How to Determine Flow Rate on High Flow Nasal Cannula (HFNC)
Start HFNC at 40-50 L/min for adults and titrate upward based on work of breathing, respiratory rate, and patient comfort, with most patients requiring flows between 40-60 L/min to achieve optimal physiological benefit. 1, 2
Initial Flow Rate Selection
Adults
- Begin at 40-50 L/min as the standard starting point 1, 2
- This flow rate matches or exceeds the peak tidal inspiratory flow (PTIF) of most patients with acute hypoxemic respiratory failure, which typically ranges between 30-40 L/min 2
- Higher initial flows (50-60 L/min) may provide greater physiological benefits including PEEP effect and dead space washout, particularly in hypoxemic respiratory failure 1
Pediatric Patients
- ≥1 L/kg/min for patients up to 10 kg 3
- ≥10 L/min for patients above 10 kg 3
- Flows below these thresholds are considered conventional oxygen therapy, not HFNC 3
Titration Algorithm
Step 1: Assess Baseline Parameters
- Measure respiratory rate, work of breathing (accessory muscle use), oxygen saturation, and patient comfort 1
- Calculate ROX index: (SpO2/FiO2)/respiratory rate 2
Step 2: Flow Adjustment Strategy
- Increase flow by 5-10 L/min increments if: 1
- Increased work of breathing persists
- Respiratory rate remains elevated
- ROX index is not improving
- Patient tolerates higher flows
- Maximum flow typically 60 L/min for adults 3, 1, 4
- Improvement in oxygenation continues up to flows that are approximately 2 times the patient's PTIF 2
- ROX index plateaus when HFNC flows reach 1.34-1.67 times the individual PTIF 2
Step 3: Monitor Response (5-10 minutes after each adjustment)
- Respiratory rate should decrease with effective therapy 1
- Work of breathing should improve (reduced accessory muscle use) 1
- SpO2 should stabilize at target range 1
- Patient comfort should improve 3, 1
Step 4: Decrease Flow if Needed
- Reduce by 5-10 L/min if patient experiences discomfort, nasal irritation, or intolerance 1
- Some patients cannot tolerate flows above 40 L/min despite potential physiological benefit 3
FiO2 Titration (Independent of Flow)
- Adjust FiO2 in 5-10% increments to maintain target SpO2 1
- Target SpO2 94-98% for patients without hypercapnic risk 1
- Target SpO2 88-92% for patients at risk of hypercapnic respiratory failure (e.g., COPD) 3, 1
Temperature Settings
- Set between 34-37°C based on patient preference 3, 1
- Higher temperatures (37°C) provide optimal humidification 1
- Lower temperatures (34°C) may improve comfort in some patients 3
Clinical Context-Specific Recommendations
Post-Extubation Support
- Start at 40-50 L/min 1
- A flow rate of 40 L/min with as-needed up-titration is reasonable and may reduce unnecessary escalation compared to routinely using 60 L/min 4
- Setting at 60 L/min versus 40 L/min did not significantly reduce reintubation rates in unselected patients 4
Hypoxemic Respiratory Failure
- Start at 50-60 L/min for more severe cases 1
- Higher flows provide greater PEEP effect and dead space washout 3, 1
Hypercapnic Respiratory Failure (COPD)
- Consider NIV first as primary therapy 3
- HFNC can be used during breaks from NIV 3
- If NIV is not tolerated, HFNC at 35-60 L/min may be attempted 3
- Maintain lower SpO2 targets (88-92%) 3, 1
Common Pitfalls to Avoid
- Inadequate initial flow rate (starting too low, <40 L/min) fails to provide sufficient PEEP effect or dead space washout 1
- Excessive FiO2 without optimizing flow first can lead to oxygen toxicity 1
- Delayed escalation to mechanical ventilation when HFNC is failing leads to worse outcomes 1
- Poor interface fit with air leaks reduces effectiveness—ensure proper nasal prong sizing 1
- Insufficient humidification causes airway dryness and patient discomfort—maintain temperature at 34-37°C 1
- Ignoring patient tolerance—some patients cannot tolerate flows >40-50 L/min despite theoretical benefit 3, 1