If you use a CPAP or BiPAP machine, the number you probably check most is your AHI — Apnea-Hypopnea Index. Most users know lower is better. But what does your CPAP AHI actually count, what do the different values represent, and — importantly — what does it miss? This guide covers the metric your nightly summary is built on, and the picture it leaves out.
If you have ever searched for what is AHI CPAP and wanted a plain-language answer rather than a clinical textbook, this is it.
Medical disclaimer: This article is for informational and educational purposes only. AirwayLab is not a medical device, and nothing here constitutes a diagnosis, therapy recommendation, or medical opinion. Always discuss your therapy data and any concerns with your sleep physician or qualified clinician.
What Is AHI?
AHI — Apnea-Hypopnea Index — is the number of apneas and hypopneas recorded per hour of sleep. It is the primary metric used in clinical sleep medicine to describe sleep-disordered breathing frequency, and it is the headline number your CPAP or BiPAP machine reports each morning.
The index combines two types of breathing events:
Apneas
A near-complete stop in airflow lasting at least 10 seconds. There are two subtypes: obstructive apneas (the airway physically collapses) and central apneas (the brain temporarily pauses the breathing signal). Both are counted in AHI.
Hypopneas
A partial reduction in airflow — typically 30% or more below baseline — lasting at least 10 seconds, accompanied by either an oxygen desaturation of 3–4% or a brief arousal from sleep. Hypopneas are the more commonly scored event on PAP therapy.
The formula is straightforward: total scored events ÷ total hours of sleep. A machine that recorded 15 events during 6 hours of sleep reports an AHI of 2.5.
What the Numbers Represent
Clinical sleep guidelines — primarily from the American Academy of Sleep Medicine (AASM) — define AHI ranges as shorthand for describing breathing event frequency during sleep. These ranges are used in diagnostic sleep studies and inform discussions between clinicians and their patients about therapy goals.
| AHI range | AASM classification |
|---|---|
| Below 5 | Below the diagnostic threshold |
| 5–15 | Mild range |
| 15–30 | Moderate range |
| Above 30 | Severe range |
* Your clinician sets therapy targets based on your clinical picture. These ranges describe diagnostic categories, not on-therapy goals.
These thresholds apply in diagnostic contexts. On PAP therapy, the goal is typically to bring residual AHI below 5 — but your clinician sets the specific target for your situation. A single elevated night is worth noting; a persistent trend is what your clinician needs to see to make informed care decisions.
On-therapy vs diagnostic AHI: The AHI your CPAP reports is your residual AHI — events recorded while the machine was running. This is typically lower than your untreated AHI from a sleep study. They are related but not the same number, and your sleep physician can explain what each means for your treatment plan.
Low AHI, Still Tired? What AHI Doesn't Count
This is one of the most common questions in CPAP communities. Your machine reports a low AHI — say, 1.5 — yet you still feel unrested. You are not imagining it.
AHI has a fundamental limitation: it only counts events that meet its strict scoring thresholds. Several types of breathing disruption remain invisible to it:
Flow limitation
Airflow that is reduced but does not meet the hypopnea threshold. When your airway narrows, the inspiratory flow waveform flattens — the typical rounded arch of a healthy breath becomes a plateau. These flow-limited breaths can persist for significant portions of the night without generating an AHI event. Understanding flow limitation covers this in depth.
RERAs — Respiratory Effort-Related Arousals
Periods where increasing respiratory effort ends in a brief arousal from sleep, without meeting the threshold for a scored hypopnea. RERAs fragment sleep architecture without appearing in your AHI. What are RERAs? covers these in detail.
Scoring variability
AASM hypopnea scoring rules have changed over time, and different labs apply different criteria. Two people with identical breathing patterns can receive different AHI values depending on which scoring rule was applied. Your device uses its own internal algorithm, which may differ from your sleep lab's criteria.
AHI is a useful count of how many qualifying events occurred. It is not a complete picture of airway behaviour during sleep. If your data shows a low AHI but you remain symptomatic, your clinician can look beyond the headline number — and tools like AirwayLab give you the underlying data to bring to that conversation.
For a longer read on this topic, see Why Your AHI Might Not Tell the Whole Story and AHI Normal But Still Tired.
AHI vs RDI: What's the Difference?
Some sleep study reports include an RDI (Respiratory Disturbance Index) alongside AHI. RDI typically adds RERAs to the apnea and hypopnea count, so it is always equal to or higher than AHI. A person with an AHI of 3 but an RDI of 12 has significant respiratory disturbances that AHI alone does not capture.
Most CPAP machines report AHI only — not RDI. The RERA-type events that contribute to RDI remain invisible in your nightly summary, even though they may be present in your raw flow waveform data.
A detailed comparison of what each metric covers — and when RDI matters more than AHI — is coming in our article AHI vs RDI: What Sleep Apnea Metrics Actually Tell You.
Tracking AHI Trends in AirwayLab
A single night's AHI is a snapshot. Trends across multiple nights are what matter most for understanding whether your therapy data is staying consistent — and for giving your clinician a meaningful picture of your nights.
AirwayLab reads your SD card data directly in your browser. No upload, no account required. It surfaces your AHI alongside the metrics that AHI misses: flow limitation scores, RERA-related breathing patterns, breath-by-breath NED analysis, and first-half vs second-half comparisons within each night.
Nightly AHI trend
AirwayLab plots your AHI across all sessions on the SD card. Persistent elevated values are easier to spot in a chart than in your device's rolling 7-day average.
Event type breakdown
AirwayLab separates obstructive and central events. Knowing how that ratio changes over time is useful context to bring to a clinician review — your sleep physician can interpret what the breakdown means for your specific situation.
Beyond the headline number
The four AirwayLab analysis engines run on your raw flow waveform — not just the device-reported events. This gives you access to flow limitation scores, breathing regularity analysis, and pattern detection that are invisible in AHI alone. All of it runs entirely in your browser; your data never leaves your device.
AirwayLab is free and always will be. The code is open source (GPL-3.0) and publicly auditable — you can verify exactly what it does with your data.
What to Bring to Your Clinician
Your AHI trend is a good starting point for a clinical conversation, but it is rarely the whole story. The most useful data to bring to a follow-up appointment:
- AHI trend over the past 30–90 days, not just the latest night's value
- Whether elevated-AHI nights cluster at specific times or appear randomly
- The breakdown between obstructive and central events, if your machine reports it
- Any nights with elevated flow limitation or RERA-related patterns
- Persistent symptoms: daytime fatigue, morning headaches, unrefreshing sleep
AirwayLab's export tools let you generate a formatted data summary from your SD card. Your sleep physician or respiratory therapist can help interpret what the patterns in your data mean for your care.
OA, CA, and Mixed: What the AHI Breakdown Actually Means
Many CPAP therapy reports show a breakdown of events — not just total AHI, but separate counts for OA, CA, and H (or similar labels). Here is what each category means in your data:
| Label | Full name | What happened |
|---|---|---|
| OA | Obstructive Apnea | Airflow stopped for ≥10 s while the airway physically obstructed. The machine was delivering pressure but airflow dropped. |
| CA | Central Apnea | Airflow stopped for ≥10 s and the effort signal also stopped — no attempt to breathe. Associated with central nervous system signalling, not airway obstruction. |
| H | Hypopnea | Airflow reduced by ≥30% for ≥10 s (with a desaturation or arousal, depending on scoring criteria). A partial reduction, not a complete stop. |
| Mixed | Mixed Apnea | Starts as a central apnea (no effort), then transitions to an obstructive pattern as effort resumes but airway remains blocked. |
Your total AHI is the sum of all these events per hour. A breakdown skewed toward central apneas is one pattern your sleep physician may want to look at more closely, as it can appear with certain pressure settings or other factors. Discuss your breakdown with your clinician rather than interpreting it in isolation.
Why does my AHI vary night to night? Sleep position, sleep stage distribution, alcohol, sedatives, nasal congestion, and pressure settings can all affect the event count from one night to the next. A single high-AHI night is less meaningful than a consistent trend over weeks. Your data is most useful when your clinician reviews a multi-week summary.
Frequently Asked Questions
What is a good AHI number for CPAP therapy?
Most clinical guidelines use an AHI below 5 as the target range on PAP therapy. Your clinician sets the specific goal for your situation — some individuals need different targets based on their clinical history. A persistent AHI above 5 on therapy is worth discussing at your next appointment.
What does a high AHI on CPAP mean?
A high residual AHI on therapy means your machine is recording more scored breathing events than the typical target range. This can happen for several reasons — pressure settings, mask fit, body position, or changes in your upper airway. Your clinician can review your data and advise on next steps.
Why does my AHI vary so much night to night?
Night-to-night AHI variability is normal. Factors like sleep position, alcohol, sedatives, nasal congestion, and sleep stage distribution can all affect how your airway behaves. A single high night is not necessarily cause for concern; a consistent upward trend is worth discussing with your clinician.
Can I have a low AHI and still feel tired?
Yes. AHI only counts events that meet specific scoring thresholds. Flow-limited breaths and RERAs can disrupt sleep without appearing in your AHI. Other factors — sleep stage distribution, periodic limb movements, circadian issues — also affect how rested you feel. Your clinician can help evaluate what is contributing.
What is the difference between AHI on a sleep study and AHI on CPAP?
Your diagnostic AHI from a sleep study reflects your untreated breathing during a supervised study. Your CPAP residual AHI reflects events recorded while the machine is running. They use related but not always identical scoring algorithms and represent different clinical contexts. Your sleep physician can explain what each means for your treatment plan.
Does AirwayLab show my AHI?
Yes. AirwayLab reads the AHI and event data from your ResMed SD card and displays it alongside additional metrics — flow limitation scores, RERA-related breathing patterns, and breath-by-breath waveform analysis — that do not appear in your device's nightly summary. Everything runs in your browser; no data is uploaded.
What does RDI mean and how is it different from AHI?
RDI (Respiratory Disturbance Index) typically adds RERAs — Respiratory Effort-Related Arousals — to the apnea and hypopnea count used in AHI. RDI is therefore always equal to or higher than AHI for the same night. Most CPAP machines report AHI only. A full comparison of AHI vs RDI is coming in the next article in this series.
See Your AHI Trends Over Time
Upload your SD card data and explore your AHI trends, event breakdowns, and the flow-limitation patterns your nightly summary doesn't show.
Related reading
What Are RERAs? — the breathing events your AHI does not count.
Understanding Flow Limitation in CPAP Data — what happens when your airway narrows without triggering an event.
Why Your AHI Might Not Tell the Whole Story — a deeper look at AHI's limits as a sleep quality metric.
AHI Normal But Still Tired — common reasons for persistent fatigue despite low AHI.
This article is for educational and informational purposes only. It has not been reviewed by a licensed clinician and is not a substitute for professional medical advice. Consult your sleep specialist or healthcare provider before making any changes to your therapy.
AirwayLab is a free, open-source tool for analysing PAP flow data. Your data never leaves your browser. Nothing on this page constitutes medical advice — always discuss your results with a qualified sleep specialist before making any changes to your therapy.