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A good AHI score and solid usage hours tell you something valuable but not everything. Here is what your CPAP data captures, what it cannot see, and why the numbers in your app are not the whole picture of your sleep health.
What CPAP Devices Are Actually Measuring
CPAP devices measure what they are designed to measure: the behaviour of the airflow they deliver. Every metric in your app is derived from changes in airflow pressure, volume, and pattern detected by the device’s internal sensors. This makes CPAP devices highly accurate at what they do and systematically blind to anything that does not produce a detectable change in airflow.
AHI: The Most Useful Number That Also Misleads
The Apnoea-Hypopnoea Index is the number most CPAP users check first and treat as the primary measure of therapeutic success. It is genuinely the most important single output of your device and it is also the most misunderstood one. Understanding both what it captures and where it falls short makes you a significantly more informed reader of your own data.
The number of complete apnoea events (full airway collapse, ≥10 seconds) and hypopnoeas (partial reduction in airflow, ≥10 seconds, with associated drop in SpO₂ or arousal) detected per hour. A well-controlled AHI below 5 is the standard clinical target on therapy.
Sub-threshold respiratory events (flow limitation, RERAs) that cause arousals without meeting the AHI definition. Whether events clustered in REM sleep. Whether the AHI represents actual sleep time or device runtime including wakeful periods lying in bed.
The total duration the CPAP device was switched on and delivering pressure in a given night. This is the metric used by NHS services and insurance systems to assess adherence typically four hours per night on 70% of nights as a minimum benchmark.
Whether you were actually asleep during those hours. A patient who lies awake with the mask on for 90 minutes before sleeping generates the same usage figure as one who falls asleep immediately. Time awake in bed with CPAP running is not therapeutic sleep time.
Total leak from the mask system, including both intentional vent leak (designed into the mask) and unintentional leak from seal failure. Devices report unintentional leak or total leak depending on the manufacturer clarify which your device reports. High unintentional leak reduces therapy effectiveness and disrupts sleep.
The cause of the leak (seal position, mouth opening, jaw drop, cushion wear) and whether the leak occurred during sleep or during wakeful lying-in-bed periods. A single reported leak rate combines all of these without distinction.
For AutoCPAP devices: the pressure delivered across the night, typically reported as median pressure, 90th percentile, and maximum. If your 90th percentile pressure is close to or at the top of your prescribed range, this suggests your device may be working hard and your range may warrant review.
Why the device chose a given pressure at a given moment which may have been a genuine apnoea event, a large mask leak, a body position change, or REM-related airway changes. Pressure alone cannot distinguish between these causes.
The Arousal Problem: Why a Good AHI Can Still Mean Poor Sleep
One of the most clinically significant things your CPAP data does not show is the arousal index the number of times per hour your brain briefly wakes from sleep in response to a stimulus. Arousals do not have to be conscious waking events to fragment sleep architecture meaningfully. Micro-arousals lasting just three to fifteen seconds, detectable only on EEG, are sufficient to prevent the brain from completing a full sleep cycle and are sufficient to cause the daytime fatigue, cognitive impairment, and mood disruption associated with poor sleep quality.
CPAP reduces arousals caused by apnoea events but it does not eliminate all arousals. Arousals triggered by PLMD, pain, noise, temperature, stress, or idiopathic cortical hyperarousal continue unchanged. A patient whose AHI is 2.1 but who has 22 arousals per hour from non-apnoea causes will wake feeling just as unrefreshed as they did before CPAP, because the underlying sleep architecture disturbance is unchanged. The CPAP data will show an excellent AHI and the clinician will see a well-controlled patient while the patient continues to suffer from a problem the data cannot see.
Sleep Position: The Data You Probably Don’t Have
Sleep position is one of the most clinically significant variables in OSA severity and one of the most poorly captured by standard CPAP data. Obstructive sleep apnoea is significantly worse in the supine (back) position for the majority of patients. In some patients, OSA is almost entirely positional: their AHI may be 2 in a lateral position and 25 when supine, making their effective average AHI highly dependent on how much of the night they spend on their back.
- Some newer devices include a positional sensor. The ResMed AirSense 11 and some Philips devices include an accelerometer that can detect and log sleep position during the night. If your device has this feature, your OSCAR data will include a position graph overlay. Check the ResMed AirView or OSCAR output for positional data if you are curious whether your events cluster in a particular position.
- Most CPAP data provides no positional information whatsoever. For the majority of users on older devices, the AHI you see is a position-averaged figure that may be masking a significantly worse positional component. If you wake more unrefreshed after nights when you tend to sleep on your back, or if a partner tells you events seem to cluster in certain positions, this is worth raising with your sleep clinic.
- Positional OSA is treatable independently of CPAP. If a sleep study confirms that your OSA is strongly positional, positional therapy devices (body-position trainers, specialist wedge pillows, vibrational warning devices) can be used to reduce or eliminate supine sleep sometimes dramatically reducing the CPAP pressure needed or even making CPAP unnecessary in mild-moderate positional cases.
- CPAP works regardless of position. The important reassurance for users who do not have positional data: AutoCPAP algorithms increase pressure in response to increased obstruction regardless of what causes it, including position. Your device is managing positional variation in real time even if it is not reporting on it explicitly.
What OSCAR Reveals That MyAir Does Not
OSCAR (Open-Source CPAP Analysis and Reporting) is a free, open-source software application that reads detailed data directly from a device’s SD card or USB and presents it at a level of granularity that no consumer app matches. For technically curious CPAP users who want to go significantly deeper into their data than any manufacturer app allows, OSCAR is the tool to use.
Breath-by-breath airflow waveform showing the shape of each inhalation and exhalation including subtle flow limitation signatures not captured in the headline AHI.
Second-by-second delivered pressure across the entire night, showing exactly when the device raised or lowered pressure and by how much with event markers overlaid.
Distinguishes obstructive apnoeas, central apnoeas, hypopnoeas, flow limitations, RERAs (on compatible devices), and snore events rather than collapsing them into a single AHI.
Leak rate plotted minute-by-minute across the night, showing whether leaks were sustained or brief, and whether they coincide with position changes or sleep cycle transitions.
On some devices: an index of short arousals detected from the flow signal that suggests sleep fragmentation beyond what the AHI captures though this is a derived estimate, not an EEG measure.
The exact time the device was started, stopped, and restarted during the night revealing whether usage hours reflect continuous sleep or fragmented mask-on/mask-off sessions.
What In-Laboratory Polysomnography Adds: The Gold Standard
In-laboratory polysomnography (PSG) is the comprehensive sleep study conducted in a sleep centre with full physiological monitoring overnight. It is the only investigation that provides the complete picture of a night's sleep, and understanding what it measures helps clarify just how much remains invisible to a home CPAP device.
| Measurement | Standard CPAP Data | In-Lab PSG |
|---|---|---|
| Obstructive apnoeas / hypopnoeas (AHI) | Measured — primary output | Measured — with EEG arousal criterion |
| Central apnoeas | Detected — but may be over-reported without EEG | Precisely classified with respiratory effort bands |
| Sleep staging (N1, N2, N3, REM) | Not measured — device has no EEG | Full hypnogram — stage-by-stage across the night |
| Cortical arousals (EEG) | Not measured | Full arousal index — every micro-arousal captured |
| Leg movements (PLMD) | Not measured | EMG electrodes on tibialis anterior muscles |
| Blood oxygen saturation (SpO₂) | Not standard — some add-on oximeters available | Continuous pulse oximetry throughout |
| Heart rate and cardiac rhythm | Not measured | ECG throughout — arrhythmia detection possible |
| Sleep position | Estimated on some newer devices (accelerometer) | Precise — video monitoring and position sensor |
| REM-specific apnoea severity | Partial — some devices flag probable REM events | Precise — AHI in each sleep stage calculable |
How to Use Your Data More Intelligently
Understanding the limits of your CPAP data does not make it less useful it makes you a more accurate interpreter of it. The right way to use CPAP metrics is to treat them as indicators of therapy delivery, not as a complete picture of sleep health.
- Use AHI as a therapy check, not a health certificate. A controlled AHI confirms your device is doing its job. It does not confirm your sleep is fully restorative. If you feel well and rested with a good AHI, great. If you feel poorly with a good AHI, the AHI is not the end of the story it is the beginning of a further investigation.
- Pay attention to the pattern across nights, not the single-night number. A single night's AHI of 6.2 is less significant than a pattern of rising AHI over four weeks. A single night of 8 hours usage is less significant than a consistent pattern of four-hour nights. Trends are diagnostically more useful than point readings.
- Log your subjective sleep quality alongside the objective data. A simple morning note of how rested you feel on a 1–5 scale, alongside your MyAir score, quickly reveals whether the two are correlated for you personally. If they diverge consistently good scores, poor feeling that divergence is clinically meaningful information to share with your sleep clinic.
- Consider using a validated pulse oximeter alongside CPAP. A clip-on overnight pulse oximeter (ensure it is medical-grade, not a fitness tracker) can add SpO₂ and heart rate data that your CPAP device lacks. Some sleep services in the UK use home oximetry to screen for residual desaturation on therapy. Ask your sleep clinic whether an overnight oximetry check would be useful in your case.
- Ask your sleep clinic what they can see in AirView beyond what you see in MyAir. Clinicians using the ResMed AirView platform can access more detailed flow waveform data, pressure curves, and event flagging than is available in the patient-facing MyAir app. Asking them to review a specific night that concerned you, or a week where your data and your experience diverged, is a reasonable and productive use of a clinic appointment.
