A bilevel PAP (often called BiPAP or bilevel CPAP) is a breathing device that delivers two different levels of air pressure: a higher pressure when you breathe in and a lower pressure when you breathe out. Standard CPAP pushes one constant pressure throughout your entire breathing cycle. That single difference changes how the therapy feels and who benefits most from it.
How Bilevel Differs From Standard CPAP
A standard CPAP machine delivers one fixed pressure, measured in centimeters of water pressure (cm H₂O). Whether you’re inhaling or exhaling, the air pushes at the same force. This works well for most people with obstructive sleep apnea, because that steady pressure acts like a splint holding the airway open.
A bilevel device uses two separate pressure settings. The higher one, called IPAP (inspiratory positive airway pressure), kicks in when you inhale. The lower one, called EPAP (expiratory positive airway pressure), activates when you exhale. A typical setup might be an IPAP of 14 cm H₂O and an EPAP of 4 cm H₂O. The gap between those two numbers, 10 cm H₂O in this example, is called the pressure support. That gap is what gives bilevel therapy its advantages: it helps pull more air into your lungs on each breath while making exhalation feel significantly easier.
Think of it this way. With CPAP, you’re always breathing against the same wall of pressure, even when you’re trying to push air out. With bilevel, the machine drops the pressure as you exhale, so your breathing muscles don’t have to work as hard. On the inhale, the higher pressure both keeps your airway open and actively assists your lungs in taking a deeper breath.
Who Needs a Bilevel Device
Most people diagnosed with obstructive sleep apnea start on standard CPAP. Bilevel is typically prescribed in specific situations where CPAP alone isn’t enough or isn’t tolerable.
- High pressure intolerance: Some people need CPAP pressures above 15 or 20 cm H₂O to keep their airway open. Exhaling against that much pressure can feel like blowing against a wall. Bilevel solves this by keeping the inhale pressure high while dropping the exhale pressure to a comfortable range.
- Obesity hypoventilation syndrome (OHS): People with OHS who have mild or moderate sleep apnea, or who experience treatment failure with CPAP, are recommended for bilevel therapy. The extra pressure support on inhalation helps move more air into lungs that are compressed by excess weight.
- COPD and other lung conditions: When someone has both sleep apnea and a chronic lung disease, their breathing muscles are already fatigued. The pressure support from a bilevel device unloads those muscles, helping them breathe more effectively during sleep.
- Central sleep apnea: In some patients, central apneas persist or emerge even after standard CPAP eliminates the obstructive events. Medicare defines complex sleep apnea as cases where, after obstructive events are treated, more than 50% of remaining events are central in nature and the central apnea index is 5 or more per hour. These patients often need a bilevel device with a backup breathing rate.
Bilevel Breathing Modes
Unlike a standard CPAP, which has essentially one mode, bilevel machines can operate in several ways depending on what your breathing needs.
In spontaneous mode (S), the device senses when you start to inhale and switches to the higher pressure, then detects your exhale and drops to the lower pressure. You control the timing of every breath. This is the most common mode for people who breathe reliably on their own but need the pressure relief on exhalation.
In timed mode (T), the machine delivers breaths at a set rate programmed by your provider, regardless of your own breathing effort. This is less common and generally reserved for people who cannot reliably trigger breaths on their own. Spontaneous/timed mode (S/T) combines both approaches: the machine follows your breathing when you’re initiating breaths normally but kicks in with a backup rate if you stop breathing for too long. A backup rate of around 12 breaths per minute is a common starting point. S/T mode is frequently used for people with central sleep apnea or hypoventilation syndromes where breathing effort can fade during sleep.
Typical Pressure Ranges
EPAP settings generally fall between 2 and 8 cm H₂O, with 4 to 5 cm H₂O being a common starting point. IPAP ranges more widely, from around 7 to 25 cm H₂O, depending on what’s needed to keep the airway open and support adequate breathing volume. The goal during titration is often to achieve an IPAP at least 8 cm H₂O above the EPAP, which gives meaningful pressure support.
Some advanced bilevel machines auto-adjust the IPAP within a prescribed range, raising or lowering it breath by breath based on what the algorithm detects you need. Your provider sets a minimum and maximum IPAP, and the machine finds the right level in real time. The minimum IPAP is usually set at or just 2 cm H₂O above the EPAP, so at its lowest the machine is barely adding pressure support. At its highest, it’s delivering the full prescribed assist.
Getting Titrated and Prescribed
Bilevel pressure settings are determined during a sleep study (polysomnography) or sometimes through a home titration period that can take anywhere from 3 to 5 days on average. The American Academy of Sleep Medicine recommends that all candidates receive education about how the device works, a hands-on demonstration, careful mask fitting, and time to get comfortable with the mask before the formal titration begins.
During titration, the technician or algorithm increases pressure until apneas, hypopneas (partial airway collapses), effort-related arousals, and snoring are eliminated. For bilevel, this means finding the right IPAP to keep the airway open and the right EPAP to maintain comfort during exhalation.
Insurance coverage for bilevel devices is more restrictive than for standard CPAP. Medicare, for example, requires documentation that CPAP has been tried and failed, or that a specific condition like complex sleep apnea or hypoventilation is present. Your sleep study results need to show the specific patterns that justify the step up from CPAP.
Common Side Effects
Most side effects are the same ones CPAP users experience, since both therapies deliver pressurized air through a mask. Dry mouth, eye irritation from mask leak, sinus congestion, and skin irritation where the mask sits on your face are all common. Claustrophobia or anxiety about wearing the mask is a real barrier for some people.
One side effect that can be more pronounced with bilevel therapy is stomach bloating (aerophagia), which happens when pressurized air gets swallowed into the stomach instead of going into the lungs. Because bilevel machines can deliver higher inspiratory pressures, this can be more noticeable. Reducing the pressure settings or adjusting your sleeping position can help. Trouble clearing mucus and mild sinus pain are also reported.
Does Bilevel Improve Adherence Over CPAP?
You might assume that because bilevel feels easier to breathe against, people would use it more consistently. The research doesn’t clearly support that. In a randomized trial comparing bilevel and CPAP in patients with obesity and obstructive airway disease, average nightly use was 4.1 hours for bilevel and 5.6 hours for CPAP over three months, a difference that was not statistically significant. There were no significant differences in daytime sleepiness, sleep quality, or cognitive function between the two groups, and no major side effects were reported with either therapy.
This doesn’t mean bilevel won’t feel better for you personally, especially if you’ve struggled with exhaling against high CPAP pressures. It does mean that the comfort advantage of bilevel doesn’t automatically translate into people wearing the mask longer. The biggest factors in adherence tend to be mask fit, nasal congestion management, and whether you notice a real difference in how you feel during the day.