CPAP vs BiPAP.
One is a splint. One is a ventilator.
A working reference for prescribing clinicians: how the two modes differ mechanistically, where each is indicated, how pressures are set and titrated, and what actually determines whether a patient stays on therapy.
A single pressure, or two.
CPAP delivers one continuous pressure across the entire respiratory cycle. It is a pneumatic splint: it holds the pharynx open and abolishes obstructive events. It does not assist inspiration, and it does not move gas on the patient’s behalf.
BiPAP delivers two pressures — a higher inspiratory positive airway pressure (IPAP) and a lower expiratory pressure (EPAP). The gradient between them is pressure support. That gradient augments tidal volume, unloads the respiratory muscles and clears CO₂. EPAP retains the splinting function; IPAP adds ventilation.
The clinical consequence follows directly: if the problem is a collapsing airway, CPAP is sufficient. If the problem is inadequate ventilation — a rising PaCO₂ — only bilevel therapy addresses it.
Match the mode to the failure, not to the severity.
Obstructive sleep apnoea (AHI ≥ 15, or ≥ 5 with symptoms)
- Upper-airway resistance syndrome
- Cardiogenic pulmonary oedema (acute setting)
- Post-extubation airway support in selected patients
- Positional and REM-predominant OSA
Hypercapnic respiratory failure requiring ventilatory assistance
- COPD exacerbation with respiratory acidosis (pH < 7.35, PaCO₂ > 45 mmHg)
- Obesity hypoventilation syndrome
- Neuromuscular disease and chest-wall restriction
- CPAP failure: intolerance, residual events, or pressures > 15 cm H₂O
- Overlap syndrome (OSA + COPD)
Titration parameters, side by side.
| Parameter | CPAP | BiPAP |
| Pressure profile | Single fixed pressure | IPAP / EPAP, two levels |
| Typical starting point | 4–5 cm H₂O, titrated up | IPAP 10 / EPAP 5 cm H₂O |
| Usual therapeutic range | 6–15 cm H₂O | IPAP 10–25, EPAP 4–10 cm H₂O |
| Pressure support | None | IPAP − EPAP; typically 4–10 cm H₂O |
| Backup rate | Not available | Available in ST mode (10–14 bpm) |
| Primary effect | Splints the upper airway | Splints airway + augments ventilation |
| Corrects hypercapnia | No (indirectly at best) | Yes — the principal indication |
| Titration endpoint | AHI < 5, no desaturation, no arousal | Normocapnia, adequate tidal volume, AHI < 5 |
Values are conventional starting points for adult practice and are not a substitute for attended polysomnographic titration or arterial blood-gas guidance where indicated.
Five reasons to move a patient to bilevel.
- 01
CPAP requirement exceeds ~15 cm H₂O and the patient cannot exhale comfortably against it.
- 02
Residual hypoventilation — daytime PaCO₂ remains elevated, or nocturnal oximetry shows sustained desaturation despite abolished obstructive events.
- 03
Documented CPAP intolerance persisting after mask re-fit, humidification and expiratory pressure relief have been trialled.
- 04
The underlying disorder is ventilatory: obesity hypoventilation, neuromuscular weakness, kyphoscoliosis, or overlap syndrome.
- 05
Acute hypercapnic exacerbation requiring pressure support and, in some patients, a timed backup rate (ST mode).
Shared cautions across both modes.
Untreated pneumothorax and undrained bullous disease
Facial trauma, recent upper-airway or oesophageal surgery
Impaired consciousness with inability to protect the airway
Copious secretions or high aspiration risk
Haemodynamic instability requiring intubation rather than NIV
Persistent, uncontrolled vomiting
The therapy only works at the hours the patient actually wears it.
Define adherence explicitly
The accepted threshold is ≥ 4 hours per night on ≥ 70% of nights. Record it at every review — reported use and device-recorded use diverge routinely.
Mask fit outranks mode selection
The majority of early abandonment traces to interface leak and pressure sores, not to the pressure profile. Re-fit before you re-titrate.
Treat aerophagia and dryness early
Heated humidification, chin straps and modest EPAP reduction resolve most first-month complaints. Unaddressed, they end therapy within 30 days.
Front-load the first 90 days
Adherence patterns set within the first week and predict twelve-month use. Schedule a day-7 and a day-30 download review, not a six-month recall.
Use expiratory pressure relief before escalating
Flexible pressure relief resolves a meaningful share of expiratory intolerance and avoids an unnecessary move to bilevel therapy.
Objective download data over self-report
Residual AHI, leak and usage curves from the device should drive every therapy adjustment.
Specifying the device, not just the mode.
When ordering, specify the mode (CPAP, auto-CPAP, bilevel S, bilevel ST), the pressure range, humidification, the interface type and size, and whether device-download reporting is required for follow-up. Ambiguity at this step is the most common cause of a returned unit.
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This guide is intended for qualified healthcare professionals and is provided for educational reference only. It does not constitute medical advice and does not replace clinical judgement, local protocols, or the manufacturer’s instructions for use.