Your settings
APAP vs CPAP
Auto-adjusting versus fixed pressure
The same physical machine can run in either mode, which is why many people do not know which one they are on. Your device settings are written to the SD card, so the answer is in your own data rather than in your memory of a conversation at the clinic.
The numbers that matter
- CPAP
- one fixed pressure, all night
- APAP, ResMed AutoSet
- moves between a minimum and a maximum
- Typical APAP range
- set by your prescriber, often 4 to 20 cmH2O
- How to check
- the Mode field in your device settings on the card
How each mode behaves
Fixed CPAP holds one number regardless of what happens. It is predictable, it is easy to interpret, and it has to be set high enough for your worst position and your worst sleep stage, which means it is higher than you need for most of the night.
APAP starts low and raises pressure when it detects obstruction, flow limitation or snoring, then lets it fall back. You spend more of the night at a lower pressure, which usually improves comfort and leak, at the cost of a moving number that is harder to read.
What this does to your report
On fixed CPAP the pressure line is flat, so every other number is easy to attribute. On APAP the pressure curve becomes information in its own right: where it climbs tells you when your airway needed help, and the 95th percentile tells you what pressure actually covered you.
This is also why comparing two periods only works if the mode and the range were the same in both. A widened range changes the pressure statistics without anything about your apnea having changed.
Which one suits which situation
APAP tends to suit people whose needs vary: positional apnea, seasonal congestion, weight that moves, alcohol some evenings. Fixed pressure tends to suit stable, well-titrated therapy, and it is sometimes preferred where central apnea is a concern, since an auto algorithm chasing centrals can be counterproductive.
Neither is better in the abstract. The mode is a prescription decision, and the useful contribution you can make is bringing your own pressure and event history to the discussion.
Common questions
How do I know if I am on CPAP or APAP?
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Read the Mode field in your device settings, which your machine writes to the SD card. ResMed shows AutoSet for auto-adjusting mode and CPAP for fixed. Do not rely on the machine model name, since most models can do both.
Is APAP better than fixed CPAP?
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Not universally. APAP usually wins on comfort and on adapting to variable needs. Fixed pressure gives cleaner, more interpretable data and is sometimes preferred clinically. It is a decision for your prescriber.
Related terms
See these numbers on your own nights
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