Anatomy, Sleep Position, and Sleep Stages All Impact Pressure Needs
If you have been prescribed continuous positive airway pressure (CPAP
) therapy to treat your obstructive sleep apnea, you may wonder: does the severity of sleep apnea measured by the AHI correlate with the needed CPAP pressure for effective treatment? 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, including anatomy, sleep position, and sleep stages.
How Sleep Apnea Severity Relates to Pressure
It is only natural to assume that there would be a relationship between the prescribed CPAP pressure 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
at hospital or in some cases at home that assesses the number of times per hour of sleep that the upper airway collapses, resulting in a drop in blood oxygen levels or awakenings from sleep (arousal). If the airway completely collapses this is called Obstructive Sleep Apnea and if it partially collapses this is called hypopnea. The total number of these events per hour of sleep is the apnea-hypopnea index (AHI)
. The AHI allows a general classification of the severity of sleep apnea
If there are fewer than 10 events per hour, this is deemed to be normal (losing body weight may suggested). If 10 to 20 events per hour are recorded, this is mild OSA(Obstructive Sleep Apnea) and it could suggested by physician to start treatment with Sleep Apnea Machine. If more than 20 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 pressure 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 cmH20). 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 20 cmH20 for CPAP Machines and maximum of 25 cmH20.
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 while you sleep. Usually this is done remotely from another room, so that disturbances do not occur if it’s home study a AutoCPAP could run in Auto Setup to resolve the Sleep Apneas. 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), when sleep apnea is often worsened. Some people are sent home with a self-adjusting CPAP machine, sometimes called AutoCPAP
. 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 airway resistance (suggesting persistent collapses of the soft tissues like the base of the tongue or soft palate into the throat). The CPAP delivers intermittent pulses of extra air pressure to evaluate for resistance and, by extension, whether the upper 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
. This is especially true in the setting of obesity or nasal obstruction. 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 by turning them down.
Alcohol, medications that relax airway muscles
(like benzodiazepines), and sleeping on your back may all transiently add to your pressure needs.
Finally, 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. It may be desirable to use an AutoCPAP machine that is able to adjust with these other variables.