Acute Severe Asthma
Life-threatening medical emergency requiring immediate bronchodilator therapy, systemic corticosteroids, and oxygen. Characterised by severe airflow obstruction with PEF 33-50% best/predicted.
Key Facts
Acute severe: PEF 33-50% best, RR ≥25, HR ≥110, inability to complete sentences in one breath Life-threatening: PEF <33%, SpO₂ <92%, silent chest, cyanosis, hypotension, exhaustion, altered consciousness Near-fatal: raised PaCO₂ and/or requiring mechanical ventilation Immediate treatment: high-flow O₂, nebulised salbutamol 5mg + ipratropium 500mcg, hydrocortisone 100mg IV or prednisolone 40-50mg PO Magnesium sulphate 1.2-2g IV over 20 min for life-threatening or poor response to initial therapy Aminophylline IV considered if life-threatening and poor response; requires cardiac monitoring ABG: request if SpO₂ <92% — normal/raised PaCO₂ is a sinister sign indicating exhaustion All patients should receive prednisolone 40-50mg daily for at least 5 days post-discharge
Overview
Key Facts
Acute severe asthma is a medical emergency characterised by progressive worsening of asthma symptoms with significant airflow limitation. It requires prompt recognition and aggressive treatment to prevent respiratory arrest.
Epidemiology
- ~77,000 emergency hospital admissions per year in the UK for asthma
- ~1,400 asthma deaths per year in the UK
- Higher mortality in females, ethnic minorities, and deprived areas
- Most deaths occur pre-hospital or within first 24 hours of admission
Aetiology
- Viral upper respiratory tract infection (most common trigger)
- Allergen exposure
- Poor medication adherence (especially ICS)
- Air pollution or irritant exposure
- Exercise
- Drugs: NSAIDs (in aspirin-sensitive asthma), beta-blockers
- Emotional stress
- Failure to recognise worsening symptoms
Pathophysiology
- Severe bronchospasm, mucosal oedema, and mucus plugging cause critical airflow obstruction
- Dynamic hyperinflation increases work of breathing
- V/Q mismatch initially causes hypoxaemia with hypocapnia (hyperventilation)
- Rising PaCO₂ indicates respiratory muscle fatigue and impending arrest
- Severe air trapping can cause pneumothorax or pneumomediastinum
Clinical Presentation
Moderate Exacerbation
- Increasing symptoms, PEF 50-75% best/predicted
- No features of acute severe asthma
Acute Severe Asthma
- PEF 33-50% best or predicted
- Respiratory rate ≥25/min
- Heart rate ≥110/min
- Inability to complete sentences in one breath
Life-Threatening Asthma
- PEF <33% best or predicted
- SpO₂ <92%
- PaO₂ <8 kPa
- Normal PaCO₂ (4.6-6.0 kPa) — indicates failing ventilation
- Silent chest
- Cyanosis
- Poor respiratory effort
- Hypotension
- Exhaustion, altered consciousness
- Arrhythmia
Near-Fatal Asthma
- Raised PaCO₂ and/or requiring mechanical ventilation with raised inflation pressures
Red Flags
- Silent chest (no wheeze despite severe obstruction)
- Altered consciousness or confusion
- Bradycardia or hypotension
- Rising PaCO₂ on serial ABGs
- Failure to improve with initial treatment
- Previous near-fatal episode or ICU admission
Differential Diagnosis
| Diagnosis | Key Features | Investigation |
|---|---|---|
| Acute exacerbation of COPD | Age >40, smoking history, chronic symptoms, less reversibility | Spirometry, CXR, ABG |
| Pneumothorax | Sudden onset, unilateral reduced breath sounds, pleuritic pain | CXR, CT if uncertain |
| Pulmonary embolism | Pleuritic pain, haemoptysis, VTE risk factors, tachycardia | CTPA, D-dimer |
| Anaphylaxis | Acute onset, urticaria, angioedema, hypotension, trigger exposure | Clinical diagnosis |
| Inhaled foreign body | Sudden onset, unilateral signs, history of choking | CXR, bronchoscopy |
| Vocal cord dysfunction | Inspiratory stridor, no response to bronchodilators | Laryngoscopy |
| Acute heart failure | Bilateral crackles, raised JVP, orthopnoea, frothy sputum | BNP, CXR, echo |
| Hyperventilation/panic | Anxiety, perioral/acral tingling, no wheeze | Clinical, ABG (respiratory alkalosis) |
Diagnosis / Investigation
Bedside
- PEF: compare to best or predicted — severity grading
- Pulse oximetry: continuous SpO₂ monitoring
- ABG: if SpO₂ <92% or life-threatening features — assess PaCO₂, PaO₂, pH, lactate
- ECG: arrhythmia, right heart strain
Bloods
- FBC: raised WCC (infection or steroid effect)
- CRP: infection assessment
- U&Es: hypokalaemia (salbutamol and steroid effect)
- Magnesium: hypomagnesaemia worsens bronchospasm
- Theophylline level: if already on theophylline before aminophylline
- Blood cultures: if sepsis suspected
Imaging
- Chest X-ray: hyperinflation, exclude pneumothorax, pneumomediastinum, consolidation
Special Tests
- Serial PEF: monitor response to treatment (15-30 minutely)
- Serial ABG: if initial PaCO₂ elevated or patient deteriorating
- Sputum culture: if infective exacerbation suspected
Management
Non-pharmacological
- Sit patient upright
- High-flow oxygen via non-rebreathe mask (target SpO₂ 94-98%)
- Continuous pulse oximetry and cardiac monitoring
- Senior clinician review immediately for life-threatening features
Pharmacological
Immediate management (BTS/SIGN):
- Oxygen: 15L/min via non-rebreathe mask (target SpO₂ 94-98%)
- Nebulised salbutamol 5mg (oxygen-driven) — repeat every 15-30 min, or continuous nebulisation for severe/life-threatening
- Nebulised ipratropium bromide 500mcg — add to salbutamol if poor initial response or life-threatening
- Corticosteroids: prednisolone 40-50mg PO or hydrocortisone 100mg IV if unable to swallow — continue for at least 5 days
- Magnesium sulphate 1.2-2g IV over 20 minutes: if life-threatening features or poor response to initial treatment
- IV aminophylline: loading dose 5mg/kg over 20 min (omit if already on theophylline) then infusion 0.5-0.7mg/kg/hr — requires cardiac monitoring
- IV salbutamol 250mcg slowly: if life-threatening and no response to nebulisers
Escalation if deteriorating:
- Involve ICU early
- Consider intubation and mechanical ventilation for near-fatal asthma
- Ketamine for bronchospasm in ventilated patients
- ECMO in refractory cases (specialist centres)
Surgical/Interventional
- Chest drain if pneumothorax develops
- Intubation and ventilation for respiratory arrest or near-fatal asthma
Referral Criteria
- ICU review: life-threatening or near-fatal features, deterioration despite treatment
- All patients admitted with acute severe asthma need respiratory follow-up within 2 working days of discharge
- Specialist referral if ≥2 admissions per year or any ICU admission
Prognosis
- In-hospital mortality for acute severe asthma: ~1-2%
- Near-fatal asthma: ICU mortality ~5-10%
- Previous ICU admission is the strongest predictor of future fatal asthma
- Most deaths occur pre-hospital — 75% are potentially preventable
- NRAD (2014): poor follow-up, lack of asthma action plans, and excessive SABA use are major factors in deaths
- Good recovery expected with prompt treatment and appropriate discharge planning
Other Relevant Information
Acute Asthma Severity Classification (BTS/SIGN)
| Severity | PEF | Features |
|---|---|---|
| Moderate | 50-75% | Increasing symptoms |
| Acute severe | 33-50% | RR ≥25, HR ≥110, cannot complete sentences |
| Life-threatening | <33% | SpO₂ <92%, silent chest, cyanosis, arrhythmia, hypotension, exhaustion |
| Near-fatal | — | Raised PaCO₂, requiring ventilation |
Discharge Criteria
| Criterion | Target |
|---|---|
| PEF | >75% best/predicted |
| Diurnal PEF variability | <25% |
| Inhaler technique | Checked and correct |
| Prednisolone course | ≥5 days supply |
| Asthma action plan | Written, reviewed |
| Follow-up | GP within 48 hours, specialist within 4 weeks |