CPAP vs APAP vs BiPAP: how the three machines differ

CPAP, APAP and BiPAP are three kinds of positive airway pressure (PAP) machine, and they all treat sleep apnea the same basic way: by blowing air through a mask to hold your airway open while you sleep. The difference is how they set the pressure. CPAP holds one fixed pressure, APAP adjusts it through the night, and BiPAP switches between a higher pressure when you breathe in and a lower one when you breathe out. For most adults with obstructive sleep apnea, guidelines treat CPAP and APAP as equally good first choices.

What is the difference between CPAP, APAP and BiPAP?

The difference between CPAP, APAP and BiPAP is how each machine delivers air pressure. CPAP gives one steady pressure all night. APAP raises or lowers pressure automatically as your breathing changes. BiPAP gives two pressures, one for breathing in and a lower one for breathing out. All three use the same kinds of masks, tubes and humidifiers.

AASM Sleep Education describes all forms of PAP therapy as keeping your airway open by providing a stream of air through a mask. Cleveland Clinic explains that the pressurized air keeps your tongue, uvula and soft palate from shifting too far into your airway, which stabilizes your breathing.

Cleveland Clinic lists a few more relatives in the same family. Auto-bilevel machines adjust both of BiPAP's pressures on their own. Adaptive servo-ventilation (ASV) is a different device used for certain types of central sleep apnea, which monitors your breathing and can deliver a breath when needed. ASV is a specialist treatment and isn't part of the usual CPAP, APAP or BiPAP choice.

How does a fixed CPAP machine work?

A fixed CPAP machine works by delivering a single, steady air pressure through your mask for the whole night. NHLBI describes CPAP as a machine that uses mild air pressure to keep your breathing airways open while you sleep. The pressure is prescribed by your sleep team, often after a titration study.

Cleveland Clinic says a CPAP machine takes in room air, filters and pressurizes it, and delivers it through a tube to your mask. Some machines add heated humidifiers and adjustable pressure settings. CPAP is the most common of the PAP devices, and Cleveland Clinic calls it one of the most common treatments for sleep apnea.

The pressure has to be right for you. One way to find it is an in-lab CPAP titration study, where a technologist adjusts pressure while you sleep. The AASM guideline notes that the lab approach gives you education from a trained technologist and the chance to fix comfort problems in real time, at the cost of a night away from home.

How does APAP work?

APAP, or auto-adjusting positive airway pressure, works by measuring your breathing and changing the pressure on its own through the night. AASM Sleep Education says APAP automatically raises or lowers your air pressure as needed. Your doctor sets a range, and the machine moves within it.

The AASM guideline names the main benefit: APAP can adjust to short-term and long-term changes in what pressure you need, such as after drinking alcohol, when you change body position, or as your weight changes. Many people need more pressure on their back than on their side, which our guide to positional sleep apnea explains.

APAP is also used as a way to start treatment at home. The guideline strongly recommends starting PAP with either APAP at home or an in-lab titration in adults with obstructive sleep apnea and no significant other conditions. Across 10 randomized trials, it found no clinically significant differences in adherence, sleepiness or quality of life between the two starts. When treatment begins at home on APAP, people can either keep using auto mode or switch to a fixed pressure based on their machine's data.

How does BiPAP work?

BiPAP, also called BPAP or bilevel PAP, works by using two pressure levels instead of one. AASM Sleep Education says there is a certain pressure when you inhale that drops when you exhale. Cleveland Clinic describes the same pattern: a higher pressure while breathing in and a lower one while breathing out.

The idea is comfort. Breathing out against steady pressure can feel like work for some people, especially at higher settings. The AASM guideline says BiPAP's main potential benefit over CPAP or APAP is improved comfort from the lower pressure on exhalation, which may then help people use it more.

There is a catch the guideline also points out. If the exhale pressure is set too low, it may fail to prevent the breathing events it's meant to stop. BiPAP machines also cost more.

Is APAP better than CPAP?

APAP is not better than CPAP for most people, according to the best evidence available. The AASM's 2019 guideline reviewed 26 randomized trials comparing ongoing APAP with fixed CPAP. Meta-analyses found no clinically significant differences in adherence, self-reported or objective sleepiness, or quality of life. The guideline strongly recommends either one.

The guideline rated the quality of this evidence as moderate and found no substantial differences in harms. It also notes that individual people may tolerate one form better than the other, and that patient preference varied between studies. So the advice is to choose based on how you do, not on a general ranking.

That's useful to know if you're nervous about your machine type. A fixed CPAP prescribed by your sleep team isn't a lesser treatment. And if fixed pressure feels wrong for you, APAP is a reasonable thing to ask about. Our guide to getting used to CPAP covers the settings and habits that help most in the early weeks.

Who gets BiPAP instead of CPAP or APAP?

BiPAP is usually given to people whose needs go beyond what CPAP or APAP can comfortably handle. The AASM's 2019 guideline suggests CPAP or APAP over BiPAP for routine obstructive sleep apnea treatment, and saves BiPAP for particular situations.

The guideline describes two main ones. The first is people who need pressures higher than 20 cm H2O, which CPAP units typically can't deliver. The second is people who can't tolerate CPAP or APAP because of high pressure needs, even with pressure-relief settings turned on. For them, a BiPAP trial may be offered during a lab titration or after a period of struggling with CPAP.

AASM Sleep Education adds a third: your doctor may recommend BiPAP if you have sleep apnea along with another breathing disorder. The guideline notes that some breathing disorders involving high carbon dioxide levels may require BiPAP and are covered by separate AASM guidelines. Its own recommendations don't apply to people with obesity hypoventilation syndrome or combined obstructive and central sleep apnea.

Why don't guidelines start with BiPAP for comfort?

Guidelines don't start with BiPAP because the research hasn't shown it helps most people use their machine more. In five randomized trials comparing BiPAP with CPAP, the AASM found no clinically significant differences in adherence, self-reported sleepiness or leftover sleep apnea. Studies of sleep quality and quality of life showed no meaningful differences either.

The guideline rated that evidence as very low quality, partly because of small studies and industry funding. It also says the old comfort advantage of BiPAP matters less now, because modern PAP devices include modified pressure profiles that also lower pressure when you breathe out. In practice this means you can often get some of BiPAP's comfort from a standard CPAP or APAP machine's pressure-relief setting.

Which is most comfortable?

The most comfortable machine is the one that fits your pressure needs, paired with a mask that seals well. The AASM guideline found no consistent winner on comfort or use between the three types in trials, and it says individual tolerance may differ for one form or another.

Most comfort problems have simpler fixes than switching machines. AASM Sleep Education says strap marks and skin sores often come from poor mask fit, and a humidifier can ease a dry nose and sore throat. NHLBI notes that your provider can help with the humidifier and pressure settings, or you may need to try a machine with multiple or auto-adjusting pressures. If your mouth dries out at night, our guide to dry mouth with CPAP walks through the usual causes and fixes.

The guideline also stresses support. It strongly recommends education when you start PAP, and suggests troubleshooting help and remote monitoring of your machine data in the first weeks. Those steps often matter more than the type of device.

How do cost and insurance affect which machine you get?

Cost and insurance often shape the choice, especially for BiPAP. We don't list prices here, because they vary widely by plan, supplier and region. What we can describe is how coverage rules work for Medicare in the US.

Medicare.gov says Part B may cover a 12-week trial of CPAP therapy, including devices and accessories, if you've been diagnosed with obstructive sleep apnea. After the Part B deductible you pay 20% of the Medicare-approved amount. Medicare pays the supplier to rent the machine for 13 months of continuous use, and after that you own it.

The Medicare national coverage determination (NCD 240.4) sets the diagnosis rules. You need a clinical evaluation and a positive sleep test, either an in-lab study or a qualifying home test. The test must show an AHI of 15 or more, or 5 to 14 with documented symptoms or conditions such as excessive sleepiness, impaired cognition, mood disorders, insomnia, high blood pressure, heart disease or a history of stroke. Our explainer on what AHI means covers these numbers.

The local coverage policy for PAP devices adds the details that affect machine type. It covers a bilevel device without a backup rate for sleep apnea only when a CPAP device has been tried and proven ineffective. Ineffective means failing to meet treatment goals despite proper mask fitting and appropriate pressure settings. A bilevel device with a backup rate isn't covered if the main diagnosis is obstructive sleep apnea.

That policy also defines use. Continued coverage after the first three months requires a follow-up visit and use of at least 4 hours a night on 70% of nights during a 30-day stretch in the first three months. This is why your machine records how long you wear it.

Private insurers set their own rules. NHLBI notes that your insurer may want to check your machine's data to confirm you're using it and that the settings are working. Ask your plan what it requires before you switch device types.

What should you ask your sleep doctor?

Ask your sleep doctor which machine they're prescribing and why. Questions that tend to help include whether you'll start with home APAP or a lab titration, what your pressure range is, and whether your machine has a pressure-relief setting for exhaling. If you have heart failure, lung disease, long-term opioid use or another condition that affects breathing, ask how that changes the plan, since the AASM's home APAP recommendation was based on studies that left those groups out.

If you're already on treatment and struggling, say so early. Pressure, mask fit and humidity can usually be adjusted, and a different machine type is an option when those don't work. Our guide to whether CPAP is safe covers side effects and when to call your team, and sleep apnea treatment options explains what's available beyond PAP.

This article is general information, not medical advice. Your sleep doctor can explain which machine fits your results.

Read next

• Getting used to CPAP
• Central sleep apnea
• CPAP cleaning and supplies

* This content is for informational and educational purposes only and is not medical advice. It is not intended to diagnose, treat, cure or prevent any disease or health condition. Talk to a qualified healthcare provider about your health.