Does Sleep Apnea Severity Correlate with the Needed CPAP Pressure?

Anatomy, Sleep Position, and Sleep Stages All Impact Pressure Needs

The CPAP pressure requirement may not directly correlate to sleep apnea severity due to other factors
The CPAP pressure requirement may not directly correlate to sleep apnea severity due to other factors. Brandon Peters, M.D.

If you have been prescribed continuous positive airway pressure (CPAP) therapy to treat your obstructive sleep apnea, you man wonder: does the severity of sleep apnea correlate with the needed CPAP pressure? Learn if and how the prescribed CPAP setting is related to the underlying degree of sleep apnea and what other factors might be involved to determine your optimal treatment.

How Sleep Apnea Severity Relates to Pressure

It is only natural to assume that there would be a relationship between the prescribed CPAP setting and the degree of obstructive sleep apnea that is being treated.

If you need a medication for blood pressure, a higher dose naturally would have a greater effect, right? Well, unfortunately, the relationship is not quite so direct when treating sleep apnea.

Obstructive sleep apnea (OSA) is diagnosed with an overnight sleep study that assesses the number of times per hour of sleep that the airway collapses, resulting in a drop in blood oxygen levels or awakenings from sleep. If the airway completely collapses this is called apnea and if it partially collapses this is called hypopnea. The total number of events per hour of sleep is the apnea-hypopnea index (AHI).

The AHI allows a general classification of the severity of sleep apnea to occur. If there are fewer than 5 events per hour, this is deemed to be normal. If 5 to 15 events per hour are recorded, this is mild OSA. If more than 15 but fewer than 30 events are observed, this is considered to be moderate OSA.

Finally, if more than 30 events are recorded per hour of sleep, this is characterized as severe OSA.

You might think that severe OSA requires a higher CPAP setting to treat it. In actuality, this is not always the case as there are multiple factors involved in determining the required setting. Typically, the treatment starts at a low setting and is gradually increased to resolve all apnea and hypopnea events as well as snoring.

The lowest setting on CPAP machines may be 4 to 5 centimeters of water pressure (abbreviated as cm of H20 or CWP). The vast majority of people require more pressure than this lowest setting. The maximum setting varies with the type of machine, but it could be as high as 25 or 30 CWP.

Determining the Required CPAP Pressure to Resolve Sleep Apnea

If the CPAP setting is determined as part of an overnight titration study in a sleep lab, a polysomnography technologist will observe the breathing patterns and adjust the setting upwards. The goal is to eliminate the sleep apnea and snoring and observe deep sleep, including rapid eye movement (REM) sleep. This setting should also be optimized while sleeping supine (on your back). 

Some people are sent home with a self-adjusting CPAP machine, sometimes called AutoCPAP or APAP. In this scenario, the prescribing physician allows a range of pressures, and the machine will start low and adjust upwards as needed in response to measured resistance. The CPAP delivers intermittent pulses of extra air pressure to evaluate for resistance and, by extension, whether the airway is collapsing.

If the airway is open at the current CPAP setting delivered, it is maintained.

The required CPAP pressure does not directly correlate with the severity of sleep apnea. Some people with mild OSA need high pressures and some people with severe OSA need relatively modest pressures. That being said, people with more severe sleep apnea do more often need higher pressures on CPAP, or even bilevel therapy. Children may require similar pressures to adults, despite the smaller size of their airways.

What factors determine the required CPAP pressure setting? The anatomy of the upper airway and the nature of the airway obstruction has the biggest role. If sleep apnea occurs because of a blocked nose due to allergies or a deviated septum, a collapsing soft palate, or a tongue that falls back into the airway, differing amounts of air are required to push these tissues out of the way. In addition, being overweight or obese may exacerbate things. In fact, when people lose about 10% of their body weight, it may be necessary to adjust the CPAP settings. Alcohol, medications that relax airway muscles (like benzodiazepines), and sleeping on your back may all transiently add to your pressure needs. REM sleep towards morning may relax muscles and exacerbate sleep apnea as well.

Proper Setting May Depend on Review of the Residual AHI

Therefore, as described above, it is not easy to guess what CPAP pressure you may need to treat your degree of sleep apnea. It may also vary somewhat during the night depending on your sleep position and sleep stage. If the pressure is too low, your sleep apnea will not be adequately controlled. If it is too high, you may experience side effects like mask leak or air swallowing.

It is important to have the machine properly set by a sleep specialist to ensure the best experience with it and the greatest benefits. Modern devices can provide information on the residual AHI and this can help to guide the pressure adjustments. If you continue to have symptoms, get reassessed to ensure your condition is adequately treated by your machine's pressure settings.


Kryger, MH et al. "Principles and Practice of Sleep Medicine." Elsevier, 5th edition.

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