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Leak
Mask leak rate
Every CPAP mask leaks a little by design: the vent holes are what flush your exhaled air out of the circuit. The leak figure on your report is the extra, unintended leak on top of that. It is the most fixable problem on the whole report, and the one that most often ruins the rest of the data.
The numbers that matter
- ResMed large leak threshold
- 24 L/min
- Comfortable target
- 95th percentile under 24 L/min
- Time over the red line
- aim for 0 percent of the night
- Most common single cause
- mouth breathing with a nasal mask
Why the threshold matters more than the average
ResMed devices flag anything above roughly 24 litres per minute as a large leak. Below it, the machine compensates and keeps working normally. Above it, two things break at once: the pressure actually delivered to your airway drops, and the airflow signal the machine uses to detect apneas becomes unusable.
This is why the average leak is a weak number and the 95th percentile is a strong one. An average of 5 L/min can hide twenty minutes at 50 L/min. Look at the 95th percentile and at the percentage of the night spent over the red line.
Where the air is actually going
Mouth leak is the most common cause with a nasal mask or nasal pillows. You breathe through your nose while awake, relax in deeper sleep, and the air takes the shortest way out. A chin strap or a switch to a full face mask usually settles it.
Seal leak comes from fit. A cushion older than three months has lost its elasticity, straps get overtightened as a reflex which deforms the cushion and makes the leak worse, and facial hair or a pillow pushing on the frame will break the seal in one position only.
What to change, in order
Start with the cushion age, then the strap tension, which almost always needs loosening rather than tightening. Then the mask size, which is not the same as your general build: many people wear one size too large. Then the mask type.
Change your pressure settings last, and only with your prescriber. Pressure is very rarely the cause of leak, and raising or lowering it to chase a leak problem usually creates a second problem.
Common questions
What is an acceptable CPAP leak rate?
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Keep the 95th percentile below the 24 L/min large-leak line, and the time spent over that line as close to zero as you can. A small steady leak that stays under the threshold is not a problem and does not need fixing.
Can leak make my AHI look better than it is?
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Yes, and this is the most important thing to know about leak. When the airflow signal is corrupted the machine stops scoring events, so a heavy-leak night can report a falsely low AHI. Always read the two numbers together.
Related terms
See these numbers on your own nights
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