NIV and high-flow nasal oxygen in the ICU
- Concept
Two ways to support breathing without a tube in the windpipe: a tight mask that gives pressure, or warm humid oxygen at high flow through the nose.
What it is
Non-invasive ventilation (NIV) delivers positive pressure through a tight-fitting face or nasal mask or a helmet. Bilevel NIV gives a higher pressure on inspiration, which unloads the breathing muscles and increases tidal volume to clear CO2, and a lower pressure on expiration that works like PEEP; CPAP gives one constant pressure that recruits lung and reduces left ventricular afterload. The strongest evidence is in acute exacerbations of COPD with hypercapnic respiratory acidosis, where NIV reduces intubation and death, and in cardiogenic pulmonary oedema. It also helps obesity hypoventilation, selected immunocompromised patients and high-risk patients after extubation. High-flow nasal oxygen delivers heated, humidified gas with a set oxygen fraction at flows high enough to match the patient's inspiratory demand. It washes CO2 out of the upper airway dead space, gives a small, variable positive pressure, keeps secretions moist and is comfortable, so patients can talk, eat and cough. It is a first choice for many patients with acute hypoxaemic respiratory failure, for preoxygenation in some settings and after extubation in lower-risk patients. Both methods need a patient who is awake, can protect the airway and has no immediate need for intubation, and both carry the same danger: delaying a needed intubation. Response is judged early, within the first one to two hours, by breathing rate, work of breathing, oxygenation, pH and CO2, with a clear plan for what happens if the patient does not improve.
Key points
- Type 1 (hypoxaemic) failure: high-flow oxygen first; type 2 (hypercapnic) failure with acidosis: bilevel NIV.
- Target saturation is lower in patients at risk of CO2 retention (about 88–92%) than in others (about 92–96%).
- Signs of failure: rising breathing rate, accessory muscle use, paradoxical breathing, falling consciousness, worsening acidosis.
- Large tidal volumes from strong efforts on NIV in hypoxaemic failure may add lung injury (patient self-inflicted lung injury).
- Mask leaks, skin pressure injury on the nose, gastric distension, dry eyes and claustrophobia are the common problems of NIV.
Components
- Bilevel NIV: inspiratory pressure to unload muscles and clear CO2, expiratory pressure to keep alveoli open.
- CPAP: a single continuous pressure, mainly for oxygenation and cardiogenic pulmonary oedema.
- Interfaces: oronasal mask (most used in acute care), full face mask, nasal mask, helmet.
- High-flow nasal cannula: heated humidifier, oxygen blender with set fraction, wide-bore nasal prongs.
- Conventional oxygen devices (nasal cannula, simple mask, reservoir mask, Venturi mask) for milder cases.
How it works
- Inspiratory pressure takes over part of the work of breathing and raises tidal volume, so CO2 falls and pH rises.
- Expiratory pressure or CPAP recruits alveoli, offsets intrinsic PEEP in COPD and lowers left ventricular afterload.
- High flow meets peak inspiratory flow so little room air is entrained, keeping the delivered oxygen fraction close to the set one.
- Flushing the nasopharynx with fresh gas lowers dead space and work of breathing; humidity protects mucociliary clearance.
Choosing between high-flow oxygen and NIV
| Situation | Usual first choice | Why |
|---|---|---|
| COPD exacerbation with respiratory acidosis | Bilevel NIV | Unloads muscles and clears CO2; reduces intubation and death |
| Cardiogenic pulmonary oedema | CPAP or bilevel NIV | Recruits alveoli, lowers preload and afterload |
| Acute hypoxaemic failure (e.g. pneumonia) | High-flow nasal oxygen | Comfortable, matches demand, may avoid intubation |
| After extubation, high risk | NIV, or NIV with high flow | Prevents reintubation in hypercapnic or high-risk patients |
| Comatose, vomiting, shocked | Intubation | Airway cannot be protected |
In practice
- Start NIV for COPD with acidosis in a monitored area, with a blood gas before and after the first one to two hours.
- Decide and document the ceiling of treatment before starting: is NIV the top, or a bridge to intubation?
- Do not use NIV to delay intubation in a patient who is tiring, confused, vomiting or haemodynamically unstable.
- For hypoxaemic failure without hypercapnia, high-flow oxygen is usually preferred to face-mask NIV.
- After extubation, high-flow oxygen suits lower-risk patients; NIV, or NIV alternating with high flow, suits high-risk or hypercapnic patients.
Next step
- Repeat the blood gas within one to two hours of starting NIV, and watch breathing rate, SpO2 and consciousness continuously.
- On high flow, follow the ROX index and the breathing rate over the first hours; do not let a failing trial drift on.
- Call a senior early if pH or CO2 do not improve, the patient tires or becomes drowsy, or oxygen needs keep rising.
- Protect the skin under the mask, give mouth care, consider a gastric tube for distension and plan breaks for drinking and eating.
- Wean NIV by lengthening the time off it during the day once the cause improves; review the need for home NIV in chronic hypercapnia.
Test yourself
Think of the answer, then tap the question to see it.
In which two conditions does NIV have the strongest evidence?
COPD exacerbation with hypercapnic respiratory acidosis and acute cardiogenic pulmonary oedema.
What are four contraindications to NIV?
Cardiac or respiratory arrest, inability to protect the airway, vomiting or copious secretions, facial trauma, shock or undrained pneumothorax.
How does high-flow nasal oxygen reduce the work of breathing?
It washes CO2 out of the upper airway dead space, meets inspiratory flow demand, adds a small PEEP and humidifies secretions.
What is the ROX index used for?
It divides SpO2/FiO2 by breathing rate to predict, over the first hours, whether high-flow oxygen will succeed or the patient needs intubation.
Elsewhere in DurgiPedia
- Starting non-invasive ventilation
- Oxygen therapy: devices, flows and safety
- Hypoxemic and hypercapnic respiratory failure
- Reading an arterial blood gas step by step
- The work of breathing
Connections
In Pharmacology1
Educational content; it does not replace supervised training, your institution's protocols or the anaesthetist's or intensivist's decision. It contains no doses and no ventilator or infusion settings.
Sources: Noninvasive Ventilation (StatPearls (NCBI Bookshelf), 2026); Oxygen Therapy (StatPearls (NCBI Bookshelf), 2025).
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Representative model, for educational purposes only.