Support without a tube
"Non-invasive ventilation is first choice in the acidotic COPD exacerbation and in cardiogenic pulmonary oedema, and is reasonable in selected others. High-flow nasal oxygen is a comfortable alternative for hypoxaemic failure. The one unbreakable rule: when NIV is failing, do not delay a needed intubation."
Summarised from Irwin & Rippe's Intensive Care Medicine; with Marino's The ICU Book, Oh's Intensive Care Manual and ERS/ATS.| Term | What it is |
|---|---|
| CPAP | One constant pressure throughout the cycle; no inspiratory boost (not true ventilation) |
| BiPAP / NIV (NPPV) | Two levels — IPAP (inspiratory) + EPAP (expiratory); the IPAP−EPAP difference actively assists breaths |
| HFNC / HFNO | Heated humidified oxygen up to ~60 L/min via nasal cannula; stable FiO₂, small PEEP effect, dead-space washout |
Physiological effects
| Effect | CPAP | BiPAP | HFNC |
|---|---|---|---|
| Improves oxygenation (recruit + FiO₂) | ✔✔ | ✔✔ | ✔ (+ washout) |
| Augments tidal volume / clears CO₂ | — | ✔✔ (pressure support) | Minimal |
| Offloads work of breathing | ✔ (counters auto-PEEP) | ✔✔ | ✔ |
| Reduces LV afterload/preload | ✔✔ (oedema) | ✔ | — |
| Comfort / tolerance | Moderate | Moderate | High |
Where each earns its place
| Indication | Modality | Strength |
|---|---|---|
| COPD exacerbation with respiratory acidosis (pH <7.35) | BiPAP | Strong — first line |
| Cardiogenic pulmonary oedema | CPAP or BiPAP | Strong |
| Immunocompromised with early hypoxaemic failure | NIV or HFNC | Reasonable (avoid intubation risks) |
| Facilitating extubation in COPD; post-op respiratory failure | NIV | Suggested |
| Obesity hypoventilation, chest-wall/neuromuscular disease, palliation | NIV (HFNC for comfort) | Suggested |
| De novo hypoxaemic failure / early ARDS / pneumonia | HFNC (± cautious NIV) | Cautious — watch closely, don't delay intubation |
When NIV is unsafe
| Absolute / strong | Relative |
|---|---|
| Cardiac/respiratory arrest; need for immediate intubation | Agitation / poor cooperation |
| Unprotected airway, coma, high aspiration risk, copious secretions | Recent upper-GI/airway surgery |
| Facial trauma/burns/surgery, fixed obstruction | Haemodynamic instability, arrhythmia |
| Undrained pneumothorax | Vomiting / bowel obstruction |
Starting and titrating
- BiPAP: start IPAP ~10–12 / EPAP ~4–5 cmH₂O; up-titrate IPAP for tidal volume/CO₂, EPAP for oxygenation/auto-PEEP. Backup rate as needed.
- CPAP: ~8–10 cmH₂O for oedema; titrate FiO₂ to SpO₂ target.
- HFNC: flow 40–60 L/min, FiO₂ to target; titrate flow to work of breathing.
- Interface: oro-nasal mask usual for NIV; protect skin; minimise leak; involve the patient.
Reassess within 1–2 hours with a repeat ABG (for NIV) and clinical response. Improving pH/CO₂/effort = continue; not improving = escalate.
Catch failure early
ROX = (SpO₂ / FiO₂) ÷ respiratory rate. A higher value means the patient is doing well. A low or falling ROX (roughly <4.88 at 2, 6 and 12 h) predicts HFNC failure and the need to escalate/intubate.
| Warning of failure | Action |
|---|---|
| No improvement in pH/PaCO₂ (NIV) at 1–2 h | Intubate — do not persist |
| Rising RR, falling ROX, worsening SpO₂ | Escalate support / intubate |
| Falling consciousness, agitation, intolerance | Reconsider airway; intubate if unsafe |
| Haemodynamic deterioration | Stop NIV; secure airway |
Using NIV/HFNC around extubation
- Prevent re-intubation: in high-risk patients (hypercapnia, obesity, cardiac, COPD), prophylactic NIV or HFNO immediately after extubation reduces re-intubation.
- Extubate-to-NIV in chronically hypercapnic COPD patients who pass a breathing trial.
- Not a rescue for established post-extubation failure — reactive NIV once distress is established can delay a needed re-intubation.
- Full weaning/SBT detail on the Mechanical Ventilation page.
Common mistakes
The commonest fatal error. Reassess at 1–2 h and intubate if not improving.
Coma, vomiting or copious secretions make NIV dangerous — secure the airway.
Prophylactic use helps; rescue use of NIV once failing can delay re-intubation and worsen outcomes.
Leaks and pressure injury cause intolerance and failure — fit, pad and involve the patient.
NIV/HFNC buys time — diuretics for oedema, bronchodilators/steroids/antibiotics for COPD still drive the outcome.
Exam pearls
Q: Strongest indications for NIV?
COPD exacerbation with respiratory acidosis (first line) and cardiogenic pulmonary oedema.
Q: CPAP vs BiPAP?
CPAP = one pressure (oxygenation, oedema); BiPAP adds pressure support (IPAP−EPAP) to augment ventilation and clear CO₂ (hypercapnic failure).
Q: What is the ROX index?
(SpO₂/FiO₂)/RR for HFNC; a low/falling value (≈<4.88 at 2–12 h) predicts failure.
Q: NIV in de novo hypoxaemic failure/ARDS?
Not clearly recommended for or against; HFNC is a reasonable alternative — monitor closely and intubate promptly if failing.
Q: Post-extubation use?
Prophylactic NIV/HFNO in high-risk patients reduces re-intubation; reactive rescue NIV can be harmful.
All the comparisons in one place
The distinctions that decide which device and answers — gathered at the end.
CPAP vs BiPAP vs HFNC
| Feature | CPAP | BiPAP | HFNC |
|---|---|---|---|
| Pressure | One level | Two (IPAP+EPAP) | Flow-based, small PEEP |
| Helps mainly | Oxygenation | Ventilation + oxygenation | Oxygenation + comfort |
| Best for | Cardiogenic oedema, OSA | Hypercapnic COPD | Hypoxaemic failure, post-extubation |
| Interface | Mask | Mask | Nasal cannula |
NIV vs HFNC
| Feature | NIV | HFNC |
|---|---|---|
| Clears CO₂ | Yes (pressure support) | Minimal |
| Comfort/tolerance | Lower (mask) | Higher |
| Best fit | Hypercapnic COPD, oedema | Hypoxaemic failure, breaks from NIV |
| Failure monitoring | ABG at 1–2 h | ROX index |
NIV vs Invasive ventilation
| Feature | NIV | Invasive |
|---|---|---|
| Airway protection | None | Yes |
| Best candidate | Awake, cooperative, reversible cause | Coma, shock, secretions, NIV failure |
| Main risk | Delay to intubation | VAP, sedation, weaning |
References
- Irwin RS, Lilly CM, Mayo PH, Rippe JM (eds). Irwin & Rippe's Intensive Care Medicine. 9th ed. Wolters Kluwer; 2023.
- Marino PL. Marino's The ICU Book. 5th ed. Wolters Kluwer; 2025.
- Bersten AD, Handy JM (eds). Oh's Intensive Care Manual. Elsevier; 2026.
- Rochwerg B, Brochard L, Elliott MW, et al. Official ERS/ATS clinical practice guidelines: non-invasive ventilation for acute respiratory failure. Eur Respir J. 2017;50:1602426.
- Frat JP, Thille AW, Mercat A, et al. (FLORALI). High-flow oxygen through nasal cannula in acute hypoxemic respiratory failure. N Engl J Med. 2015;372:2185–2196.
- Roca O, Caralt B, Messika J, et al. The ROX index to predict outcome of high-flow nasal oxygen. Am J Respir Crit Care Med. 2019;199:1368–1376.
- Hernández G, Vaquero C, Colinas L, et al. Post-extubation high-flow nasal cannula vs noninvasive ventilation on reintubation. JAMA. 2016;316:1565–1574.