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AHI
Apnea-Hypopnea Index
AHI stands for Apnea-Hypopnea Index. It is the number of apneas and hypopneas your machine detected, divided by the hours you actually wore the mask. It is the headline figure on your report, and it is also the one most often misread.
The numbers that matter
- Well controlled therapy
- under 5 events per hour
- Excellent control
- under 1 event per hour
- Untreated mild apnea
- 5 to 15 events per hour
- Untreated moderate apnea
- 15 to 30 events per hour
- Untreated severe apnea
- over 30 events per hour
What the number actually counts
An apnea is a breathing pause of at least ten seconds. A hypopnea is a partial reduction in airflow, also lasting at least ten seconds, deep enough for your machine to register it. AHI adds both together and expresses them as an hourly rate.
Because it is a rate and not a total, short nights distort it badly. Twelve events across seven hours gives an AHI of 1.7. The same twelve events across one hour of restless sleep before you gave up gives an AHI of 12. The therapy did not get worse, the denominator did.
Why a single night tells you almost nothing
AHI moves night to night for reasons that have nothing to do with your therapy: a glass of wine, a cold, sleeping on your back instead of your side, a late meal, a bad mask night. A single reading of 8 after weeks at 2 is noise, not a signal.
What matters is the shape of the curve over several weeks. A steady climb over a month is worth a conversation. One spike is worth a shrug. This is exactly why a tool that stores your history is more useful than the number your machine shows you at breakfast.
The trap: a low AHI that means nothing
A CPAP machine detects events by watching your airflow signal. When the mask leaks heavily, that signal is corrupted, and the machine simply stops finding events it can score. The result is a reassuringly low AHI on a night that was genuinely bad.
So AHI should never be read alone. Always check the leak figure for the same night first. If leak was high, treat that night's AHI as unreliable rather than as good news.
Common questions
Is an AHI of 5 on CPAP good or bad?
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It sits right at the boundary most clinicians use for controlled therapy. On its own it is acceptable. If your usual figure is 1 or 2 and you have drifted to 5 over several weeks, that drift is the thing worth raising, not the absolute value.
Why is my AHI higher than before with the same settings?
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The common causes are weight change, alcohol, nasal congestion, sleeping position, a mask cushion that has softened with age, or a genuine change in your apnea. Look at leak and at your usage hours for the same period before assuming the settings are wrong.
Related terms
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