CPAP vs APAP vs BiPAP: What the Difference Actually Means for You

The naming is unhelpful. “CPAP” is used both as the name of a specific machine type and as a catch-all for the whole category, so people end up comparing a thing to itself.

The NIH describes three types under the umbrella of positive airway pressure: CPAP, which “delivers constant pressure”; BPAP, which uses “variable pressure during inhalation and exhalation”; and APAP, which is “self-adjusting”¹ .

That’s the whole distinction. Everything else is detail.

CPAP — one pressure, all night

A fixed pressure, usually set after a titration study or a period of auto-titration, delivered constantly whether you’re breathing in, breathing out, on your back, or dreaming.

Good: simple, well-studied, cheap, and completely predictable. There’s nothing to second-guess.

Less good: your pressure needs aren’t constant. You need more pressure on your back and in REM sleep than you do on your side in light sleep. A fixed pressure has to be set high enough for your worst moments, which means it’s higher than necessary for much of the night — and unnecessary pressure is what drives aerophagia, leaks, and general discomfort.

APAP — pressure that moves with you

An auto-adjusting machine is given a range (say 6 to 14 cm H₂O) and moves within it, sensing flow limitation and snoring and raising pressure when it detects the airway starting to close, then backing off.

This is what most new patients in the US get. It means you don’t need a lab titration night to find a single number, and it handles the fact that your needs change with position, sleep stage, alcohol, congestion, and weight.

Good: lower average pressure across the night for most people, better tolerance, adapts as your body changes.

Less good: it’s reacting to events rather than preventing them, so some people see a slightly higher residual AHI than a well-set fixed pressure would produce. A few people find the pressure changes themselves noticeable enough to be annoying.

BiPAP — two pressures

Also written BPAP or bilevel. It delivers a higher pressure when you inhale (IPAP) and a lower one when you exhale (EPAP).

The reason that matters: at high pressures, exhaling against the airflow starts to feel like real work. Bilevel drops the pressure for the out-breath, which makes high therapeutic pressures tolerable.

Bilevel tends to be prescribed when:

• Someone needs pressures high enough that plain CPAP is genuinely uncomfortable.
• There’s central sleep apnea in the picture — the NIH notes central apnea is a problem with “the way your brain controls your breathing while you sleep” and is associated with heart failure, stroke, and opioid use² .
• There’s hypoventilation — conditions where you’re not just obstructing but under-breathing, such as obesity hypoventilation syndrome or some neuromuscular disease.
• Someone has failed CPAP despite a genuine effort with mask and settings.

Bilevel devices are a separate coverage category with their own documentation requirements, which is part of why you can’t simply request one because it sounds nicer³ .

Exhale relief is not the same as bilevel

Nearly every modern CPAP and APAP has a comfort feature that drops pressure slightly during exhalation — ResMed calls it EPR, Philips calls it Flex, others have their own names.

It helps, and it’s worth asking about if exhaling feels like effort. But it’s a small reduction over the first part of the out-breath, not the sustained two-level therapy that bilevel provides. Don’t let a salesperson blur the two.

Adaptive servo-ventilation, briefly

ASV is a more sophisticated bilevel device that adjusts breath by breath, developed largely for central and complex sleep apnea.

It comes with an important safety history. A large randomised trial of ASV in patients with central sleep apnea and heart failure with reduced ejection fraction was stopped early after finding increased mortality in the treatment arm, and that finding changed prescribing guidance and produced manufacturer safety notices. The practical upshot: ASV is contraindicated in certain heart failure patients, and it is a specialist decision made with echocardiographic information — not something to request.

So which is better?

For straightforward obstructive sleep apnea, the honest answer is that a well-set machine of any of these types works, and the mask matters more than the machine. People agonise over device choice and then quit because a cushion leaks into their eye.

If you’re being offered a choice at all, an auto-adjusting machine is the flexible default: it can run in fixed mode if needed, adapts as your body changes, and doesn’t require a lab titration to get started.

What actually determines whether therapy works is whether you wear it. That’s mask fit, the first thirty days, and fixing the small miseries before they become reasons to stop.