CPAP vs BiPAP — What Is the Difference?
CPAP vs BiPAP —
what is the difference
and when does it matter?
CPAP delivers one pressure all night. BiPAP delivers two — a higher pressure when you breathe in, a lower one when you breathe out. Most people with sleep apnoea start on CPAP. BiPAP is prescribed when CPAP is not enough, or not tolerable.
CPAP (Continuous Positive Airway Pressure) delivers a single fixed pressure every time you breathe — the same pressure on inhalation and exhalation, all night. It keeps your airway open by maintaining continuous positive pressure against the walls of the throat.
BiPAP (Bilevel Positive Airway Pressure) delivers two separate pressures. A higher pressure when you breathe in (IPAP — Inspiratory Positive Airway Pressure) and a lower pressure when you breathe out (EPAP — Expiratory Positive Airway Pressure). This makes exhalation significantly easier and less effortful than breathing out against continuous CPAP pressure.
Which do you need? Most people with obstructive sleep apnoea start on CPAP — it has the most extensive evidence base and is effective for the majority of presentations. BiPAP is prescribed when CPAP pressure is too high to exhale against comfortably, when there are central or complex apnoea components, or when the patient has a respiratory condition that needs ventilatory support alongside airway pressure.
How CPAP works
A CPAP machine generates a continuous stream of pressurised air, delivered through a mask that seals around the nose, mouth, or both. The air pressure acts as a pneumatic splint — maintaining enough positive pressure in the upper airway to prevent the soft tissue walls from collapsing during sleep, which is the mechanism behind obstructive sleep apnoea.
The pressure is fixed: the same on every inhalation and every exhalation, every breath, all night. It is set by your consultant based on your sleep study results and does not change unless represcribed.
For most patients with straightforward obstructive sleep apnoea, CPAP works extremely well. Forty years of clinical evidence supports its efficacy. Patients who use it consistently — typically four or more hours per night — show significant improvement in AHI, oxygen levels, daytime alertness, and cardiovascular risk markers.
The most common difficulty with CPAP is the sensation of exhaling against continuous pressure. At lower prescribed pressures (below 10 cmH₂O), most patients adapt within a few weeks. At higher pressures, exhaling against the constant flow can feel effortful enough to disrupt sleep — and this is where BiPAP has a distinct clinical advantage.
How BiPAP works — and why two pressures matter.
BiPAP solves the exhalation problem by delivering two separate pressures. When the device detects you are breathing in, it delivers the higher IPAP pressure — sufficient to hold the airway open. When it detects you are breathing out, it drops to the lower EPAP pressure — making exhalation feel much more natural.
The difference between IPAP and EPAP is called the pressure support. A pressure support of 4 cmH₂O, for example, means the device drops 4 cmH₂O on exhalation. A higher pressure support makes breathing feel more like normal respiration and can also assist patients who have difficulty generating sufficient respiratory effort on their own.
CPAP vs BiPAP — side by side.
| Feature | CPAP | BiPAP |
|---|---|---|
| Pressures delivered | One fixed pressure | Two — IPAP (inhale) and EPAP (exhale) |
| Exhalation effort | Against full prescribed pressure | Against lower EPAP — significantly easier |
| Pressure support | None | IPAP minus EPAP (e.g. 4–10 cmH₂O) |
| Ventilatory support | — No | ✓ Yes — via pressure support |
| For obstructive sleep apnoea | ✓ First-line therapy | ✓ When CPAP is insufficient or intolerable |
| For central sleep apnoea | Not first line | ✓ With appropriate variant |
| For overlap syndrome (OSA + COPD) | Limited | ✓ Preferred |
| For CPAP intolerance at high pressure | — | ✓ Primary indication |
| Evidence base for OSA | Most extensive — 40+ years | Strong — specific indications well evidenced |
| Device complexity | Simpler | More complex — more settings |
| Consultant prescription required | ✓ | ✓ |
| Prior sleep study required | ✓ | ✓ |
When BiPAP is prescribed — and when it is not.
- Moderate to severe OSA confirmed on sleep study
- Predominantly obstructive events — not central
- Required pressure is tolerable (typically below 14 cmH₂O)
- No significant respiratory comorbidity
- No ventilatory support needed
- No prior failed CPAP trial at adequate support
- Required CPAP pressure is high and exhalation is difficult
- Central or complex sleep apnoea components present
- Overlap syndrome — OSA combined with COPD or other lung disease
- Obesity hypoventilation syndrome (OHS)
- Neuromuscular conditions affecting respiratory effort
- CPAP has failed despite good mask fit and clinical support
CPAP failure is not always a reason to switch to BiPAP. The most common reason CPAP fails is mask fit — not pressure mode. Before considering BiPAP, our CPAP Therapy Service addresses mask fitting, pressure adjustment, and adherence support systematically.
BiPAP variants — not all BiPAP is the same.
Standard BiPAP (sometimes called BiPAP-S or spontaneous mode) delivers two pressures triggered by the patient’s own breathing effort. This is the most common form and is appropriate for CPAP intolerance and most OSA presentations requiring BiPAP.
BiPAP-ST (spontaneous/timed) adds a backup respiratory rate — if the patient does not breathe within a set time interval, the device triggers a breath at IPAP pressure. This is used for patients with reduced respiratory drive, central apnoea components, or neuromuscular conditions.
ASV (Adaptive Servo-Ventilation) is a more advanced form of bilevel therapy that continuously adjusts both pressures and backup rate in response to the patient’s breathing pattern. It is specifically indicated for complex and central sleep apnoea, including treatment-emergent central apnoea (where central events appear after starting CPAP).
The appropriate BiPAP variant is determined by your consultant based on your sleep study findings. Starting on the wrong variant can be ineffective or, in the case of ASV in patients with reduced cardiac ejection fraction, potentially harmful.
Can I switch from CPAP to BiPAP?
Yes — and it is one of the most common escalation decisions in sleep medicine. The most appropriate reasons to switch are persistent pressure intolerance at required CPAP settings, residual apnoea events that CPAP is not controlling, or identification of central apnoea components that were not apparent on the initial study.
In most cases switching does not require a full repeat sleep study. Your Grace Sleep consultant reviews your therapy data, your current pressure, your residual AHI, and your symptom profile — and can represcribe BiPAP with appropriate IPAP and EPAP settings based on that information.
Your full clinical record is retained through any therapy mode change. The mode is an attribute of your care, not a separate episode.
If you are considering switching, contact our team — a brief consultation is all it takes to assess whether BiPAP is the right next step.
Frequently asked questions
Start with a diagnosis.
We prescribe what fits.
A home sleep study tells your consultant exactly what your airway is doing and which therapy mode — CPAP, APAP or BiPAP — is clinically appropriate for your presentation.
Book a sleep study → Speak to the team