CPAP vs APAP — What Is the Difference?
CPAP vs APAP —
what is the difference
and which do you need?
CPAP delivers a fixed pressure all night. APAP adjusts automatically with every breath. Both treat obstructive sleep apnoea — but they suit different presentations. Here is the clinical difference explained clearly.
CPAP (Continuous Positive Airway Pressure) delivers a single fixed pressure every time you breathe in and out — all night, every night. The pressure is set by your consultant based on your sleep study results and stays constant regardless of what your airway is doing.
APAP (Automatic Positive Airway Pressure) adjusts the delivered pressure breath by breath. When it detects airway narrowing or an obstruction, it increases the pressure. When your airway is open, it reduces it. The result is that APAP typically delivers lower average pressure than fixed CPAP — and many patients find it more comfortable.
Which is better? Neither is universally superior. CPAP is the most extensively evidenced therapy and is preferred when OSA is consistent and well-characterised. APAP suits variable or positional sleep apnoea, and patients who struggle with fixed pressure. Your consultant will prescribe the right one based on your sleep study data — not a default.
How CPAP works
A CPAP machine pumps a continuous stream of pressurised air through a mask — maintaining enough air pressure in the upper airway to prevent the tissues from collapsing during sleep. The pressure is constant: the same when you breathe in as when you breathe out, and the same at 11pm as it is at 5am.
The therapeutic pressure — measured in centimetres of water pressure (cmH₂O) — is prescribed by a sleep consultant after reviewing your sleep study. Most patients with moderate OSA require pressures between 8 and 12 cmH₂O; severe OSA may require higher settings.
CPAP has been in clinical use for over 40 years and has the most extensive evidence base of any sleep apnoea therapy. For the right patient, it is highly effective and well-tolerated.
The most common complaint about fixed CPAP is the sensation of exhaling against continuous pressure — particularly at higher settings. This is where APAP has a clinical advantage for some patients.
How APAP works
An APAP machine works within a prescribed pressure range — a minimum and maximum set by your consultant. Within that range, the device adjusts pressure automatically on a breath-by-breath basis, responding to what your airway actually needs at that moment.
When the device detects resistance — the early sign of airway narrowing — it increases pressure. When it detects no obstruction, it allows pressure to reduce toward the minimum. The result is that most patients receive lower average pressure than they would on fixed CPAP, which many find more comfortable to breathe against.
APAP devices also generate detailed nightly data — showing the pressure range used, the hours the device operated at each pressure level, and the residual AHI. This data is reviewed by your Grace Sleep clinician at each clinical touchpoint and used to guide any adjustments.
APAP is particularly valuable for patients whose airway resistance changes significantly across the night — including those with positional apnoea (worse when sleeping on their back) or REM-related apnoea (worse during REM sleep, which is concentrated in the early morning hours).
CPAP vs APAP — side by side
| Feature | CPAP | APAP |
|---|---|---|
| Pressure delivery | Single fixed pressure | Auto-adjusting — breath by breath |
| Pressure range | One set value | Min and max set by consultant — device operates within range |
| Average pressure delivered | Constant | Typically lower than equivalent fixed CPAP |
| Adapts to body position | — No | ✓ Yes |
| Adapts to REM sleep | — No | ✓ Yes |
| Nightly data detail | Standard usage data | Detailed pressure histogram + residual AHI |
| Suitable for central apnoea | Not first line | Not appropriate |
| Evidence base | Most extensive — 40+ years | Equivalent for OSA — multiple RCTs |
| Consultant prescription required | ✓ Yes | ✓ Yes |
| Sleep study required first | ✓ Yes | ✓ Yes |
Who should use CPAP — and who should use APAP?
- Moderate to severe OSA with a stable, consistent AHI
- Your required therapeutic pressure has been established
- No significant positional or REM-related variation in your study
- You have tolerated CPAP pressure well during initial therapy
- Central apnoea is present — APAP may worsen central events
- Complex sleep apnoea where APAP auto-titration is inappropriate
- OSA severity varies significantly across the night
- Positional apnoea — worse when sleeping on your back
- REM-related apnoea — concentrated in early morning hours
- You find exhalation against fixed CPAP pressure uncomfortable
- Initial therapy while optimal pressure range is being established
- Weight changes affecting required pressure over time
The right choice between CPAP and APAP is a clinical decision — not a preference. Your Grace Sleep consultant will review your sleep study results and prescribe the mode that matches your specific presentation.
Is APAP as effective as CPAP?
Yes — for most patients with obstructive sleep apnoea. Multiple randomised controlled trials and systematic reviews have compared CPAP and APAP head-to-head and found broadly equivalent effectiveness: similar AHI reduction, similar improvement in daytime sleepiness, and similar patient satisfaction scores.
A 2023 Cochrane review concluded that for patients with OSA, auto-titrating PAP (APAP) is as effective as fixed CPAP in reducing respiratory events and improving sleep quality — with the additional advantage of lower average delivered pressure and better tolerance in some patients.
The exception is central sleep apnoea and complex sleep apnoea. APAP devices are not appropriate for significant central apnoea components — in these cases, CPAP or BiPAP is more appropriate, and the choice requires careful clinical assessment.
Can I switch from CPAP to APAP?
Yes, and it is one of the most common therapy adjustments made in sleep medicine. If you are on fixed CPAP and finding it difficult to tolerate — particularly at higher pressures — your Grace Sleep consultant can reassess and switch your prescription to APAP.
In most cases this does not require a new sleep study. Your consultant will review your therapy data, your current pressure, and your residual AHI — and can represcribe APAP with an appropriate pressure range based on that information. The switch is straightforward and your clinical record is retained throughout.
The reverse is also possible — if you are on APAP and your consultant determines that a fixed pressure would give better control, CPAP can be represcribed.
Already have a CPAP device that is not working? Our CPAP Therapy Service includes a dedicated troubleshooting pathway — covering mask fit, pressure, and therapy mode — for patients who are struggling with existing treatment.
What about BiPAP?
BiPAP (Bilevel Positive Airway Pressure) is a third mode of PAP therapy that delivers two separate pressures — a higher pressure when you breathe in (IPAP) and a lower pressure when you breathe out (EPAP). This makes exhalation feel significantly more natural and reduces the effort of breathing against continuous pressure.
BiPAP is typically reserved for patients who cannot tolerate CPAP or APAP at the required pressure, those with central or complex sleep apnoea, or patients with respiratory conditions requiring ventilatory support. It is not a first-line therapy for straightforward obstructive sleep apnoea.
For a full comparison of all three modes, see our Sleep Apnoea Treatment page.
Frequently asked questions
Start with a diagnosis.
Then get the right device.
A home sleep study tells your consultant exactly which therapy mode and pressure range is right for you.
Book a sleep study → Speak to the team