CPAP Event Types Decoded: Obstructive, Central, Hypopnea & RERA

Your AHI is a single number, but it's built from several very different events stacked together. Two people can both report an AHI of 6, yet one is doing great on therapy and the other needs a different machine entirely. The difference is the event mix — what kinds of breathing disruptions your CPAP is recording, and which ones it can actually fix.

This guide decodes the four event types your CPAP data tracks: obstructive apneas, central (clear-airway) apneas, hypopneas, and RERAs. Understanding them turns your AHI from a mystery score into something you can actually reason about. (For the bigger picture, start with our pillar guide on Reading Your CPAP Data.)

The event hierarchy — flow limitation, hypopnea, apnea

Sleep-disordered breathing isn't all-or-nothing. It lives on a spectrum from a slight narrowing of your airway all the way to a complete stop in airflow. Picture it as a ladder:

1. Flow limitation — the mildest rung. Your airway narrows enough to deform the shape of each breath, but airflow only dips a little. It usually doesn't make it into your AHI at all. (More on this in CPAP Flow Limitation: The Hidden Metric Beyond AHI.)
2. Hypopnea — a partial reduction in airflow that's big enough and long enough to count, typically paired with a drop in blood oxygen or a brief arousal from sleep.
3. Apnea — a near-complete or complete stop in airflow for at least 10 seconds.

The AHI (Apnea-Hypopnea Index) is the number of apneas plus hypopneas per hour of sleep. Everything below the hypopnea threshold — including most flow limitation and all RERAs — is invisible to that number, which is exactly why two identical AHIs can hide very different nights.

Obstructive apnea (airway closed, effort present)

An obstructive apnea is what most people picture when they think of sleep apnea: the throat collapses and blocks airflow, but your body keeps trying to breathe. Your chest and diaphragm strain against a closed door.

The defining signature is continued respiratory effort during the blockage. You're working to breathe; the air just can't get through. This is the event CPAP was built for. By delivering a steady column of pressurized air, CPAP acts as a pneumatic splint that holds the airway open so it can't collapse in the first place.

On your data, obstructive apneas often appear as a flat line in the flow rate waveform with effort still visible. If you're seeing a lot of them on therapy, it usually points to a fixable cause — pressure that's too low for the night, positional collapse, or a leak masking the real picture. Our guide to Why Is My AHI High on CPAP? walks through the usual suspects.

Central / clear-airway apnea (no effort)

A central apnea — your machine may label it a "clear-airway" (CA) event — is the mirror image of an obstructive one. Airflow stops, but so does the effort. For a few seconds, your brain simply doesn't send the signal to breathe. The airway is open; there's just no drive behind it.

This distinction matters enormously for therapy:

• CPAP reliably treats obstructive events by splinting the airway open.
• CPAP does not directly correct the unstable breathing drive behind central events. Holding an already-open airway open does nothing for a missing breath signal.

Sometimes central events appear after someone starts CPAP. This is called treatment-emergent central sleep apnea (TECSA), or complex sleep apnea — the obstructive events resolve on pressure, but central events emerge. TECSA shows up in roughly 5–15% of PAP titrations and, encouragingly, resolves on its own in about 60–80% of cases within weeks to a few months of continued CPAP. Management often means watchful waiting on CPAP under clinician guidance, and if it persists, switching to BiPAP or ASV, adding oxygen, or other measures.

What you should not do is raise your own pressure to "chase" central events — higher pressure doesn't fix them and can sometimes make breathing instability worse. A central-heavy AHI is a flag to bring to your sleep clinician, especially if symptoms return, you're very sleepy in the daytime, you see deep oxygen drops, or you've started new medications (such as opioids) or have a heart or kidney condition. We cover this in depth in Central Apneas Showing Up on CPAP: Treatment-Emergent CSA Explained and Central vs Obstructive Apnea: What 'Clear Airway' (CA) Events Mean.

Hypopnea — partial reduction and how it's scored

A hypopnea sits between a normal breath and a full apnea. Airflow drops substantially but doesn't stop. Because it's a partial event, scoring it requires a specific rulebook — and that rulebook has two versions.

Per the American Academy of Sleep Medicine (AASM), a hypopnea is:

• At least a 30% drop in airflow from baseline,
• Lasting 10 seconds or longer,
• Accompanied by either a drop in blood oxygen or a brief arousal from sleep.

That last condition — what counts as "accompanied" — is where the two rules diverge.

AASM Rule 1A (3% or arousal) vs 1B (4% desaturation)

Rule 1A is the AASM's recommended definition. It catches events that fragment your sleep via an arousal even without a big oxygen drop, so it counts more events. Rule 1B is the more conservative, acceptable alternative often tied to U.S. insurance (CMS) requirements; it only counts hypopneas with a 4% desaturation. The same night of sleep can yield two different AHIs depending on which rule a lab applies.

Here's the catch for home users: your CPAP machine doesn't run either rule the way a sleep lab does. It has no EEG to detect arousals and, in most cases, no pulse oximeter wired in to measure desaturation. Instead it estimates hypopneas from airflow patterns alone. That's why your device-reported AHI can differ from a lab-scored AHI — the machine is making an educated guess, not a clinical scoring decision. It's still useful for spotting trends, but it isn't the same measurement.

RERA — the arousal event AHI doesn't count

A RERA (Respiratory Effort-Related Arousal) is real sleep disruption that your AHI ignores. In a RERA, your airway narrows and your breathing effort ramps up to compensate — not enough to qualify as an apnea or even a hypopnea, but enough to jolt you into a brief arousal. You don't fully wake up, but your sleep gets fragmented, and over a night that adds up to genuine daytime fatigue.

Because RERAs are arousal-based, confirming them properly requires EEG — the brainwave monitoring done in a polysomnogram (in-lab sleep study). A home CPAP machine can't see arousals, so RERAs generally don't appear in your AHI and may be only loosely approximated (if at all) by flow-limitation flags.

RERAs are why a second metric exists: the RDI (Respiratory Disturbance Index), which adds RERAs on top of apneas and hypopneas. An AHI can look reassuringly low while the RDI tells a different story — a common situation in UARS (Upper Airway Resistance Syndrome). If you're still tired on CPAP despite good numbers, RERAs and flow limitation are prime suspects. See AHI vs RDI: Why Two Numbers Measure Your Sleep Apnea and RERA and Flow Limitation: The Events That Don't Show in Your AHI for the full breakdown.

Why the event MIX matters more than the total

Now the payoff. The treatment goal for adults on CPAP is generally a residual AHI below 5 events per hour; some clinicians aim lower (e.g., under 1–2) when it's comfortably achievable, though targets are individualized with your provider. But that single number can't tell you what kind of trouble is left over.

Consider two people, both at AHI 6:

• Person A: mostly obstructive events. This is the kind CPAP can usually fix — a pressure or mask conversation with the clinician may bring it down.
• Person B: mostly central / clear-airway events. More CPAP pressure won't help, and chasing it yourself could backfire. This pattern warrants a clinician's review and possibly a different therapy mode (BiPAP or ASV).

Same total, completely different story — and completely different next steps.

One more trust check before you read too much into any event count: a large mask leak can invalidate or under-report your AHI, because the machine can't reliably detect events when air is escaping. Always confirm your leak is in range first (ResMed flags excess leak above 24 L/min at the 95th percentile). Our CPAP Leak Rate guide explains the thresholds.

This is exactly where staring at one blended number falls short. SomniCharts splits your AHI by event type and color-codes the timeline, so you can see at a glance whether your residual events are obstructive (often fixable on CPAP) or central (the kind CPAP can't directly fix). It imports data from ResMed, Philips Respironics (including the encrypted DreamStation 2), and Löwenstein prisma devices and explains it in plain language automatically — no spreadsheets, no guesswork.

And remember the recurring rule of CPAP data: a single night is noise. One night heavy on central events, or one night with a leak spike, rarely means anything. Watch the trend in your event mix over weeks — that's the signal worth bringing to your provider. (Why night-to-night swings are normal is covered in Why Does My AHI Change Night to Night?.)

Frequently asked questions

What's the difference between an apnea and a hypopnea? An apnea is a near-complete or total stop in airflow for at least 10 seconds. A hypopnea is a partial reduction — at least a 30% drop for 10+ seconds — paired with an oxygen desaturation or an arousal. Both count toward your AHI.

Is obstructive or central apnea worse? Neither is universally "worse," but they need different treatments. Obstructive apneas respond well to CPAP. Central apneas don't respond to more pressure and may require a clinician's evaluation and a different device mode, so a central-heavy pattern deserves attention.

Why doesn't my machine count RERAs? RERAs are defined by an arousal in your brainwaves, which only EEG (done in a sleep lab) can detect. Home CPAP machines have no EEG, so RERAs generally don't appear in your reported AHI.

Why is my CPAP's AHI different from my sleep study's? Sleep labs score events using EEG and oximetry under specific AASM rules (1A or 1B). Your machine estimates hypopneas from airflow alone, with no arousals and usually no oxygen data — so the two numbers measure slightly different things.