Your PAP machine records thousands of data points every night. Breathing flow, leak rates, pressure changes, events per hour. All of it sitting on an SD card, waiting to be read.
But most people never look at it. And the ones who do? They see a single number: AHI.
Here's the thing. An AHI of 2 doesn't mean your therapy is sorted. Millions of PAP users have an AHI under 5 and still wake up exhausted. The reason is that AHI only counts complete airway closures and significant partial closures. It misses the subtler stuff -- flow limitation, breathing pattern instability, respiratory effort-related arousals (RERAs) -- that can fragment your sleep just as effectively.
If you want to actually understand what's happening while you sleep, you need to learn how to read your CPAP data properly. All of it. Not just the summary screen on your machine.
What Your PAP Machine Actually Records
Every modern ResMed and Philips machine writes detailed session data to its SD card. The exact data varies by model, but typically includes:
Flow waveform data
The raw airflow signal, measured breath by breath, usually at 25 samples per second.
Pressure data
The pressure your machine delivered throughout the night.
Leak rate
How much air escaped from your mask seal.
Events
Apneas (complete airway closures), hypopneas (partial closures), and sometimes central events.
Machine settings
Your prescribed pressure, mode, and response settings.
Session timing
When you put the mask on, when you took it off, total usage time.
This data is stored in EDF (European Data Format) files. Your machine's summary screen only shows a fraction of what's in there.
The AHI Problem
AHI (Apnea-Hypopnea Index) is the standard metric. It counts the number of apneas and hypopneas per hour of sleep. An AHI under 5 is generally considered in the typical range for treated PAP users.
But AHI has blind spots:
Flow limitation
When your airway narrows but doesn't collapse enough to register as an event, your AHI stays low while your breathing quality degrades.
RERAs
Respiratory effort-related arousals cause brief wake-ups that fragment sleep, but most machines don't count them in AHI.
Breathing pattern instability
Periodic breathing, irregular tidal volume, and variable breath timing all affect sleep quality without showing up in the AHI count.
Positional and temporal patterns
Your AHI might average 2 for the whole night but spike to 8 in the first half, suggesting something specific is happening in early sleep.
This is why the PAP community has been reading raw data for years. People on forums like ApneaBoard and r/SleepApnea learned long ago that AHI alone doesn't tell the full story.
How to Read Your CPAP Data: Tools and Approaches
There are several ways to dig into your data. Each has strengths depending on what you're looking for.
OSCAR (Open Source CPAP Analysis Reporter)
OSCAR is the gold standard for viewing raw PAP data. It's a desktop application that reads your SD card and displays detailed waveforms, event flags, pressure graphs, and session summaries. If you want to see every individual breath and manually inspect your flow data, OSCAR is the tool for that. (For a detailed comparison, see AirwayLab vs OSCAR.)
OSCAR's strength is depth. You can zoom into a specific two-minute window and see exactly what your airflow looked like during a cluster of events. It's invaluable for understanding what's happening breath by breath.
The tradeoff is that OSCAR shows you everything and leaves interpretation up to you. Reading raw waveforms requires time and knowledge. Many people download OSCAR, see the dense graphs, and aren't sure what they're looking at.
AirwayLab (Browser-Based Analysis)
AirwayLab takes a different approach. Instead of showing you raw waveforms, it runs your flow data through scoring algorithms -- the Glasgow Index for breath shape analysis, NED for negative effort dependence, WAT for breathing regularity and periodicity -- and presents the results as scored metrics with context.
You drag your SD card folder into your browser. AirwayLab parses the EDF files, runs all four analysis engines, and gives you a dashboard with traffic-light indicators, trend charts, and night-by-night comparisons. Everything runs in your browser. Your data never leaves your device.
AirwayLab complements OSCAR. OSCAR shows you the raw waveforms. AirwayLab scores and interprets the patterns across nights. Many users use both -- OSCAR for deep-dive investigation and AirwayLab for the bigger picture.
Your Machine's App (myAir, DreamMapper)
Manufacturer apps like myAir provide a simplified summary. They're convenient but limited to what the manufacturer chooses to show -- usually AHI, usage hours, leak rate, and a compliance score. They don't expose the raw flow data or any analysis beyond basic event counting.
Key Metrics Beyond AHI
Once you start reading your CPAP data beyond the summary screen, here are the metrics that tell you more:
Flow Limitation Metrics
Glasgow Index
Scores the shape of each inspiratory flow curve on a 0 to 9 scale. A normal breath has a rounded inspiratory flow shape. As the airway narrows, the shape flattens, skews, or develops multiple peaks. The Glasgow Index captures this with nine components: skew, spike, flat top, top-heavy, multi-peak, no-pause, inspiratory rate, multi-breath, and variable amplitude. Higher scores indicate more distorted breath shapes.
FL Score
Measures inspiratory flatness on a 0 to 100 scale. Flat-topped inspiratory flow curves are a characteristic pattern seen in flow-limited breathing. Higher values indicate flatter, more restricted breathing patterns.
Breathing Pattern Metrics
Regularity (Sample Entropy)
Quantifies how predictable your minute-by-minute breathing pattern is. Lower entropy means more regular breathing. Higher values indicate more variable breathing patterns.
Periodicity Index
Uses frequency analysis to detect cyclical breathing patterns in the 30 to 100 second range. This is the frequency range associated with periodic breathing patterns, where ventilation rises and falls in a repeating wave pattern.
Effort and Arousal Metrics
NED (Negative Effort Dependence)
Measures whether airflow decreases during a breath despite continued effort. High NED values indicate that airflow decreases during the breath despite continued inspiratory effort.
Flatness Index
The ratio of mean to peak flow within each breath. Lower values mean more peaked, less restricted breathing. Higher values indicate flatter, more restricted flow profiles.
Estimated Arousal Index (EAI)
Identifies brief spikes in respiratory rate and tidal volume that coincide with changes in breathing pattern, even when they don't trigger a scored event.
Oximetry Metrics (If You Have a Pulse Oximeter)
ODI-3 and ODI-4
Oxygen Desaturation Indices counting drops of 3% or 4% from a rolling baseline. These correlate with respiratory events but are measured independently from airflow.
Heart rate surges
Brief increases in heart rate that often accompany respiratory arousals. Counted using clinical thresholds (8, 10, 12, and 15 bpm above baseline).
Coupled events
When an oxygen desaturation and a heart rate surge occur within 30 seconds of each other, they are counted as a coupled event.
Reading Your Data: A Practical Example
Say you pull your SD card and load your data. Your AHI is 1.8-- well within the typical range. But you're still tired.
Here's what deeper analysis might show:
Glasgow Index of 4.2 -- Your breath shapes are moderately distorted. This is higher than the lower range where most unrestricted breathing falls.
FL Score of 62 -- Inspiratory flow curves are substantially flattened, as reflected in an elevated FL Score. These patterns are not captured by event-based AHI scoring.
Periodicity Index of 0.15 -- There's a cyclical component to your breathing in the 30 to 100 second range.
First-half vs second-half split -- Your NED values are 28% in the first half of the night and 12% in the second half, showing the pattern is concentrated in early sleep.
None of these show up in AHI. But together, they paint a much more detailed picture of your breathing patterns -- the kind of information your clinician can interpret alongside your clinical history.
Getting Started
Reading your data takes three things:
Your SD card
Remove it from your PAP machine. Most ResMed machines use a standard SD card in a slot on the side or back.
A card reader
Any USB SD card reader works. Many laptops have built-in slots.
An analysis tool
Load your data into OSCAR for raw waveform viewing, AirwayLab for automated scoring and pattern analysis, or both for the complete picture.
With AirwayLab, you drag your SD card's DATALOG folder into the browser window. Analysis runs in about 30 seconds for a typical card with weeks of data. No account required, no data uploaded, no installation needed. Your results are saved locally and persist for 30 days.
If you also use a Viatom or Checkme O2 Max pulse oximeter, you can upload that CSV alongside your SD card data for combined respiratory and oximetry analysis.
Sharing Your Results
Your data gives you a detailed picture of your breathing patterns. This information is most valuable when shared with your sleep physician or clinician, who can interpret it alongside your full clinical context -- your sleep study results, symptoms, medical history, and physical examination.
The PAP community is also a great resource. Forums like ApneaBoard, r/SleepApnea, and r/CPAP have knowledgeable members who've been reading their own data for years. AirwayLab includes a forum export feature that formats your results for sharing in these communities, complete with all relevant metrics and context.
Your Data Belongs to You
Your PAP machine generates detailed information about your breathing every single night. That data belongs to you. Reading it, understanding it, and sharing it with your clinician is how you become an informed participant in your own care.
AHI is a starting point, not the finish line. The real story is in the flow shapes, the breathing patterns, the temporal trends across nights and weeks. Learning how to read your CPAP data is the first step toward understanding that story.
Medical disclaimer
AirwayLab is not a medical device. The analysis provided is informational and educational. Always discuss your results with your sleep physician or clinician. AirwayLab does not diagnose, treat, or provide clinical recommendations.
Related reading
Understanding Flow Limitation -- what flow limitation is, how the Glasgow Index scores it, and how to detect it.
Why Your AHI Is Lying to You -- the evidence that AHI misses the majority of breathing problems.
Your PAP Data Belongs to You -- who can see your sleep data and how to keep control of it.
See Your Breathing Patterns Scored and Visualised
Drag your SD card folder into AirwayLab and see your breathing patterns scored and visualised in 30 seconds. Free, open source, and your data never leaves your browser.