๐ŸŒฌ๏ธ Chapter 36: Acute Severe Asthma

Pathophysiology (air trapping, auto-PEEP) ยท Severity classification (mild, acute severe, life-threatening) ยท Pharmacotherapy (ฮฒ2 agonists, ipratropium, steroids, magnesium, ketamine) ยท Ventilatory strategy (low rate, long expiration, permissive hypercapnia) ยท Intubation indications ยท Inhaled anaesthetics ยท Heliox ยท Manual chest compression for refractory air trapping

๐Ÿ” Core Concepts: Acute Severe Asthma

๐Ÿ“Œ Pathophysiology
Chronic inflammation โ†’ oedema, mucus plugging, bronchospasm. Airway obstruction during expiration โ†’ air trapping โ†’ dynamic hyperinflation โ†’ auto-PEEP โ†’ โ†“ venous return, hypotension, barotrauma.
๐Ÿ“Š Severity Grades
Mild: SpO2 >94%, normal speech. Acute severe: SpO2 90-94%, sentences to words, use of accessory muscles. Life-threatening: SpO2 <90%, silent chest, altered sensorium, pulsus paradoxus, hypercapnia.
๐Ÿ’Š Pharmacotherapy
Continuous nebulised salbutamol + ipratropium, IV steroids (hydrocortisone/methylprednisolone). Magnesium (25-50 mg/kg IV). Ketamine (1-2 mg/kg then infusion) for refractory bronchospasm.
๐Ÿซ Ventilatory Strategy (if intubated)
Low rate (10-12/min), low tidal volume (5-7 mL/kg), prolonged expiratory time (I:E 1:3-1:4), permissive hypercapnia (pH โ‰ฅ7.2), set external PEEP at ~2/3 auto-PEEP. Avoid hyperinflation and hypotension.
โš ๏ธ Intubation Indications
Cardiac/respiratory arrest, severe hypoxia, altered sensorium, no improvement despite maximum therapy, rising PaCO2 with acidosis, exhaustion.
๐Ÿ†˜ Rescue Therapies
Manual chest compression during expiration for severe air trapping. Heliox (80/20) if FiO2 โ‰ค0.3. Inhaled anaesthetics (sevoflurane, isoflurane) for refractory status asthmaticus.

๐Ÿฉบ Stepwise Approach: Acute Severe Asthma

1
Assess severity & risk factors
Stridor? Speech? Accessory muscles? SpO2? Pulsus paradoxus? Previous ICU admission/intubation? Poor compliance? Life-threatening features: silent chest, cyanosis, altered sensorium, hypercapnia.
2
Initial therapy (all severe)
High-flow oxygen to maintain SpO2 94-98%. Nebulised salbutamol (continuous) + ipratropium. IV steroids (hydrocortisone 4 mg/kg or methylprednisolone 1 mg/kg).
3
Add second-line agents if poor response
IV magnesium sulphate (25-50 mg/kg over 20 min). IV ketamine (1-2 mg/kg bolus then 1-4 mg/kg/h). Consider terbutaline infusion or aminophylline (levels 10-20 mg/L).
4
NIV trial (if no life-threatening features)
CPAP (5-8 cmH2O) or BiPAP may reduce work of breathing. Avoid in life-threatening asthma; do not delay intubation if worsening.
5
Intubation (if deterioration)
Modified RSI with ketamine (bronchodilator) + glycopyrrolate. Avoid bag-mask hyperventilation. Ventilator: low rate (10-12), low TV (5-7), long expiratory time, permissive hypercapnia. Set external PEEP ~2/3 of auto-PEEP.
6
Refractory measures
Manual chest compression during expiration (life-threatening air trapping). Inhaled anaesthetics (sevoflurane) in ICU. Heliox if FiO2 โ‰ค0.3. ECMO as last resort.