High-Flow Nasal Cannula (HFNC) 

1. Introduction

High-Flow Nasal Cannula (HFNC) is an advanced oxygen delivery system that delivers heated, humidified oxygen–air mixture at high flow rates (up to 60–70 L/min) with a precise and adjustable FiO₂ (21–100%).

➡️ In conventional nasal prongs → YES, flow determines FiO₂.

➡️ In HFNC → NO, FiO₂ is independent of flow (when flow ≥ patient inspiratory demand).

2. Components of HFNC System

HFNC consists of:

1. Air–oxygen blender → Delivers precise FiO₂ (21–100%)
2. Flow generator → Provides high flow up to 60–70 L/min
3. Heated humidifier → Maintains gas at 31–37°C
4. Heated circuit tubing → Prevents condensation
5. Wide-bore nasal cannula → Comfortable high-flow interface

Why heating and humidification are crucial?

• Prevents mucosal injury
• Maintains mucociliary function
• Improves secretion clearance
• Enhances tolerance

Exam point: Dry high-flow oxygen causes epithelial damage and ciliary dysfunction.

3. Physiological Effects of HFNC

HFNC is not “just high oxygen.” It has multiple physiological mechanisms:

3.1 High FiO₂ Delivery

Unlike conventional nasal prongs:

• Delivers stable and predictable FiO₂
• Minimizes room air entrainment

Useful in:

• Severe hypoxemia
• ARDS
• Pneumonia

3.2 Dead Space Washout

HFNC flushes nasopharyngeal dead space:

• Reduces CO₂ rebreathing
• Improves ventilatory efficiency
• Reduces work of breathing

Important in:

• Mild hypercapnia
• Post-extubation support

3.3 Generation of Positive Airway Pressure

HFNC generates low-level PEEP (3–5 cm H₂O):

• Increases end-expiratory lung volume
• Prevents alveolar collapse
• Improves oxygenation

PEEP effect increases with:

• Higher flow
• Closed mouth breathing

Exam point: HFNC ≠ NIV, but provides mild CPAP-like effect.

3.4 Reduced Work of Breathing

Mechanisms:

• Reduced inspiratory resistance
• Better lung compliance
• Improved oxygenation

Clinical markers:

• ↓ Respiratory rate
• ↓ Accessory muscle use
• ↓ Dyspnea score

3.5 Improved Secretion Clearance

Humidification:

• Maintains mucus viscosity
• Enhances ciliary activity
• Prevents crust formation

Very useful in:

• Post-extubation
• Bronchiectasis
• Pneumonia

4. Indications of HFNC

4.1 Acute Hypoxemic Respiratory Failure (AHRF)

Strongest Evidence Area

Landmark trial:

• FLORALI trial

Key findings:

• Lower intubation rates (especially PaO₂/FiO₂ < 200)
• Lower 90-day mortality vs NIV

Preferred in:

• ARDS (mild–moderate)
• Severe pneumonia
• Viral pneumonitis (including COVID era practice)

4.2 Post-Extubation Support

• Reduces reintubation in low–moderate risk patients
• Alternative to NIV in selected cases

High-risk extubation:

• HFNC + NIV may be superior

4.3 Preoxygenation Before Intubation

Advantages:

• Provides apneic oxygenation
• Delays desaturation
• Improves safe apnea time

Especially useful in:

• Obese patients
• Hypoxemic patients
• ARDS

4.4 Postoperative Hypoxemia

Useful after:

• Major abdominal surgery
• Thoracic surgery
• Cardiac surgery

Improves comfort compared to NIV.

4.5 Palliative / DNI Patients

HFNC:

• More comfortable than NIV
• Allows talking and eating
• Reduces dyspnea

5. Contraindications

Absolute:

• Immediate need for intubation
• Respiratory arrest
• Severe hemodynamic instability
• Altered sensorium with aspiration risk

Relative:

• Severe hypercapnic respiratory failure (unless mild and monitored)
• Facial trauma

 HFNC is NOT first-line for severe COPD exacerbation with acidosis → NIV preferred.

6. HFNC vs NIV – Comparison

7. Starting Settings in ICU

Initial Settings:

• Flow: 40–60 L/min
• FiO₂: 1.0 initially in severe hypoxemia
• Temperature: 34–37°C

Then titrate:

• Target SpO₂: 92–96% (88–92% in COPD)

Higher flow = better PEEP + better washout.

8. Monitoring HFNC Therapy

Monitor:

• Respiratory rate
• Work of breathing
• SpO₂
• ABG
• Hemodynamics

8.1 ROX Index 

ROX = (SpO₂ / FiO₂) ÷ Respiratory Rate

Interpretation:

• 4.88 at 12 hours → Success likely
• <3.85 → High failure risk

Derived from:

• ROX index study

Weaning from High-Flow Nasal Cannula

1. When to Consider Weaning?

Clinical Stability Criteria

Patient should have:

• RR < 22–24/min
• Minimal accessory muscle use
• Hemodynamic stability
• Improving primary pathology
• Stable mental status
• SpO₂ target achieved

Oxygenation Criteria

• FiO₂ ≤ 0.4–0.5
• SpO₂ ≥ 92–96% (88–92% in COPD)
• ROX index > 4.88

(ROX = [SpO₂/FiO₂] ÷ RR)

2. Core Principle: What to Reduce First?

 Always reduce FiO₂ first, then flow.

Why?

• FiO₂ reduction confirms oxygenation recovery.
• Flow provides:
• Dead space washout
• Low-level PEEP
• Work-of-breathing reduction

If you reduce flow first → risk alveolar derecruitment.

3. Stepwise Weaning Protocol

Step 1 – Reduce FiO₂

If FiO₂ > 0.4:

• Reduce by 5–10% every 30–60 min
• Maintain target SpO₂

Target before moving to flow reduction:

• FiO₂ ≤ 0.4

Step 2 – Reduce Flow

Once FiO₂ ≤ 0.4:

• Reduce flow in steps of 5–10 L/min
• Reassess after each reduction (30–60 min)

Typical sequence:

• 60 → 50 → 40 → 35 → 30 L/min

Most patients tolerate 25–30 L/min before switching.

Step 3 – Transition to Conventional Oxygen

When:

• Flow ≤ 25–30 L/min
• FiO₂ ≤ 0.4
• RR stable
• Minimal distress

Switch to:

• Nasal prongs 2–5 L/min
• Venturi mask if controlled FiO₂ needed

Monitor closely for 1–2 hours after switch.