You load your CPAP data and see two numbers sitting side by side in your event summary: OA (obstructive apneas) and CA (central apneas). Most CPAP guides explain obstructive events at length and say almost nothing about centrals. This article covers what central apnea is on CPAP, how to read central events in your data, and what to make of them.
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 event counts with a qualified sleep specialist or physician.
What Is a Central Apnea on CPAP?
A central apnea is a pause in breathing that lasts 10 seconds or more where airflow stops — but not because the airway is blocked. The breathing drive from the brain briefly pauses, and the respiratory muscles stop working as a result. The airway itself is open; it is just not receiving the signal to breathe.
That is the key distinction. In an obstructive apnea, the airway physically collapses or is blocked — the brain keeps trying to breathe but cannot get air through. In a central apnea, the respiratory drive itself pauses. There is no effort against obstruction because the brain has not sent the signal to breathe.
The AASM (American Academy of Sleep Medicine) defines a central apnea as an airflow cessation of 10 or more seconds with an absent or very low respiratory effort signal. That "absent effort" is how central events are distinguished from obstructive ones in scored sleep studies. Your CPAP machine uses an algorithm-based version of this distinction to categorise the events it records.
Central vs Obstructive: The Key Data Difference
Both event types show a flat period in the flow waveform where airflow drops to near zero. The context around that flat period differs:
Obstructive apnea
Airflow stops but respiratory effort continues. The airway is physically blocked. CPAP pressure is applied specifically to prevent airway collapse and keep the passage open during sleep.
Central apnea
Airflow stops AND respiratory effort stops. The airway is clear. ResMed devices use the label "clear airway apnea" for this reason — the airway is unobstructed during the event, distinguishing it from obstructive apneas.
CPAP pressure is effective at holding the airway open during obstructive events. It does not directly address events where the breathing drive pauses — which is why central events can remain in data even when obstructive events are well controlled.
If you want to understand the broader taxonomy of apneas and hypopneas in your data, that article covers the full spectrum of scored events. Your AHI and RDI numbers both combine obstructive and central events into headline counts.
What Central Events Look Like in Your CPAP Data
ResMed machines record central apnea events separately from obstructive ones in the EDF data stored on your SD card. When you load your data in OSCAR, you will see distinct event markers — labelled CA (central apnea) or "clear airway" — plotted alongside your OA (obstructive apnea) and H (hypopnea) markers.
In the flow waveform, a central apnea appears as a flat period where the trace drops to near zero and stays there for the duration of the event. Unlike an obstructive apnea — which may show partial airflow attempts as the machine works against a blocked airway — a central event is typically a clean, undisturbed flat line. No struggle, no partial airflow.
In AirwayLab, central apnea counts are surfaced from the same EDF data your machine records. You will see your CA count alongside your OA and AHI breakdown in the overview dashboard, and you can track how central event counts change across nights using the trend view.
Note on device labelling
ResMed uses the term "clear airway apnea" to describe central events in their EDF data. OSCAR and AirwayLab both map this to the CA label. The machine identifies these events based on the absence of respiratory effort during the event — it is the device's algorithm-based categorisation of what it observed in the flow signal, not a clinical diagnosis.
Treatment-Emergent Central Events: A Data Pattern to Know
Some people's data shows elevated central event counts during the early weeks of CPAP therapy, having been low or absent before treatment began. This data pattern is common enough that it has a name: treatment-emergent central events (sometimes called complex sleep apnea in older literature).
It is discussed frequently in CPAP forums, and finding elevated CA counts early in therapy can be unsettling. In some people's recorded data, central event counts are lower after the first weeks to months of therapy; in others they remain at a steady level. The pattern varies.
What the data shows and what it means clinically are separate questions. Central event data — including trends across nights — is something your sleep physician can assess in the context of your individual history and therapy.
Data vs diagnosis: Treatment-emergent central events are a label for a data pattern, not a clinical diagnosis. Whether elevated CA counts are clinically significant — and what, if anything, to do about them — is a conversation for you and your sleep physician.
How Many Central Events Per Night?
A few central apneas scattered across a night is a common finding in CPAP data. Many users with otherwise well-controlled obstructive events see a handful of CA events per night. Their presence alone is not unusual.
If you want to bring your central event data to your sleep physician, AirwayLab's trend view gives you the multi-night CA count chart to take to that conversation. Whether any particular pattern is significant depends on individual context only your clinician can assess.
The central apnea index — CA events per hour of therapy — is the metric most commonly used to describe the volume of central events in CPAP data. Higher values are a data observation. Whether a given value is clinically significant depends on individual context that only your sleep physician can assess.
There are no universal thresholds that translate from a CA count to clinical meaning without individual context. Your data gives you the count. Your clinician gives you the interpretation.
Reading Central Events in AirwayLab
AirwayLab reads CA event data directly from your ResMed EDF files — the same source OSCAR uses. All parsing and analysis runs locally in your browser. Your data never leaves your device.
Event breakdown in overview
The overview dashboard breaks your total AHI into its components: obstructive apneas, central apneas, hypopneas, and (where available) estimated RERAs. You can see exactly how much of your total event count is driven by central events versus obstructive ones.
Night-by-night trend
The trend tab shows CA counts across your loaded sessions. If you have several weeks of data, you can see whether your central event count is stable, has shifted, or varies with changes in your environment or therapy timing.
Central events alongside flow limitation data
Central apneas and flow limitation are different patterns in the same data. RERAs and flow limitation patterns are associated with obstructive physiology. Central apneas operate through a different mechanism. Both are measured independently in your EDF data.
AirwayLab is open source (GPL-3.0), free, and always will be. The analysis code is publicly auditable. Your breathing data is yours.
Track Your Central Event Trends in AirwayLab
Load your ResMed SD card data to see your CA breakdown, how central events compare with your obstructive counts, and how the pattern changes across nights.
Related reading
Hypopnea vs Apnea: Understanding the Difference in Your CPAP Data — covers the full event spectrum your AHI counts.
AHI vs RDI: What's the Difference? — how central and obstructive events combine into your AHI and RDI.
Understanding Flow Limitation in CPAP Data — the obstructive-side waveform patterns that complement central event data.
What Are RERAs? — respiratory events that don't appear in AHI but show up in flow data.
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 and event data with a qualified sleep physician.