If you've been on CPAP for a while and started wondering whether BiPAP might be relevant to your situation — or if you've just been prescribed BiPAP and want to understand what's different — this article covers the key distinctions between the two device types and how their therapy data looks when you analyse it.
The Core Difference: One Pressure vs. Two
CPAP (Continuous Positive Airway Pressure) delivers a single, continuous pressure throughout the entire breathing cycle. Whether you're inhaling or exhaling, the pressure stays the same. On auto-adjusting CPAP (APAP), the pressure varies based on detected events — but it still delivers one pressure level at any given moment.
BiPAP (Bilevel Positive Airway Pressure) delivers two separate pressure levels:
IPAP — Inspiratory Positive Airway Pressure
The higher pressure level, active during inhalation. Supports the breathing effort as air flows in.
EPAP — Expiratory Positive Airway Pressure
The lower pressure level, active during exhalation. Keeps the airway open while reducing the effort needed to breathe out against the machine.
The gap between IPAP and EPAP — called pressure support (PS) — is set by the prescribing clinician. This pressure difference actively assists the breathing effort during inhalation.
Why Two Pressures Instead of One?
For most people with obstructive sleep apnea, CPAP at an appropriate pressure is effective at maintaining airway patency. BiPAP is typically prescribed in situations where:
- High pressure is required and exhaling against it is uncomfortable or difficult — the lower EPAP makes exhalation easier
- There is a component of hypoventilation — conditions where breathing volume (tidal volume) needs support, not just airway patency
- Central or complex sleep apnea patterns are present — though specific device selection in these cases is complex and highly individualised
- Respiratory muscle weakness or certain neuromuscular conditions — where breathing assistance is needed beyond just keeping the airway open
- CPAP intolerance — when patients cannot tolerate single fixed pressure despite adequate time and fitting adjustments
The decision about which device is appropriate for a given person is made by a sleep physician based on diagnostic study results, clinical history, and response to therapy.
What the Therapy Data Looks Like: CPAP vs. BiPAP
When you review therapy data from each device type, some of what you see stays exactly the same and some is new.
Some things look exactly the same as CPAP:
AHI
Still your headline number: apneas and hypopneas per hour.
Leak rate
Unintentional air escaping from the mask or via mouth breathing.
Flow waveform
The shape of each breath, useful for spotting flow limitation and RERAs.
Snore index
Vibration-based snore detection (device-dependent).
What's new or more prominent in BiPAP data:
- IPAP and EPAP traces — Instead of one pressure line you get two, plus a pressure support channel on most modern devices.
- Tidal volume — The volume of air per breath. More clinically relevant here because pressure support directly affects it.
- Minute ventilation — Breaths per minute × tidal volume; shows total breathing work across the session.
- Respiratory rate — Often logged more granularly on BiPAP machines.
- Ti (inspiratory time) — How long each inhale lasts; useful for spotting breathing pattern irregularities.
Not all BiPAP machines log all of these. ResMed AirCurve devices export the richest data; some older or simpler BiPAP units produce only basic summary statistics.
One practical note: BiPAP devices — particularly those in the S/T (spontaneous/timed) category — may not export the same EDF data fields as APAP devices. If you're loading BiPAP data into AirwayLab, the available metrics may differ from what you'd see with a ResMed AirSense.
How Flow Patterns Differ
On a CPAP waveform, the inspiratory flow shape reflects the patient's own respiratory effort modulated by the constant applied pressure. Tools like the Glasgow Index and FL Score analyse these shapes to characterise breath quality — flatness, limitation patterns, and shape irregularities.
On BiPAP, the pressure support component actively augments inhalation. The resulting flow shapes look different: inhalation is typically faster and more assisted. Flow limitation analysis tools designed for CPAP waveforms interpret BiPAP waveforms with this context in mind.
Flow waveform and flow limitation
Flow limitation applies to both CPAP and BiPAP, but pressure support makes it more visible in the data. A clipped or flattened peak on the inspiratory flow waveform is called flow limitation — a data pattern you may see flagged in OSCAR or AirwayLab. Flow limitation scores may vary across sessions depending on pressure support settings and other factors. If you notice this pattern in your data, that's a specific observation to bring to your next appointment.
Viewing and Loading Your BiPAP Data
OSCAR
OSCAR handles BiPAP data well and has a long history in the PAP community. It displays IPAP/EPAP pressure graphs, leak rate, flow waveform, tidal volume (where the device logs it), and event flags. If you're familiar with OSCAR for CPAP, you'll find the same interface with additional pressure channels in the session view.
AirwayLab
AirwayLab reads the same SD card data OSCAR uses, scores each night for you, and opens the waveform behind any flag in the browser. A few things it adds for BiPAP users:
- Pressure support visualisation — see the IPAP/EPAP gap across the full night at a glance
- Flow limitation scoring alongside your dual-pressure trace
- RERAs, breathing pattern irregularities, and AHI, all in your browser with your data staying local
Loading your data
.edf files — into the drop zone. AirwayLab processes everything in your browser; nothing is uploaded.What you'll see
Session overview
Date, duration, and a top-line AHI summary.
Pressure support
How inspiratory support varied across the night.
Flow limitation
Described as a frequency measure across the session.
Flow waveform
Your full breathing signal, scrollable night-by-night.
RERA patterns
Respiratory effort-related arousals displayed as a visual timeline.
Common Questions
“My AHI is fine on CPAP — why would I switch to BiPAP?”
AHI measures event count. It doesn't fully capture comfort, breathing effort, or ventilation adequacy. Whether AHI alone is sufficient to evaluate therapy outcome is a clinical determination. Your sleep physician considers the full picture.
“Can I request BiPAP if I find CPAP uncomfortable?”
Discomfort with CPAP is worth discussing with your sleep clinician — there are fitting, humidity, and pressure adjustment approaches that sometimes resolve tolerance issues without changing device type. Your clinician can advise which approach fits your situation.
“Is BiPAP more expensive?”
Generally yes — BiPAP hardware and supplies typically cost more than equivalent CPAP. Insurance coverage varies. Your equipment provider and clinician can walk through the options.
“My IPAP trace looks like it's shifting around all night — is that a problem?”
If your device auto-adjusts IPAP while holding EPAP fixed (as in ASV or BiPAP Auto modes), you'll see IPAP vary across the night. A stable, narrow IPAP trace means your breathing was regular; a wide, shifting trace means the machine was working harder to compensate for irregular breathing. Neither reading is a conclusion on its own — it's context to bring to your next appointment, not a cause for alarm.
What to Ask Your Clinician
If you're wondering whether your current device type is the right fit for your situation, useful questions include:
- “Based on my recent data, does my event and pressure pattern suggest my current device is well-matched to my needs?”
- “Are there any aspects of my data — central events, pressure levels, flow patterns — that would warrant a review of my device type?”
- “If I'm finding exhalation difficult at my current pressure, is bilevel worth exploring?”
These questions give your clinician the opening to review your data in context. They are in the best position to compare your diagnostic study, your current data trends, and your symptoms together.
“I pulled my SD card data and noticed X” is a more productive starting point for that conversation than “I don't think my therapy is working.” A specific data point gives your clinician something concrete to look at.
Frequently Asked Questions
What is the difference between CPAP and BiPAP?
CPAP (Continuous Positive Airway Pressure) delivers a single continuous pressure during both inhalation and exhalation. BiPAP (Bilevel Positive Airway Pressure) delivers two separate pressure levels — a higher IPAP during inhalation and a lower EPAP during exhalation. The pressure difference (pressure support) actively assists the breathing effort.
When is BiPAP prescribed instead of CPAP?
BiPAP is typically considered when high pressure is required and exhaling against it is uncomfortable, when there is a component of hypoventilation, when central or complex sleep apnea patterns are present, or when CPAP cannot be tolerated despite adequate time and fitting adjustments. The decision is made by a sleep physician based on sleep study results and clinical history.
Is BiPAP better than CPAP for sleep apnea?
Neither device type is universally better — the appropriate choice depends on each person's clinical situation. CPAP is effective for most people with obstructive sleep apnea. BiPAP addresses specific scenarios where single-pressure therapy is not adequate or well-tolerated. Your sleep physician can advise which approach fits your situation.
Does AirwayLab work with BiPAP data?
AirwayLab currently reads ResMed EDF format. Some ResMed BiPAP models write compatible EDF files and the available metrics depend on what your device records. Check the community forum for device-specific compatibility notes.
What is pressure support in BiPAP data?
Pressure support is the difference between IPAP and EPAP — the extra pressure applied to assist each inhale.
Does tidal volume appear in all BiPAP data?
It depends on the device. ResMed AirCurve devices log tidal volume in SD card data; some other BiPAP machines do not export it.
What does a shifting IPAP trace mean in auto BiPAP mode?
It means the machine adjusted the inspiratory pressure during the night — typically in response to detected breathing changes. This is information to discuss with your clinician, not a cause for alarm on its own.
What is the difference between CPAP and BiPAP EDF data?
BiPAP machines deliver two pressures (IPAP and EPAP). The EDF format is the same as CPAP, but the waveform includes pressure support data alongside the flow signal. AirwayLab parses both correctly.
Ready to look at your CPAP or BiPAP data?
Upload your SD card data on AirwayLab — no account needed, nothing uploaded, works entirely in your browser.
Further Reading
What Is Flow Limitation on CPAP? — the breath-shape analysis that goes beyond AHI.
AHI Normal but Still Tired? Understanding the Gap — why a normal AHI doesn't always mean therapy is complete.
How to Read Your CPAP Data — full guide to PAP data metrics including AHI and flow limitation.
How to Read Your ResMed SD Card Data in Your Browser — step-by-step guide to finding and opening EDF files.
Analyse CPAP Data in Your Browser — No Download, No Cloud, No Account — how AirwayLab protects your privacy.
Glossary
Flow limitation — what it means and how AirwayLab measures it.
RERA — respiratory effort-related arousals explained.
NED Mean — per-breath negative effort dependence metric.
A note on device decisions
AirwayLab is a data-visualisation tool, not a medical device. This article describes how CPAP and BiPAP devices work and how their therapy data differs — it is not a recommendation for any particular device or treatment. Device selection and prescription decisions belong with your sleep physician. Your clinician can help interpret these findings in context.