What is the indication for High Flow Nasal Oxygen (HFNO) therapy in asthma?

Indications for High-Flow Nasal Oxygen (HFNO) in Asthma

HFNO is not specifically indicated as a first-line therapy for asthma exacerbations but may be considered as a rescue therapy when conventional oxygen therapy fails and before escalation to non-invasive or invasive ventilation.

Current Evidence on HFNO in Asthma

The evidence for HFNO use specifically in asthma is limited, as most guidelines and studies focus on its use in acute respiratory failure (ARF) of various etiologies:

• The American College of Physicians (ACP) guidelines do not specifically address asthma as a primary indication for HFNO therapy 1
• Most research has focused on HFNO in hypoxemic respiratory failure, post-extubation management, and acute-on-chronic respiratory failure 1
• Limited pediatric evidence suggests HFNO may be used in children with asthma exacerbations who don't respond to conventional therapy, as an alternative before escalation to non-invasive positive pressure ventilation (NIPPV) 2

Potential Role of HFNO in Asthma Management

HFNO may be considered in asthma patients with:

1. Acute hypoxemic respiratory failure despite conventional oxygen therapy
2. Increased work of breathing that hasn't responded to standard asthma treatments
3. Risk of intubation where a trial of HFNO might prevent escalation to mechanical ventilation

Monitoring Parameters When Using HFNO in Asthma

If HFNO is initiated for an asthma patient:

• Continuous oxygen saturation monitoring is essential with target SpO2 of 88-92% for most patients 3
• Regular assessment of respiratory status including respiratory rate, work of breathing, and patient comfort 3
• Intermittent measurement of pCO2 and pH to assess ventilatory status 3
• ECG monitoring if heart rate >120 bpm, dysrhythmia present, or known/suspected cardiomyopathy 3

Flow Rate and FiO2 Settings

• Flow rates may be adjusted between 20-60 L/min based on patient response and comfort 3
• FiO2 should be titrated to maintain target SpO2 (typically 88-92% for most patients) 3
• Heated humidification (34-37°C) is essential to prevent mucosal dryness and aid secretion clearance 3

Warning Signs of HFNO Failure in Asthma

HFNO should be considered failing and escalation of care should be considered if:

• Persistent or worsening hypoxemia despite optimized HFNO
• Increasing respiratory rate or work of breathing
• Development of hypercapnia or worsening hypercapnic respiratory failure
• Deteriorating mental status
• Hemodynamic instability 3

Advantages of HFNO Over Conventional Oxygen Therapy

HFNO may offer several physiological benefits in asthma:

• Washout of anatomic dead space
• Generation of low-level positive end-expiratory pressure (PEEP)
• Delivery of higher concentrations of heated humidified oxygen
• Improved patient comfort compared to mask oxygen or NIV 1
• Reduced need for escalation to NIV or invasive mechanical ventilation 4

Common Pitfalls When Using HFNO in Asthma

• Delayed escalation of care when HFNO is failing
• Setting inappropriate oxygen targets
• Inadequate humidification
• Overlooking hypercapnia development
• Using insufficient flow rates 3

Algorithm for HFNO Use in Asthma

1. First-line treatment: Standard asthma therapy (bronchodilators, systemic corticosteroids)
2. If hypoxemia persists: Trial conventional oxygen therapy
3. If respiratory distress continues despite conventional oxygen:
   • Consider HFNO with close monitoring
   • Initial settings: Flow 30-40 L/min, FiO2 to achieve SpO2 88-92%
4. Reassess within 1-2 hours:
   • If improving: Continue HFNO and gradually wean
   • If no improvement or worsening: Consider escalation to NIV or intubation

While HFNO may be beneficial in selected asthma patients with respiratory failure, it should not delay appropriate escalation to NIV or intubation when indicated, as delayed intubation is associated with worse outcomes in critically ill patients.