A 32-year-old male with acute severe asthma exacerbation requiring emergency intubation arrives in the ICU. During rapid sequence intubation, the anaesthetist avoids propofol and thiopentone. Which induction agent is most appropriate, and what is the mechanism that makes it safe in this scenario?

Explanation

## Clinical Scenario: Acute Asthma & Induction Agent Selection In acute asthma exacerbation, the induction agent must: 1. Avoid histamine release (propofol, thiopentone, atracurium trigger mast cells) 2. Preserve or improve bronchial tone 3. Maintain airway reflexes 4. Preserve cardiovascular stability ## Why Ketamine is the Agent of Choice **Key Point:** Ketamine is the gold standard induction agent in asthma because it: - **Preserves airway reflexes** — maintains spontaneous ventilation and cough reflex - **Does NOT trigger histamine release** — no mast cell degranulation - **Causes bronchodilation** — via direct smooth muscle relaxation and sympathomimetic effects - **Maintains cardiovascular stability** — sympathomimetic effects preserve BP and HR **High-Yield:** Ketamine is preferred in: - Acute asthma/COPD exacerbation - Anaphylaxis (intraoperative) - Septic shock - Hypovolaemia - Patients with reactive airway disease ## Comparison: Induction Agents in Asthma | Agent | Histamine Release | Bronchial Effect | Airway Reflexes | Safety in Asthma | |-------|-------------------|------------------|-----------------|------------------| | **Ketamine** | No | ↑ Bronchodilation | Preserved | **Gold standard** | | **Propofol** | Yes (mild) | Bronchodilation | Preserved | Contraindicated | | **Thiopentone** | Yes | Bronchoconstriction | Preserved | **Contraindicated** | | **Etomidate** | No | Neutral | Preserved | Acceptable alternative | **Clinical Pearl:** Ketamine's sympathomimetic effects (catecholamine release) are actually beneficial in asthma—they promote bronchodilation and maintain airway patency. The preserved airway reflexes are critical for emergency airway management. **Warning:** Do not confuse propofol's bronchodilator property with safety in asthma—it causes mild histamine release and is relatively contraindicated. Thiopentone actively causes bronchoconstriction and is absolutely contraindicated.