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Understand AHI, Event Flags, Pressure, Leak Rate and Flow Limitation with real chart examples explained in plain English.
What Is OSCAR?
OSCAR (Open Source CPAP Analysis Reporter) is free software that reads therapy data from your CPAP machine’s SD card and displays it as detailed charts. It shows far more information than the summary on your machine screen or in apps such as myAir.
It is compatible with:
- ResMed AirSense 11 and AirSense 10
- ResMed AirMini (travel machine)
- Certain Philips DreamStation and Löwenstein Medical devices
- BMC and other SD-card recording CPAP machines
The Best Order to Review Your OSCAR Charts
Rather than jumping into every panel at once, review in this sequence to avoid information overload:
- AHI Your headline summary number
- Event Flags When did breathing events happen?
- Leak Rate Was your mask sealing properly?
- Pressure How did the machine respond?
- Flow Limitation Any partial obstruction signals?
- Flow Rate Zoom in if needed for detail
1. AHI Your Headline Number
AHI (Apnoea-Hypopnoea Index) tells you how many breathing disruptions occurred per hour of sleep. It is the single most-watched number in CPAP therapy, but it should never be read in isolation.
| AHI Range | Classification | What It Suggests |
|---|---|---|
| Under 5 | ✓ Treated | Therapy is generally working well |
| 5 – 14 | Mild residual | May warrant mask or pressure review |
| 15 – 29 | Moderate | Discuss with your sleep clinic |
| 30 or above | High | Therapy needs attention promptly |
2. Event Flags Most Important Starting Point
The Event Flags chart shows when breathing events happened throughout the night. Each coloured tick mark represents one event at the exact time it occurred. The green background makes it easy to spot clusters at a glance.
What to look for in Event Flags
- A few scattered events throughout the night is completely normal and expected
- Clusters of OA events in the early morning often indicate REM-related obstruction
- Events grouped at a single time point may suggest positional apnoea try a different pillow height
- OA events that coincide with LL flags may be false detections caused by leaks, not real apnoeas
3. Pressure Chart How Your Machine Responded
The Pressure chart shows how your CPAP machine adjusted its output pressure throughout the night. The red IPAP line is the pressure you receive on each breath. The green EPAP line is the baseline support. On auto-adjusting (APAP) machines, the red line fluctuates constantly as the machine detects and responds to breathing changes.
- Pressure constantly hitting your maximum setting suggests the machine wants more room to respond discuss raising the ceiling with your sleep clinic
- Frequent sudden pressure spikes followed by drops often correlate with event clusters visible in the Event Flags panel
- A completely flat pressure line indicates fixed-pressure (CPAP) mode rather than auto-adjusting (APAP) mode
- The P95 figure (95th percentile pressure) in the left panel statistics is the most clinically relevant single number it tells you the pressure needed to control 95% of events
4. Leak Rate One of the Most Important Charts
The Leak Rate chart shows unintentional air escaping your mask seal, measured in litres per minute. OSCAR automatically subtracts the intentional vent flow built into your mask, so what you see is genuine mask leak. This chart directly affects the accuracy of everything else high leaks can create false AHI events and prevent the machine from titrating correctly.
Common causes of mask leaks
- Worn or degraded cushion — silicone degrades over time, particularly with natural skin oils
- Wrong cushion size — if the seal gaps around the nose or chin, try a different size
- Mouth opening during sleep — visible as sudden brief spikes on the leak chart
- Headgear too loose or too tight — both cause leaks in different ways
- Hose connection not fully seated — check the elbow connector is clicked in securely
5. Flow Rate Chart Every Breath Shown
The Flow Rate chart displays every single breath as a dense waveform each oscillation is one inhalation and exhalation. Normal breathing appears as a regular, rhythmic pattern. Apnoeic events appear as flat sections where the waveform disappears. Hypopnoeas appear as noticeably shallower oscillations. Advanced users zoom in on specific sections to examine individual events in detail.
6. Flow Limitation Often Overlooked by Beginners
Flow Limitation (FL) is scored on a scale of 0 to 1. It measures how restricted each breath is even when airflow is not fully stopped. High or repeated flow limitation can indicate partial airway restriction even when your AHI looks perfectly normal. This is why experienced CPAP users track it alongside AHI rather than instead of it.
Good Night vs Needs Attention A Visual Comparison
After a few weeks of using OSCAR, you will start to recognise patterns. Here is what a well-controlled night typically looks like compared to one that warrants attention.
Common Mistakes Beginners Make
One central apnoea flag is not automatically a problem. A single event per night is within normal variation for many users.
Zero is neither realistic nor necessary. An AHI under 5 with good sleep quality is the actual goal.
Leaks make every other chart harder to interpret and can artificially inflate your AHI. Fix the seal first.
One night of data is never enough. Look for trends across at least one to two weeks before drawing conclusions.
OSCAR data supports a clinical conversation it does not replace one. Always discuss changes with your sleep team.
A session of only 3 hours may show elevated AHI simply because it captured only REM-heavy early morning sleep.
Can OSCAR Help Improve Your Therapy?
Yes many CPAP users use OSCAR to have far more productive conversations with their sleep clinicians. Rather than describing symptoms in vague terms, you can share screenshots of specific nights, export multi-week trend summaries, and flag patterns that might not be visible from a brief clinic appointment.
- Use the Overview tab to export a multi-week AHI trend this is the most useful single item to share with your sleep clinic
- Screenshot specific bad nights to show event clustering or pressure behaviour
- The Statistics tab summary across a date range gives clinicians the averaged picture they need for a prescription review
- Back up your SD card data regularly some machines overwrite sessions older than 30 days
Final Thoughts
OSCAR can seem technical at first, but you do not need to analyse every graph at once. Start with three:
Those three charts alone often tell most of the story. Over time, you can add Flow Limitation and the breath-by-breath Flow Rate trace to your regular review.
Remember: do not chase perfect numbers focus on better sleep, good therapy, and understanding trends over time.
