💨 What is HFNC?
High-Flow Nasal Cannula (HFNC) — also called high-flow oxygen therapy or Optiflow — delivers heated, humidified oxygen at flow rates of up to 60 L/min via wide-bore nasal prongs. This is far above conventional oxygen therapy (typically 1–15 L/min).
Advantages of HFNC over conventional oxygen
- Accurate FiO₂: Delivers consistent FiO₂ as flow exceeds patient's peak inspiratory flow (~30–60 L/min); no dilution with room air
- PEEP effect: Generates 3–5 cmH₂O of positive end-expiratory pressure, improving alveolar recruitment
- CO₂ washout: High flow flushes anatomical dead space in nasopharynx (150 mL), reducing CO₂ rebreathing and work of breathing
- Mucociliary clearance: Heated, humidified gas (37°C, 100% RH) prevents secretion drying and cilia damage
- Patient comfort: More comfortable than NIV mask; allows speech and oral intake
Indications for HFNC
- Hypoxaemic respiratory failure (Type 1): SpO₂ <92% despite ≥15 L/min via non-rebreather mask
- COVID-19 pneumonia (widely used during pandemic)
- Community-acquired pneumonia
- Post-extubation respiratory support (reduces reintubation risk)
- Cardiogenic pulmonary oedema (adjunct to NIV)
- Do-Not-Intubate (DNI) patients requiring maximal non-invasive support
- Pre-oxygenation before intubation in high-risk patients
HFNC for hypercapnic respiratory failure (Type 2): Generally NOT first-line. NIV/BiPAP is preferred for COPD exacerbation with CO₂ retention. HFNC has a limited role in mild hypercapnia only.
🔬 Contraindications & Cautions
Relative Contraindications
- Severe hypercapnia (PaCO₂ >55 mmHg with acidosis) — use NIV
- Apnoea / respiratory arrest
- Severe facial trauma / recent nasal surgery
- Inability to clear secretions / impaired consciousness (GCS <8)
- Haemodynamic instability requiring immediate intubation
Cautions
- Avoid delaying intubation in rapidly deteriorating patients
- Facial pain, epistaxis from high-flow nasal delivery
- Aerosol-generating procedure (COVID-19) — side room, PPE
- Gastric distension if mouth breathing at high flows (rare)
⚙️ HFNC Setup and Initial Settings
Equipment Checklist
- HFNC device (e.g., Fisher & Paykel Optiflow, Airvo 2, Vapotherm)
- Heated humidifier circuit
- Appropriately-sized wide-bore nasal cannula (should fill ~50% of nare)
- Oxygen source with sufficient flow capacity (piped O₂ with high-flow regulator)
- SpO₂ probe and continuous monitoring
- ABG capability for assessment
Initial Settings
| Parameter | Starting Setting | Rationale |
|---|
| Flow rate | 30–40 L/min (titrate up to 60) | Higher flow = greater CO₂ washout + PEEP effect |
| FiO₂ | Start 100%, wean to maintain SpO₂ 92–96% | Titrate to minimum FiO₂ to achieve target SpO₂ |
| Temperature | 34–37°C | 37°C for comfort; 34°C if patient feels too warm |
| Humidification | Fully humidified (Airvo circuit auto-humidifies) | Mucociliary clearance, comfort |
Target SpO₂: 92–96% for most patients. In COPD (known CO₂ retainer): 88–92%. Avoid hyperoxia (SpO₂ >98% on high FiO₂) — associated with harm in critically ill.
Cannula Sizing
The nasal cannula should fill approximately 50% of the nare. Too small = gas leaks = reduced delivered FiO₂ and PEEP effect. Too large = discomfort, mucosal pressure injury. Ensure prongs face downward and sit comfortably in nares — do NOT insert deeply.
Patient Positioning
- Semi-upright (30–45° head of bed elevation) — reduces aspiration risk, improves diaphragmatic excursion
- Prone positioning with HFNC ("awake proning") — increasingly used in COVID-19 and ARDS to improve V/Q matching
Awake Proning + HFNC: Patients proned for 1–3 hours 3–4 times/day with HFNC showed improved oxygenation and reduced intubation rates in COVID-19 trials. Requires patient cooperation and adequate nursing observation.
📊 Monitoring on HFNC
Frequency of Assessment
| Parameter | Frequency | Action if Abnormal |
|---|
| SpO₂ | Continuous | Adjust FiO₂; if <88% on 100% FiO₂ → escalate |
| Respiratory rate | Every 1 hour | RR >30–35 = consider escalation |
| Work of breathing | Every 1 hour | Accessory muscle use, paradoxical movement → escalate |
| GCS / consciousness | Every 2 hours | Declining GCS → intubate |
| ABG | At 1–2 hours, then 4–6 hourly | Guide FiO₂ and flow; rising PaCO₂ = switch to NIV |
| Heart rate, BP | Every 1–4 hours | Haemodynamic instability → intubation discussion |
⭐ The ROX Index — Key Assessment Tool
ROX Index = (SpO₂/FiO₂) ÷ Respiratory Rate
Developed to predict HFNC failure and need for intubation in pneumonia and hypoxaemic failure.
| ROX Index at 2, 6, or 12 hours | Interpretation | Action |
|---|
| >4.88 | HFNC likely to succeed | Continue HFNC, reassess |
| 3.85–4.88 | Indeterminate — high-risk zone | Close monitoring, consider ICU |
| <3.85 | HFNC failure likely | Prepare for NIV or intubation |
A falling ROX index over time (even if still above 4.88) is also a warning sign of deterioration.
Example calculation: SpO₂ 92%, FiO₂ 0.60 (60%), RR 28 breaths/min
ROX = (92/60) ÷ 28 = 1.53 ÷ 28 = 0.055 — this is very low. Wait… correct formula: SpO₂ as %, FiO₂ as decimal:
ROX = (92/0.60) ÷ 28 = 153.3 ÷ 28 = 5.48 — success likely. (Always use FiO₂ as a decimal in denominator.)
Signs of HFNC Failure Requiring Urgent Escalation
- SpO₂ <88% despite FiO₂ 100% and flow 60 L/min
- Respiratory rate >35 breaths/min with increasing work of breathing
- Rising PaCO₂ on serial ABGs with respiratory acidosis
- Haemodynamic instability (BP <90 systolic, HR >130)
- Declining level of consciousness (GCS falling)
- Patient exhaustion / inability to protect airway
- ROX index <3.85 at 2, 6, or 12 hours
HFNC must not delay intubation. If there are signs of failure, escalate promptly. Delayed intubation in a fatigued patient is more dangerous than timely elective intubation.
📉 Weaning HFNC
Criteria to Consider Weaning
- SpO₂ ≥94% on FiO₂ ≤40% (0.40) at current flow
- Respiratory rate <25 breaths/min without distress
- No accessory muscle use
- ABG: PaO₂/FiO₂ ratio improving
- Underlying cause of respiratory failure improving
Weaning Strategy
- Wean FiO₂ first: reduce by 5–10% every 1–4 hours to minimum FiO₂ while maintaining SpO₂ target
- Once FiO₂ ≤40%, begin reducing flow rate: decrease by 5 L/min every 2–4 hours
- When at 20–25 L/min flow and FiO₂ ≤40%, trial conventional nasal cannula or Venturi mask
- If SpO₂ maintained for 2–4 hours on conventional O₂ → HFNC discontinued
HFNC holiday (brief trials off therapy): Short periods off HFNC (e.g., during meals) can be used to assess readiness for weaning. Patient should not deteriorate during these trials.
Post-Extubation HFNC
- HFNC started immediately post-extubation in high-risk patients significantly reduces reintubation risk
- High-risk features: BMI >30, COPD, prolonged ventilation, ≥2 comorbidities, heart failure, weak cough
- Prophylactic HFNC 24 hours post-extubation is now standard in many GCC ICUs
HFNC vs NIV — When to Choose What
| Scenario | Preferred Therapy |
|---|
| Hypoxaemic failure (Type 1), alert patient | HFNC first-line |
| COPD exacerbation with CO₂ retention | NIV (BiPAP) first-line |
| Cardiogenic pulmonary oedema | NIV (CPAP) first-line, HFNC adjunct |
| Post-extubation prophylaxis | HFNC preferred over NIV |
| Immunocompromised with respiratory failure | HFNC (reduces VAP risk) |
| Obstructive sleep apnoea exacerbation | NIV preferred |