In an adult with severe acute asthma exacerbation who remains hypoxemic or hypercapnic despite optimized therapy (high‑flow oxygen, frequent short‑acting β2‑agonist nebulisations, systemic corticosteroids, and intravenous magnesium sulfate), when is non‑invasive ventilation (NIV) indicated, and what are the recommended settings and monitoring parameters?

Non-Invasive Ventilation in Acute Severe Asthma

NIV should not be used routinely in acute asthma exacerbations, but may be considered as a therapeutic trial in highly selected patients with respiratory acidosis (pH <7.35) who remain cooperative, can protect their airway, and are managed in an ICU/HDU setting with immediate intubation capability. 1

Primary Guideline Position

The British Thoracic Society explicitly states that NIV should not be used routinely in acute asthma (Grade C recommendation). 1 This reflects insufficient evidence to support standard use and the very low mortality rate with invasive mechanical ventilation in asthma, making the risk-benefit ratio favor early intubation in truly life-threatening cases. 1, 2

When NIV May Be Considered (Highly Selective)

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

• Respiratory acidosis with pH between 7.25 and 7.35 and rising PaCO₂ despite maximal medical therapy (high-flow oxygen, frequent β2-agonist nebulizations, systemic corticosteroids, IV magnesium sulfate). 1, 2

• Patient remains cooperative, able to follow commands, and can protect the airway (intact gag reflex, no vomiting). 1, 3

• Minimal respiratory secretions—copious secretions constitute an absolute contraindication. 1, 2

• ICU or HDU setting with immediate access to endotracheal intubation. 1, 2

• Pre-treatment intubation plan documented—either as a therapeutic trial with backup intubation or as ceiling of care for patients who are not intubation candidates. 1, 3

Absolute Contraindications to NIV in Asthma

Do not use NIV if any of the following are present:

• Inability to protect the airway, copious respiratory secretions, or active vomiting. 1, 2

• Life-threatening hypoxemia, severe hemodynamic instability, or depressed mental status/coma. 1, 2

• Recent facial or upper airway surgery, facial trauma/burns, or fixed upper airway obstruction. 1, 3

• Recent upper gastrointestinal surgery or bowel obstruction. 1, 3

NIV Settings and Interface

If NIV is initiated:

• Use a full-face mask initially in the acute setting, changing to nasal mask after 24 hours as the patient improves. 1

• Typical BiPAP settings: IPAP 10-15 cm H₂O, EPAP 4-6 cm H₂O, titrated to patient comfort and gas exchange. 4, 5

• BiPAP is preferred over CPAP for hypercapnic respiratory failure because it provides two pressure levels and ventilatory assistance during inspiration. 3, 4

Monitoring Parameters and Failure Criteria

Arterial blood gas must be measured 1-2 hours after NIV initiation and again at 4-6 hours if initial improvement is minimal. 2, 3

Continuous clinical assessment includes:

• Respiratory rate, heart rate, work of breathing, and patient comfort. 2

• Level of consciousness—deterioration mandates immediate intubation. 3

• Patient-ventilator synchrony—persistent asynchrony despite adjustments indicates failure. 3

Discontinue NIV and proceed to intubation if:

• No improvement in pH or PaCO₂ after 4-6 hours despite optimal settings. 2, 3

• Deteriorating level of consciousness, exhaustion, or severe distress. 2, 3

• Development of complications (pneumothorax, aspiration). 3

• Persistent pH <7.25 or worsening hemodynamic instability. 5

Evidence Base and Nuances

Five small RCTs of NIV in acute asthma have been published, but importantly, none included patients with hypercapnia and intubation rates were low. 1 Most showed faster improvement in FEV₁ and shorter ICU/hospital stay, but all have important design weaknesses. 1, 6

A retrospective cohort study by Meduri et al. of 17 patients with mean pH 7.25 reported NIV success in avoiding intubation in 15 patients, but the NIV failure rate was 33%. 1 A more recent case series demonstrated that NIV was safe and effective in 20 asthma patients with pH <7.25 or PaCO₂ >60 mmHg, avoiding intubation in all cases, including 8 who were obtunded. 5 However, a 2015 case-control study showed zero mortality in the NIV group but also zero escalation to invasive ventilation, suggesting highly selective patient inclusion. 7

The critical pitfall is delayed intubation—patients with acute asthma tend to deteriorate rapidly and require high inflation pressures. 2, 3 The greatest risk of NIV is postponing necessary intubation; do not continue beyond 1-4 hours without clear improvement. 3

Special Populations

For patients with chronic asthma and acute-on-chronic hypercapnia (resembling COPD phenotype), manage according to COPD protocols with the same NIV criteria. 2

For patients who are not candidates for intubation (ceiling of care decision), NIV represents the maximum intervention and may be used in a controlled ICU/HDU environment, accepting that it is palliative rather than curative. 1, 2

Practical Algorithm

1. Optimize medical therapy first: high-flow oxygen (target SaO₂ 96%), frequent β2-agonist nebulizations, systemic corticosteroids, IV magnesium sulfate. 2

2. If pH <7.35 and rising PaCO₂ persist, assess for NIV contraindications (secretions, inability to protect airway, hemodynamic instability). 1, 2

3. If no contraindications and patient in ICU/HDU, document intubation plan and initiate NIV trial with full-face mask. 1, 3

4. Reassess ABG at 1-2 hours—if no improvement, repeat at 4-6 hours. 2, 3

5. If no improvement by 4-6 hours or any clinical deterioration, proceed immediately to endotracheal intubation. 2, 3