When should High Flow Nasal Cannula (HFNC) be started in patients with respiratory distress or failure?

When to Start High Flow Nasal Cannula (HFNC)

Primary Indications for Initiating HFNC

Start HFNC immediately as first-line therapy in adults with acute hypoxemic respiratory failure requiring oxygen support beyond conventional oxygen therapy. 1, 2 This represents the strongest evidence-based indication, with HFNC demonstrating reduced intubation rates (risk ratio 0.89) and superior patient comfort compared to conventional oxygen therapy. 1

Acute Hypoxemic Respiratory Failure

• Initiate HFNC when patients have PaO2/FiO2 ≤ 200 mmHg, as this threshold demonstrates improved survival compared to both standard oxygen and noninvasive ventilation. 2
• Start HFNC in patients with acute respiratory distress syndrome (ARDS), pneumonia, COVID-19, interstitial lung disease, or immunocompromised states presenting with hypoxemic respiratory failure. 1, 3
• HFNC should be preferred over conventional oxygen therapy when patients demonstrate increased work of breathing, tachypnea, or hypoxemia despite standard oxygen delivery. 1

Post-Extubation Respiratory Support

• Start HFNC immediately after extubation in nonsurgical patients at low or moderate risk of extubation failure (defined as absence of weak cough, poor neurological status, or severe cardiac/respiratory disease). 1
• HFNC reduces reintubation rates (risk ratio 0.62) and need for escalation to NIV (risk ratio 0.38) in this population. 1
• For patients at high risk of extubation failure, start NIV instead of HFNC unless contraindications to NIV exist (intolerance, facial trauma, claustrophobia). 1, 2

Post-Operative Respiratory Support

• Initiate HFNC prophylactically in high-risk post-operative patients, particularly following cardiac or thoracic surgery, to prevent pulmonary complications. 1, 2
• Start HFNC in obese patients (BMI >30 kg/m²) after major surgery who are at elevated risk of respiratory failure. 1

When NOT to Start HFNC as First-Line

Do not start HFNC as first-line therapy in patients with COPD and acute hypercapnic respiratory failure with acidosis (pH ≤7.35). 1 Instead, initiate a trial of NIV first, as NIV remains the evidence-based standard for hypercapnic acidotic respiratory failure. 1

• Avoid HFNC as initial therapy in patients with baseline PaCO2 >65 mmHg and pH <7.28. 4
• Do not use HFNC in patients who cannot protect their airway, have depressed mental status requiring immediate intubation, or are immediately deteriorating. 2
• For high-risk extubation patients, start NIV rather than HFNC to avoid the increased reintubation risk (approximately 4% higher with HFNC). 1

Initial HFNC Settings

When initiating HFNC, use the following parameters: 2, 5

• Flow rate: 50-60 L/min for adults (titrate to patient tolerance)
• Temperature: 37°C with 100% relative humidity
• FiO2: Titrate to achieve SpO2 92-97% (or 88-92% in patients at risk of hypercapnia)

Critical Monitoring After Initiation

Reassess patients 30-60 minutes after initiating HFNC to evaluate response. 2 This early assessment is crucial to identify treatment failure and avoid delayed intubation, which increases mortality. 2

Predictors of HFNC Failure Requiring Escalation

Monitor for these high-risk features within the first 1-2 hours: 2

• Failure to improve respiratory rate or persistent tachypnea (>30 breaths/min)
• Rapid shallow breathing index (RSBI) >105 breaths/min/L
• Tidal volumes persistently >9.5 mL/kg predicted body weight
• Thoracoabdominal asynchrony or paradoxical breathing
• Worsening hypoxemia despite FiO2 titration
• Altered mental status or inability to cooperate
• Hemodynamic instability or tachycardia

If no substantial improvement occurs within 1-2 hours, escalate to NIV or intubation immediately rather than prolonging inadequate support. 2 Delayed intubation in failing patients significantly worsens hospital mortality. 2

Adjunctive Use of HFNC

• Use HFNC during breaks from NIV rather than conventional oxygen therapy to maintain adequate oxygenation and respiratory support. 1, 2
• In COPD patients on NIV, HFNC during breaks significantly reduces diaphragm activation and improves comfort without affecting gas exchange. 1

Common Pitfalls to Avoid

• Do not prolong HFNC therapy in patients showing signs of failure. 2 The most critical error is delaying intubation when HFNC is inadequate, as this increases mortality risk.
• Do not use HFNC in hypercapnic acidotic respiratory failure without first attempting NIV. 1
• Do not assume HFNC is appropriate for all post-extubation patients—high-risk patients require NIV. 1
• Avoid starting HFNC at low flow rates (<40 L/min in adults), as this negates the physiological benefits of dead space washout and PEEP effect. 3, 6