If you look at a CPAP summary report — whether from ResMed's MyAir app, OSCAR, or your machine's own display — you will usually see your AHI broken into components: obstructive apneas, central apneas, and hypopneas. The totals are combined into one number, but the individual categories tell different stories. The difference between a hypopnea and an apnea is not just about severity. The two event types have distinct definitions, involve different airway mechanics, and appear differently in your flow waveform data. Understanding what you are looking at helps you have a more specific conversation with your clinician about what your data shows.
Medical disclaimer: This article is for informational purposes only. AirwayLab is not a medical device, and nothing here constitutes a diagnosis or treatment recommendation. Always discuss your therapy data and any concerns about your metrics with a qualified sleep specialist.
What Is an Apnea?
An apnea is a near-complete stop in airflow lasting at least 10 seconds. There are two main types:
Obstructive apnea (OA)
The airway physically collapses during sleep. Airflow stops because the upper airway — typically the soft palate, tongue base, or pharyngeal walls — closes completely. Breathing effort continues; the problem is mechanical, not neurological. In your flow data, this appears as a flat line: airflow drops to near zero.
Central apnea (CA)
The brain temporarily stops sending the signal to breathe. Unlike obstructive apneas, there is no mechanical obstruction — the airway is open. Airflow stops because the drive to breathe pauses. In CPAP flow data, central apneas also appear as near-zero airflow, but breathing typically resumes gradually rather than abruptly.
Mixed apneas
Some people have mixed apneas — events that begin without respiratory effort and become obstructive as the effort-free period progresses. ResMed devices count and report these separately.
Your CPAP device counts and categorises these events automatically and reports them in your nightly summary.
What Is a Hypopnea?
A hypopnea is a partial reduction in airflow — not a complete stop, but a significant dip — lasting at least 10 seconds, accompanied by either an oxygen desaturation or an arousal from sleep.
Key scoring thresholds
- —Airflow reduction: typically 30% or more below baseline
- —Duration: at least 10 seconds
- —Associated marker: either a 3–4% oxygen desaturation or an EEG-verified arousal
Hypopneas are, by definition, less complete in terms of airflow reduction than apneas — but they can still fragment sleep and affect oxygen levels depending on their frequency and depth.
The scoring rules have changed
The AASM (American Academy of Sleep Medicine) revised its hypopnea scoring criteria between 2007 and 2012. Under the older rules (AASM 2007), events with a 3% desaturation oran arousal counted as hypopneas. Under the newer rules (AASM 2012), only events with a 4% desaturation counted. Events that qualified as hypopneas under the 2007 rules but don't reach the 4% desaturation threshold under the 2012 rules are instead classified as RERAs — events that don't appear in your AHI at all. If your initial sleep study was scored under different criteria than your current follow-up data, the numbers may not be directly comparable even if nothing in your breathing has changed. Your sleep physician can help interpret these differences in context.
How They Combine Into AHI
The Apnea-Hypopnea Index is the sum of all apneas (obstructive + central + mixed) and all hypopneas, divided by hours of sleep:
AHI = (Obstructive Apneas + Central Apneas + Mixed Apneas + Hypopneas) ÷ Hours of sleep
The severity thresholds most commonly used:
| AHI range | Category |
|---|---|
| 0–4 | Typical range |
| 5–14 | Mild |
| 15–29 | Moderate |
| ≥30 | Severe |
These thresholds were established for diagnostic use in polysomnography. For a closer look at what AHI misses and why it can be misleading as a standalone metric, see Why Your AHI Is Lying to You.
Why the Apnea vs Hypopnea Split Matters
The AHI headline number collapses all event types into one. But the split between apneas and hypopneas — and between obstructive and central apneas — contains information that the single number doesn't.
A night with AHI 8 composed of 7 hypopneas and 1 obstructive apnea looks very different in the flow data from a night with AHI 8 composed of 6 central apneas and 2 obstructive apneas. The event types, the waveform signatures, and the patterns in the data are distinct.
The obstructive vs central distinction in particular
Obstructive events are the primary target of CPAP therapy — pressure splints the airway open. Central events are driven by respiratory control rather than airway anatomy, and the clinical picture is different. Your sleep physician can help interpret the balance between obstructive and central events in the context of your specific data.
The same AHI, different picture:
| Metric | Person A | Person B |
|---|---|---|
| AHI | 12 | 12 |
| Obstructive apneas/hour | 10 | 1 |
| Central apneas/hour | 0 | 8 |
| Hypopneas/hour | 2 | 3 |
The headline number is identical. The flow data looks completely different. The breakdown — not just the total — gives a more complete picture of what the data shows.
How They Look in the Flow Waveform
If you look at your raw CPAP flow data — in OSCAR, AirwayLab, or any tool that reads SD card data — the two event types look different:
Apnea signature
Near-flat airflow for at least 10 seconds. The signal may hover near zero or drift slightly. The resumption of breathing is often abrupt (obstructive) or gradual (central).
Hypopnea signature
Airflow drops significantly but doesn't reach zero. Breath shapes are reduced in amplitude. You may see flat-topped waveforms (flow limitation) or reduced but still-present airflow. This is where the line between a hypopnea and a RERA can be difficult to see without knowing the exact scoring thresholds applied.
The grey zone
Events that don't quite reach the hypopnea threshold — not enough airflow drop, no sufficient desaturation, no EEG-confirmed arousal — may still appear in the flow waveform as notable patterns. These are the events most likely to be RERAs, and they are explored in What Are RERAs?
What AirwayLab Shows You
AirwayLab reads your ResMed SD card data directly in your browser. Your data never leaves your device. All analysis runs locally.
Event breakdown in context
The overview dashboard shows your AHI alongside the event breakdown from your device — obstructive apneas, central apneas, and hypopneas per night. Trending across multiple nights lets you see whether the composition shifts over time.
Flow waveform analysis
The waveform tab gives you access to the breath-by-breath flow data, where you can see the actual signatures of events: the flat-line apneas, the reduced-amplitude hypopneas, and the flow-limited breaths that sit between scored events and normal breathing.
NED engine (per-breath analysis)
AirwayLab's NED engine scores each breath for negative effort dependence — a per-breath measure of inspiratory effort patterns in the flow signal. This gives you a measure of how many breaths showed flow-limited patterns, independent of whether those breaths were scored as formal apneas or hypopneas by your device. A night with low AHI but high NED percentage shows a different picture than a night with the same AHI and low NED.
Glasgow Index (breath shape scoring)
The Glasgow engine scores the shape of each inspiratory waveform on 9 components: flatness, skew, multi-peak patterns, and more. These shape scores give you a quantitative way to look at what the waveforms in your data look like, beyond the binary scored/not-scored classification your device applies.
H1/H2 split
AirwayLab splits your night into first-half and second-half to show whether event patterns shift across the session. Obstructive events often cluster in specific sleep stages; seeing whether your apneas and hypopneas are concentrated in the first or second half of the night is a useful data point for discussing sleep architecture with your clinician.
This data is for informational purposes. Your sleep physician can help interpret these patterns in the context of your full clinical picture.
Practical Takeaways
Apnea = near-complete airflow stop (≥10 seconds)
Obstructive means the airway is blocked. Central means the breathing drive paused.
Hypopnea = partial reduction (≥30%) for ≥10 seconds
With either desaturation or arousal. A scored event, but not as complete an airflow stop as an apnea.
Both count in AHI
The single headline number combines obstructive apneas, central apneas, mixed apneas, and hypopneas.
The split between types contains information the total doesn't
A high central apnea percentage in your data is a different clinical picture from a high obstructive apnea percentage. Your clinician can help interpret which pattern your data shows.
Scoring rules matter
If your sleep study and your current CPAP data were scored under different AASM criteria, some events may have moved between the hypopnea and RERA categories.
Hypopneas shade into RERAs at the margin
Events that nearly meet the hypopnea threshold may still appear as notable patterns in your flow data. These are explored in detail in What Are RERAs?
References
Berry et al. (2012). "Rules for Scoring Respiratory Events in Sleep: Update of the 2007 AASM Manual." Journal of Clinical Sleep Medicine, 8(5), 597–619.
Ruehland et al. (2009). "The New AASM Criteria for Scoring Hypopneas: Impact on the Apnea Hypopnea Index." Sleep, 32(2), 150–157.
Morgenthaler et al. (2006). "Complex Sleep Apnea Syndrome: Is It a Unique Clinical Syndrome?" Sleep, 29(9), 1203–1209.
AASM Manual for the Scoring of Sleep and Associated Events (2017). Version 2.4.
Related articles
Why Your AHI Is Lying to You — the design limitations of AHI as a monitoring metric.
What Are RERAs in Sleep Apnea? — the events between hypopneas and normal breathing.
AHI vs RDI: What's the Difference? — why these two numbers don't match on your reports.
Understanding Flow Limitation in CPAP Data — the waveform patterns behind scored and unscored events.
Beyond AHI: Why Your Sleep Apnea Score Might Be Misleading You — the research case for multi-dimensional sleep assessment.
Medical disclaimer
AirwayLab is an educational tool, not a medical device. The analysis provided is based on published research methodologies applied to your PAP device's flow data. It is not a substitute for polysomnography or clinical evaluation. Always discuss your therapy data with your sleep physician. The metrics described here are for educational purposes and to support informed conversations with your clinician.
See Your Apnea and Hypopnea Breakdown in AirwayLab
AirwayLab analyses your ResMed SD card data in your browser — no upload, no account. See your event breakdown, flow waveforms, and NED per-breath analysis alongside your AHI, free and open-source.