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EPR
Expiratory Pressure Relief
Breathing out against a steady pressure is the single most common reason people give up on CPAP in the first month. EPR is ResMed's answer: the machine detects the start of your exhalation and drops the pressure by a set amount, then brings it back up before you inhale.
The numbers that matter
- Available levels
- 1, 2 or 3 cmH2O
- Modes
- off, ramp only, or full time
- Effect on exhale pressure
- subtracts the level from your set pressure
- Philips equivalent
- C-Flex and A-Flex
What it actually changes
Set to 3 with a fixed pressure of 10, you inhale against 10 and exhale against 7. The machine follows your breathing cycle, so the drop lands where the effort is, not on a timer.
Ramp-only mode applies the relief while you fall asleep and then switches it off. Full time keeps it all night. Off means you get the same pressure in both directions.
The trade-off nobody mentions
Your airway does not only collapse when you inhale. For many people the critical moment is the end of exhalation, when airway pressure is at its lowest. EPR lowers exactly that moment, which is why a high EPR level can let events through that a lower level would have prevented.
If your AHI rose when comfort improved, EPR is a reasonable suspect. On an APAP the machine will often compensate by raising the baseline pressure, which can leave you with more comfort and a higher average pressure at the same time.
Reading it on your report
EPR is written to the SD card as part of your device settings, so a tool that reads the card can show you the mode and level actually in use rather than what you remember setting. That matters, because settings get changed at a service visit and not always mentioned.
When you compare two periods of your own history, check that EPR was identical in both. A change of one level is enough to move your pressure and event numbers and make the comparison meaningless.
Common questions
Should EPR be on or off?
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If you tolerate therapy comfortably without it, off gives you the cleanest pressure delivery. If exhaling is a struggle, comfort wins, because a machine you actually wear all night beats a theoretically perfect setting you abandon.
Does EPR reduce the effectiveness of my CPAP?
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It lowers the pressure at exhalation by the level you set, so in principle yes. Whether it matters for you shows up in your own event data, which is the point of looking at it. Any adjustment belongs to your prescriber.
Related terms
See these numbers on your own nights
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