Why should High Flow Nasal Cannula (HFNC) be avoided in patients with asthma?

High Flow Nasal Cannula Should Be Avoided in Asthma Exacerbations

High flow nasal cannula (HFNC) should be avoided in patients with asthma exacerbations as it may delay appropriate escalation of care and potentially worsen outcomes by creating positive airway pressure that can increase air trapping in obstructive conditions. While HFNC has shown benefits in hypoxemic respiratory failure, its use in asthma requires careful consideration due to the different pathophysiology involved.

Pathophysiological Concerns with HFNC in Asthma

Mechanism of Concern

• HFNC delivers positive airway pressure (approximately 7 cm H2O at 50 L/min) 1
• This positive pressure can potentially:
  • Increase air trapping in already obstructed airways
  • Worsen dynamic hyperinflation
  • Increase work of breathing in asthmatic patients

Preferred Oxygen Delivery in Asthma

• For patients with acute severe asthma, nebulizers should be driven by piped oxygen or from an oxygen cylinder fitted with a high-flow regulator capable of delivering >6 L/min 1, 2
• After nebulizer therapy, patients should be returned to their usual oxygen mask or cannulae 1

Evidence-Based Recommendations for Asthma Exacerbations

First-Line Approach

1. Use conventional oxygen therapy with bronchodilators and systemic corticosteroids 3
2. For nebulized treatments in asthma:
   • Use oxygen as the driving gas at 6-8 L/min flow rate 1, 2
   • If oxygen cylinders cannot produce adequate flow, use an air-driven nebulizer with electrical compressor and provide supplemental oxygen via nasal cannulae at 2-6 L/min 1

Escalation of Care

• If patients do not respond to conventional therapy, consider escalation to non-invasive positive pressure ventilation (NIPPV) before invasive ventilation 3
• HFNC should not be used as an intermediate step in this escalation pathway for asthma

Limited Evidence for HFNC in Asthma

While some small studies have investigated HFNC in asthma, the evidence is limited:

• A small randomized study (n=40) showed some improvement in dyspnea and FEV1 with HFNC, but these were not clinically significant advantages over conventional oxygen therapy 4
• Another study suggested HFNC might be more efficient than conventional oxygen therapy in elevating PO₂ in severe bronchial asthma with respiratory failure, but showed no significant difference in PCO₂ reduction 5

Important Clinical Considerations

Monitoring Requirements

• If HFNC is used despite concerns, patients require continuous monitoring of:
  • Oxygen saturation
  • Respiratory rate
  • Work of breathing
  • Mental status
  • Signs of clinical deterioration

Risk of Delayed Intubation

• One of the primary concerns with HFNC in asthma is the potential delay in recognizing the need for intubation
• A delay in intubation in patients failing non-invasive respiratory support worsens outcomes 1
• Emergency intubation carries higher risks and should be avoided through appropriate early intervention 1

Conclusion

Based on the current evidence and understanding of asthma pathophysiology, conventional oxygen therapy with appropriate bronchodilator treatment remains the standard of care for asthma exacerbations. HFNC should generally be avoided in asthmatic patients due to concerns about increased air trapping and delayed recognition of clinical deterioration requiring more invasive ventilatory support.