When is non‑invasive ventilation indicated for an adult with acute severe asthma exacerbation who is alert, cooperative, has a respiratory rate >25/min, rising PaCO₂ or pH <7.35 despite optimal medical therapy, and what are the recommended BiPAP settings and failure criteria?

Non-Invasive Ventilation for Acute Severe Asthma

NIV is NOT routinely recommended for acute severe asthma, but may be cautiously attempted in highly selected patients with respiratory acidosis (pH <7.35, rising PaCO₂) who remain alert and cooperative, only in an ICU/HDU setting with immediate intubation capability. 1

Critical Guideline Position

The British Thoracic Society explicitly states NIV should not be used routinely in acute asthma (Grade C recommendation) due to insufficient evidence. 1 This reflects the fundamental concern that asthma patients tend to deteriorate rapidly, require high inflation pressures, and need high inspired oxygen concentrations—making NIV trials potentially dangerous. 1

When NIV May Be Considered (Highly Selective)

NIV may be attempted only when ALL of the following criteria are met:

• Patient remains alert and cooperative with ability to protect airway 1
• pH 7.25-7.35 with rising PaCO₂ despite optimal medical therapy 2
• ICU/HDU setting with immediate intubation capability available 1
• No copious respiratory secretions 2
• Pre-treatment intubation plan documented before starting NIV 1

Special Consideration for Altered Mental Status

While traditional teaching considers altered consciousness a contraindication, emerging evidence suggests selected patients with GCS ≤10 may tolerate NIV successfully. 3 One series reported 25 patients with GCS ≤10 managed with NIV, with only one requiring subsequent intubation, achieving average PaCO₂ reduction of 5.9 kPa over 2 hours. 3 However, this requires extremely close monitoring and should only be attempted by experienced teams.

Initial BiPAP Settings

Start conservatively and titrate based on patient response:

• IPAP: 10-12 cmH₂O initially 4
• EPAP: 4-5 cmH₂O 4
• Titrate IPAP upward by 2-3 cmH₂O increments based on patient comfort, respiratory rate, and work of breathing 4
• Target SpO₂: 96% (higher than COPD due to asthma pathophysiology) 1
• FiO₂: Adjust to maintain target saturation 4

The rationale for starting low is to assess tolerance and avoid excessive pressures that may worsen air trapping or cause barotrauma in the setting of severe bronchospasm.

Monitoring Requirements

Continuous clinical assessment is mandatory:

• Arterial blood gas at baseline, 1-2 hours, and 4-6 hours after NIV initiation 4, 1
• Continuous monitoring: respiratory rate, heart rate, work of breathing, patient comfort, ventilator synchrony 4
• Neurological status: GCS, agitation, confusion 1
• Hemodynamic stability: blood pressure, perfusion 1

Failure Criteria—When to Intubate

Proceed immediately to intubation if ANY of the following occur:

• No improvement in PaCO₂ and pH after 4-6 hours despite optimal ventilator settings 1
• Worsening or persistent hypercapnia 1
• Apnea or respiratory arrest 1
• Depressed mental status, coma, or exhaustion 1
• Severe distress or inability to tolerate NIV 1
• Hemodynamic instability 1
• Inability to protect airway or copious secretions 1

Absolute Contraindications to NIV in Asthma

Do NOT attempt NIV if any of the following are present:

• Impaired consciousness (unless highly selected cases with experienced team) 1
• Inability to protect airway 1
• Copious respiratory secretions 2, 1
• Life-threatening hypoxemia 1
• Hemodynamic instability 1
• Recent facial/upper airway surgery or trauma 1
• Vomiting or recent upper GI surgery 1
• Bowel obstruction 1
• Patient refusal or inability to cooperate 1

Critical Pitfalls to Avoid

The most dangerous error is delaying intubation when NIV is failing. 1 Asthma patients can deteriorate precipitously, and the window for safe intubation may close rapidly. NIV failure rates in asthma are approximately 33%, with mortality of 15.4% in NIV failures versus 2.3% in NIV successes. 1

Never use NIV as a substitute for intubation in truly life-threatening asthma. 1 Intubation and invasive mechanical ventilation should be the primary ventilatory support method for life-threatening asthma with respiratory failure. 1

Document the ceiling of care decision before starting NIV. 1 Clarify whether NIV is a bridge to intubation if it fails, or represents the maximum intervention (ceiling of care). 1

Evidence Quality and Limitations

The evidence supporting NIV in asthma is weak compared to COPD. 5, 6 Small studies suggest potential benefits in respiratory rate and airflow, 5, 6 and case series report successful use even in severe respiratory acidosis (pH as low as 6.89-6.95). 7, 8 However, these represent insufficient evidence for routine recommendation, and all emphasize the need for close monitoring in critical care settings. 5, 6, 7, 8