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Central vs obstructive apnea on CPAP

An obstructive apnea is a pause in breathing because the airway has closed. A central apnea is a pause because the brain briefly stopped sending the signal to breathe, with the airway still open. On CPAP the distinction matters, because pressure fixes the first and not the second. This guide explains how your machine tells them apart and what the breakdown on your dashboard means.

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Side-by-side panels of an obstructive and a central apnea: airflow is flat in both; the chest effort band continues in the obstructive event and is absent in the central one; the pressure pulse echo is flat for obstructive and present for central

Two different problems

In obstructive sleep apnea (OSA), the muscles of the throat relax during sleep and the soft tissue collapses inward. You keep trying to breathe; your chest and abdomen move, often harder and harder, but little or no air gets through. The event ends when you rouse slightly and muscle tone returns. CPAP treats this directly by holding the airway open with air pressure.

In central sleep apnea (CSA), the airway is open but the respiratory drive pauses. There is no effort: the chest is still, the abdomen is still, nothing is being blocked. The event ends when carbon dioxide builds up enough to restart the drive. Pressure does not help, because there was nothing to splint open.

Most people on CPAP have OSA. But a proportion of residual events on therapy are central, and for a minority of people they are the main thing left.

How a sleep lab tells them apart

In polysomnography, the technologist watches the effort belts around the chest and abdomen. Airflow stops in both kinds of apnea; the difference is whether effort continues. Obstructive: effort continues or increases. Central: effort stops. A mixed apnea starts central and ends obstructive, with effort returning before airflow does.

How a CPAP machine tells them apart

Your machine has no effort belts. It has a flow sensor and a blower. So how does a ResMed AirSense decide an apnea was obstructive or central?

It uses a technique called the forced oscillation technique (FOT). When the machine detects that airflow has stopped for long enough to count as an apnea, it superimposes a small, rapid oscillation on the delivered pressure, a gentle pulse at a few hertz that you would not notice. It then measures how much flow that pulse produces.

If the airway is open, the pressure pulse pushes a small amount of air in and out of the lungs, and the flow sensor sees the oscillation echoed back. ResMed calls this a clear airway (CA) apnea, its label for a central event.
If the airway is closed, the pulse goes nowhere. The flow sensor sees almost no response. That is scored as an obstructive apnea (OA).

This is clever and generally works well, but it is an inference, not a direct measurement of effort. There are situations where it is less reliable, and the main one is leak.

Unclassified apneas

When unintentional leak is high, the pressure oscillation leaks out through the gap in the mask rather than into your airway, and the machine cannot tell whether the response it sees came from your lungs or from the leak. In that case ResMed marks the event as an unclassified apnea (UA). It knows breathing stopped; it does not know why.

A night with a large share of unclassified events is almost always a night with a leak problem. Fix the leak and the events will sort themselves into the right columns, or disappear. See CPAP leak rate explained.

Why centrals appear on CPAP

A few scenarios account for most central events on therapy.

Treatment-emergent central sleep apnea. Some people with pure OSA start showing central apneas once their obstruction is treated. The usual explanation is that CPAP improves ventilation, carbon dioxide falls, and in people with a sensitive respiratory controller it dips below the threshold that keeps breathing going. For most of them this settles over weeks to months as the body recalibrates. For a few it persists and needs a different approach.

Pressure-related. Higher pressures can increase central events in susceptible people, for the same reason: more effective ventilation, lower CO₂. An auto machine will not raise pressure for a clear-airway event, but if obstructive events or flow limitation push pressure up, centrals can follow.

Sleep-wake transitions. Breathing is less regular when you are drifting off or briefly waking. A few central events at the start of the night, or after an arousal, are normal and not a concern.

Underlying conditions. Heart failure, opioid medications, stroke and living at high altitude all predispose to central apnea. Cheyne-Stokes breathing, a waxing-and-waning pattern seen in heart failure, is a specific form that ResMed machines flag separately.

Why the distinction changes what happens next

If your residual events are obstructive, the conversation with your clinician is about pressure, mask and position: the standard levers.

If they are central, more pressure is not the answer and may make things worse. Clinicians may wait to see whether treatment-emergent centrals settle, lower pressure, adjust EPR, review medications, or in persistent cases move to adaptive servo-ventilation (ASV), a bilevel machine that adjusts breath-by-breath to smooth out the pauses. ASV has specific contraindications and is a specialist decision.

Two people with an AHI of 8 can therefore be in completely different situations. The number alone does not tell you which.

How Apnealytics shows the breakdown

The dashboard does not stop at AHI. Beneath the headline tile sits the event composition ring: obstructive, central, hypopnea and unclassified, each as a share of the night's events. The night detail page lists the counts.

In Trends you can chart each event type separately over 30, 90 or 365 nights. A central share that climbs after a pressure increase, or an unclassified share that tracks your leak P95, is visible as two lines moving together. The "What changed?" panel dates your settings changes on the same axis.

On Pro, the breath-by-breath viewer shows each event as a labelled flag on the airflow trace. A clear-airway event looks like a flat line with no struggle before or after; an obstructive event often shows breaths shrinking into the pause and a large recovery breath coming out of it. You can also see the FOT pulse itself as a small ripple on the pressure trace during the pause.

The compliance report and share links include the event breakdown, so your clinician sees the composition, not just the total.

Patterns that merit a conversation

Central events making up more than a quarter of your residual AHI, consistently.
A central share that rose after a pressure or EPR change.
Central events clustered through the night rather than only at sleep onset.
Any sign of a waxing-and-waning breathing pattern on the waveform.
Unclassified events that persist after you have fixed leak.

Bring the trend and a few screenshots. "My centrals went from 5 percent to 30 percent of events after my max pressure went up" is a sentence a clinician can act on.

FAQ

Questions people ask

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What is a clear airway apnea on ResMed?

Clear airway (CA) is ResMed's term for a central apnea: a pause in breathing where the machine determined the airway was open. It tests this by sending a small pressure oscillation during the pause and checking whether flow responds. An open airway lets the oscillation through; a closed one does not. CA events are not treated by more pressure.

How does a CPAP know if an apnea is central or obstructive?

ResMed machines use the forced oscillation technique. When airflow stops, the machine adds a small, rapid pressure pulse and measures the flow response. If the pulse produces flow, the airway is open and the event is scored as clear airway (central). If it produces none, the airway is closed and it is scored as obstructive. High leak defeats the test, producing an unclassified apnea.

Why am I having central apneas on CPAP when my diagnosis was obstructive?

This is called treatment-emergent central sleep apnea and it is fairly common in the early months of therapy. Treating the obstruction improves ventilation, carbon dioxide falls, and in some people the breathing drive pauses as a result. For most it settles within weeks to months. If it persists or your central share is large, tell your clinician.

Does more pressure help central apnea?

No. Central apneas happen with the airway open, so there is nothing for pressure to hold open, and higher pressure can increase them in susceptible people by lowering carbon dioxide further. Auto machines are designed not to raise pressure for clear-airway events. If centrals dominate your residual events, the approach is different and is a clinician decision.

What does unclassified apnea mean?

An unclassified apnea is a pause in breathing the machine detected but could not sort into obstructive or central, almost always because leak was too high for the airway test to work. A night with many unclassified events is usually a night with a mask leak problem. Reduce leak and the events will either classify properly or disappear.