CPAP vs BiPAP vs ASV vs APAP: Modes Explained

Five Modes, One Goal

Every positive airway pressure device has the same fundamental job: push air into your airway to keep it open while you sleep. The differences between modes come down to how the machine decides what pressure to deliver and when.

Understanding your therapy mode helps you read your SD card data, interpret your pressure charts, and have better conversations with your sleep physician. It also explains why some machines cost $700 and others cost $2,800.

CPAP: Fixed Continuous Positive Airway Pressure

What it does: delivers one constant pressure all night. If your prescription is 10 cmH2O, you get 10 cmH2O on every breath, inhale and exhale.

Who needs it: patients with straightforward obstructive sleep apnea whose optimal pressure was determined in a titration study. CPAP is the simplest and least expensive therapy mode.

Machines: ResMed AirSense 11 Elite, AirSense 10 Elite.

What the data shows: a flat pressure line on your chart. No variation (except during ramp-up at the start of the night). If your pressure chart shows a perfectly horizontal line, you are on fixed CPAP.

Limitations: the pressure that keeps your airway open at 2 AM in a supine position may be too high when you are on your side at midnight. Fixed pressure cannot adapt to positional changes, sleep stage differences, or alcohol-related airway relaxation. Many patients find exhaling against a constant high pressure uncomfortable.

Comfort feature: EPR (Expiratory Pressure Relief) reduces pressure by 1 to 3 cmH2O during exhalation. EPR does not change the therapy mode. It is a comfort overlay available on most ResMed CPAP and APAP devices.

APAP: Auto-Adjusting Positive Airway Pressure

What it does: automatically adjusts pressure within a prescribed range (e.g., 6 to 14 cmH2O) based on detected breathing events. When your airway is stable, pressure drops to the minimum. When flow limitation, snoring, or apneas are detected, pressure increases until the events resolve.

Who needs it: the majority of new OSA patients. APAP is now the default first-line prescription for most sleep physicians because it adapts to night-to-night variability without requiring a full titration study.

Machines: ResMed AirSense 11 AutoSet, AirSense 10 AutoSet, BMC E-20A (AutoCPAP).

What the data shows: a pressure line that varies throughout the night. You will see pressure rise during periods of instability and fall during stable breathing. The 95th percentile pressure in your CPAP Clarity dashboard tells you what pressure your machine needed for most of the night. If your P95 pressure is consistently near your maximum, discuss a range adjustment with your provider.

Advantages over fixed CPAP: lower average pressure (more comfortable), adapts to positional changes, adjusts for factors like alcohol or allergies that vary night to night. Research by Ip et al. (2012, Sleep) found that APAP and fixed CPAP produce equivalent AHI reduction, but APAP users report slightly better comfort and adherence.

When APAP is not appropriate: central sleep apnea (the machine's algorithm cannot distinguish central from obstructive events reliably), Cheyne-Stokes respiration, or significant air leak that confuses the auto-titration algorithm.

BiPAP: Bilevel Positive Airway Pressure

What it does: delivers two different pressures. A higher pressure on inhalation (IPAP) and a lower pressure on exhalation (EPAP). The difference between IPAP and EPAP is called pressure support (PS).

Who needs it: patients who cannot tolerate the exhale pressure on CPAP/APAP, patients who need higher pressures (above 15 to 20 cmH2O), patients with obesity hypoventilation syndrome, COPD overlap, or neuromuscular conditions affecting breathing.

Machines: ResMed AirCurve 11 S, AirCurve 10 S. Fixed bilevel: both IPAP and EPAP are set to specific values.

What the data shows: two pressure lines on your chart (IPAP and EPAP) with a consistent gap between them. Your SD card data includes I:E ratio and inspiratory time that CPAP/APAP devices do not record.

Key distinction from EPR: EPR reduces exhale pressure by 1 to 3 cmH2O as a comfort feature. BiPAP pressure support can be 4, 6, 8, or more cmH2O, and it actively assists ventilation. A patient on BiPAP with IPAP 16/EPAP 10 gets 6 cmH2O of pressure support, which is genuinely augmenting each breath, not just making exhalation easier.

VAuto: Variable Auto-Bilevel

What it does: combines bilevel therapy with auto-titration. EPAP auto-adjusts to keep the airway open (like APAP), while IPAP adjusts to maintain a target pressure support. Both pressures vary throughout the night within prescribed ranges.

Who needs it: patients who need bilevel therapy but whose optimal pressures vary night to night. VAuto is the most commonly prescribed AirCurve mode.

Machines: ResMed AirCurve 11 VAuto, AirCurve 10 VAuto.

What the data shows: two varying pressure lines (IPAP and EPAP) that both move independently. The pressure support gap between them may also vary. This produces the most complex pressure charts of any non-ASV mode.

Advantages: adapts like APAP but with bilevel comfort. Patients who failed APAP due to exhale pressure intolerance and who also have variable pressure needs are the target population.

ASV: Adaptive Servo-Ventilation

What it does: dynamically adjusts pressure support on a breath-by-breath basis. It learns your recent breathing pattern and intervenes when breathing becomes unstable. During normal breathing, it provides minimal support. When it detects central apneas or periodic breathing, it increases support to stabilize ventilation.

Who needs it: patients with central sleep apnea, complex sleep apnea, treatment-emergent central sleep apnea (central events that appear after starting CPAP), or periodic breathing patterns.

Machines: ResMed AirCurve 11 ASV, AirCurve 10 ASV.

What the data shows: highly variable pressure support that responds to each breath. The pressure chart looks more active than any other mode. Your data includes target ventilation values that show what the machine is trying to achieve.

Critical safety warning: ASV is contraindicated in patients with heart failure and reduced ejection fraction (LVEF below 45%). The SERVE-HF trial (Cowie et al., 2015, New England Journal of Medicine) found increased cardiovascular mortality in this population. Your cardiologist and sleep physician must both be involved in the decision to use ASV.

Cost: ASV devices are the most expensive positive airway pressure machines, typically $2,500 to $3,000 or more without insurance.

ST: Bilevel with Backup Rate

What it does: delivers bilevel pressure (IPAP/EPAP) with an added safety feature: a timed backup respiratory rate. If you stop breathing for longer than a set interval, the machine triggers a breath for you by cycling to IPAP.

Who needs it: patients with central apneas, hypoventilation syndromes, or neuromuscular conditions where the brain may not reliably trigger breaths. Often prescribed for conditions like ALS, muscular dystrophy, or severe obesity hypoventilation.

Machines: ResMed AirCurve 11 ST, AirCurve 10 ST.

What the data shows: similar to bilevel, but with machine-triggered breaths visible in the event data. The backup rate setting (e.g., 12 breaths per minute) determines the minimum breathing frequency the machine enforces.

iVAPS: Intelligent Volume-Assured Pressure Support

What it does: targets a specific tidal volume (the amount of air per breath) rather than a fixed pressure support. The machine adjusts IPAP to maintain your target ventilation, even as your breathing effort changes throughout the night.

Who needs it: patients with chronic respiratory failure, obesity hypoventilation, or neuromuscular disease where maintaining adequate ventilation is the primary goal. This is a specialized mode typically managed by pulmonologists.

Machines: ResMed AirCurve 11 VAuto (iVAPS mode available), AirCurve 10 VAuto.

Quick Reference

Mode	Pressures	Auto-Adjusts?	Best For	Typical Cost
CPAP	One fixed	No	Simple OSA, known optimal pressure	$500-800
APAP	One, variable	Yes	Most new OSA patients	$700-1,000
BiPAP (S)	Two fixed	No	High pressure intolerance, COPD overlap	$1,500-2,000
VAuto	Two, variable	Yes (both)	Variable bilevel needs	$1,500-2,200
ASV	Two, breath-by-breath	Yes (dynamic)	Central/complex apnea	$2,500-3,000
ST	Two + backup rate	Optional	Hypoventilation, neuromuscular	$2,000-2,500
iVAPS	Volume-targeted	Yes (targets TV)	Chronic respiratory failure	$2,000-2,500

How to Know Which Mode You Are On

Check your machine's settings screen or your SD card data. In CPAP Clarity, your machine settings card shows the therapy mode. In the pressure chart:

• Flat line: fixed CPAP
• Single varying line: APAP
• Two lines with fixed gap: bilevel (S)
• Two lines, both varying: VAuto
• Rapidly varying support: ASV

If you are unsure, import your SD card into CPAP Clarity and check the machine settings display. It reads your mode directly from the SD card data.

Can You Switch Modes Yourself?

No. Your therapy mode is part of your prescription. Changing modes requires a physician order because:

• Different modes treat different conditions. Switching from APAP to ASV when you have obstructive (not central) apnea would be inappropriate therapy.
• ASV has a contraindication (heart failure with reduced EF) that makes unsupervised mode changes potentially dangerous.
• Insurance coverage is tied to your diagnosis and prescribed mode. Using a device in a mode not covered by your prescription can create billing issues.

If you think a different mode might help (for example, if you are on fixed CPAP and struggling with exhale pressure), discuss it with your sleep physician. Bring your CPAP data to the appointment so your provider can see the specific patterns driving your discomfort.

Equipment for Your Therapy Mode

Regardless of your therapy mode, the right accessories make a difference. A CPAP pillow (opens in new tab) accommodates your mask without breaking the seal. A pulse oximeter (opens in new tab) lets you monitor overnight oxygen levels at home, which can be valuable context alongside your AHI data. And an SD card reader (opens in new tab) lets you import your data into CPAP Clarity to see exactly how your machine is performing in your prescribed mode.

Prices vary by retailer. As of April 2026.

Related Tools and Articles

• BiPAP vs CPAP: detailed comparison
• AirSense 11 vs AirCurve 11
• Understanding EPR
• Central vs Obstructive Apnea
• Treatment-Emergent Central Apnea
• AirCurve 10 VAuto Data Guide
• STOP-BANG Sleep Apnea Screener
• Analyze your CPAP data

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