CPAP and BiPAP look identical from a distance. Same kind of mask, same kind of tubing, same kind of machine on the nightstand. The difference is what's happening inside: CPAP delivers one constant pressure all night. BiPAP delivers two — a higher pressure when you inhale, a lower pressure when you exhale. That single difference matters a lot for a specific subset of patients and not at all for most.
The short answer
CPAP is the right choice for the large majority of OSA patients. BiPAP is for users who can't tolerate the high-pressure exhale of CPAP, who have very high prescription pressures (typically 15+ cmH₂O), or who have specific conditions like obesity hypoventilation syndrome or complex sleep apnea.
What each one actually does
CPAP — Continuous Positive Airway Pressure
Delivers a single, constant pressure throughout the breath cycle. Whether you're breathing in or out, the pressure is the same. This works because the airway collapse that CPAP is treating happens during the relaxation phase of breathing — keeping the pressure constant means the airway never gets a chance to collapse.
Default first-line therapy for the large majority of OSA patients. Less complex, less expensive, simpler to maintain.
BiPAP — Bilevel Positive Airway Pressure
Delivers two separate pressures:
- IPAP (Inspiratory Positive Airway Pressure): higher pressure when you're breathing in.
- EPAP (Expiratory Positive Airway Pressure): lower pressure when you're breathing out.
The machine senses your breath cycle and adjusts pressure in real time — typically 50-100 times per minute. The differential between IPAP and EPAP is called the "pressure support" or "pressure delta," usually 4-6 cmH₂O.
What about APAP?
APAP (Automatic Positive Airway Pressure) is a third category that often gets confused with both. APAP is CPAP that adjusts its single pressure throughout the night based on what it detects — opening events, snoring, leak. It's still one pressure at any given moment, but that one pressure varies over time.
Most modern "CPAP" machines (ResMed AirSense AutoSet, Philips DreamStation Auto) are actually APAP machines. The terminology has blurred. When a sleep doctor prescribes "CPAP," they typically mean APAP unless they specify a fixed pressure.
| Mode | Pressure during inhale | Pressure during exhale | Pressure adjusts overnight? |
|---|---|---|---|
| CPAP (fixed) | 8 cmH₂O (example) | 8 cmH₂O | No |
| APAP (auto) | 5-15 cmH₂O range | Same as inhale at that moment | Yes, gradually |
| BiPAP | 12 cmH₂O (example) | 8 cmH₂O | Can be fixed or auto |
Why most people don't need BiPAP
At typical OSA prescription pressures (6-12 cmH₂O), the difference between inhaling and exhaling at the same pressure isn't difficult — most users acclimate within a few nights. Modern CPAP machines also have a feature called EPR (Expiratory Pressure Relief) or A-Flex / C-Flex (Philips terminology) that drops pressure slightly during exhalation — essentially giving you a small BiPAP-like effect within a CPAP machine.
EPR is usually configurable from 0-3. Most CPAP users find level 2 makes exhalation feel essentially natural without losing therapy effect. If you haven't enabled it, that's the first thing to try — see our CPAP side effects guide for the setting walkthrough.
When BiPAP is the right answer
1. Very high prescription pressures
Once your CPAP pressure hits 15 cmH₂O or higher, breathing out against that constant pressure becomes genuinely difficult. The exhale feels like exhaling through a straw. BiPAP solves this completely by dropping pressure during exhalation.
The cutoff isn't a hard threshold — some users tolerate 18 cmH₂O on CPAP with EPR enabled; others find 13 cmH₂O fatiguing. But the higher your pressure, the more meaningful the BiPAP advantage.
2. Aerophagia (air in the stomach)
About 30% of CPAP users experience some level of swallowed air going into the stomach. At higher pressures, this can be significant — morning bloating, belching, abdominal pain. BiPAP, by lowering exhalation pressure, reduces the pressure differential at the back of the throat and meaningfully decreases aerophagia for most affected users.
3. Complex / mixed sleep apnea
Some patients have both obstructive events (airway collapse) and central events (brain failing to signal breathing). CPAP can treat the obstructive component but doesn't help — and sometimes worsens — central events. Specialized BiPAP variants like BiPAP ST or ASV (Adaptive Servo-Ventilation) deliver timed breaths and adapt to central apnea patterns.
This requires careful diagnosis — ASV in particular has contraindications in heart failure patients, so the prescription is medical-specialist territory.
4. Obesity hypoventilation syndrome (OHS)
Some obese patients have not just OSA but also chronically reduced ventilation — they don't move enough air per breath, leading to elevated overnight CO₂. CPAP doesn't fix this; the patient needs assisted ventilation to actually move more air. BiPAP with a backup rate (BiPAP ST) is the standard treatment.
5. Neuromuscular disease or chest wall disorders
Conditions like ALS, muscular dystrophy, severe scoliosis, or post-polio syndrome can leave a patient unable to generate enough inspiratory effort on their own. BiPAP supports the breath in a way CPAP doesn't.
The cost question
BiPAP machines are more expensive than CPAP — typically $1500-3500 vs. $600-1500 for CPAP. Insurance covers both, but BiPAP requires stronger medical documentation. Without insurance coverage, the price differential is real.
Supplies are the same. Filters, hoses, cushions, headgear are interchangeable between most CPAP and BiPAP machines from the same manufacturer family — our ResMed filters work for both AirSense (CPAP) and AirCurve (BiPAP) machines, and our water chambers for the AirSense 10 also fit the AirCurve 10.
How to know if you should switch
Bring these specific signals to your doctor:
- Your prescription pressure has crept above 15 cmH₂O and you're finding it hard to fall asleep or stay asleep.
- You've enabled EPR at level 2-3 and exhalation still feels labored. EPR is the first-line fix; if it isn't enough, BiPAP is the next step.
- You have ongoing aerophagia that hasn't resolved with sleep position changes or pressure tuning.
- Your treated AHI hasn't dropped below 5 on CPAP despite well-fitted equipment and proper pressure settings — could indicate central events that BiPAP variants address.
- You have OHS, heart failure, COPD, or a neuromuscular disorder — these are conditions where bilevel therapy is often warranted from the start.
Switching machines
If your doctor agrees BiPAP is appropriate, the transition is relatively smooth. Same masks fit (the mask connector is universal). Same accessories. Most DME providers will swap your CPAP for a BiPAP under insurance with the right documentation.
Your prescription will look different on the BiPAP — you'll have two pressure numbers (IPAP and EPAP) rather than one. The first night feels noticeably different — exhaling is dramatically easier — and adjustment is usually faster than the first night on CPAP.
If you're new to CPAP and considering BiPAP based on what friends or forums say: don't jump. Start with CPAP, tune EPR, give it 30 days. Most users acclimate, and switching to BiPAP from a poorly-tuned CPAP setup often doesn't solve what you thought it would.
Bottom line
BiPAP is a real and useful therapy for the right indications — high pressures, aerophagia, complex apnea, obesity hypoventilation, neuromuscular conditions. For the typical OSA patient at typical pressures, CPAP with EPR enabled does the same job at a lower cost with simpler equipment. The decision is medical and worth a real conversation with your sleep physician, not a self-prescription from internet research.