correct or adjust the pressure settings yourself, if you are a suitable candidate for this team effort. See the next article on Changing Your CPAP Pressure Setting.

After you have mastered hose, mask, leaks, comfort, humidity, ramp/settling, exhalation relief, and been stable on PAP for a few weeks or months, borrow an overnight recording pulse oximeter from your doctor or RT, or buy one online. Use it to check your blood oxygen saturation levels at night as another indicator that your PAP is working well.

Even better, get a machine with software to more precisely show how well your therapy is working.

With your doctor, discuss a safe range for pressure adjustments in your treatment based on your sleep study, especially the higher pressure. For example, if your titrated pressure is 10 cm/H2O and you have an APAP, how suitable is a range from 7 to 15 cm/H2O for starters? Or does your doctor recommend 9 to 12 cm/H2O? If you haven’t been titrated, is a range from 6 to 16 cm/H2O appropriate for starters or not?

Next, for APAP, some people find a pressure to use as a central number for a range of pressure. If they have a titrated pressure, they use that number. Some people use the median or mean pressure as the central number.

Continuing the above about APAP, some people use their central number and add three points above and below it for a range. For example, if the titrated pressure is 10 cm/H2O, the range is 7 to 13 cm/H2O. Some people start with their titrated setting and go 3 cm/H2O under and 2 cm/H2O above the titrated pressure; for example, 7 to 12 cm/H2O. Some people benefit from an even narrower range, since that may help the machine to respond faster to events; for example, 9 to 11 cm/H2O. Remember, this is tricky business related to your health, throat anatomy, and the capabilities of each machine, so consult a doctor. For example, if you go 2 or 3 above your titrated pressure, is this likely to lead to a pressure-induced central apnea?

Some people on APAP use their titrated pressure as their lowest setting and go up 2 or 3 cm/H2O to catch events. For example, if the titrated pressure is 10 cm/H2O, the range is 10 to 12 or 13 cm/H2O. Again, work with your doctor, based on your titration study and health conditions.