A 38-year-old woman with acute anaphylaxis due to penicillin allergy presents with severe hypotension, bronchospasm, and angioedema. Which adrenergic agonist is the most commonly recommended first-line agent for immediate management of anaphylaxis?

Explanation

## First-Line Agent in Anaphylaxis: Adrenaline **Key Point:** Intramuscular adrenaline (epinephrine) 0.3–0.5 mg IM is the gold standard, most commonly used, and most effective first-line treatment for acute anaphylaxis. ### Why Adrenaline is Optimal in Anaphylaxis Adrenaline is a non-selective α and β adrenergic agonist with balanced effects: | Effect | Mechanism | Benefit in Anaphylaxis | |--------|-----------|------------------------| | **α₁ vasoconstriction** | Direct vascular smooth muscle | Reverses hypotension, reduces angioedema | | **β₁ inotropy** | Increased cardiac contractility | Maintains cardiac output | | **β₂ bronchodilation** | Relaxes airway smooth muscle | Relieves bronchospasm | | **β₂ mast cell stabilization** | Inhibits mediator release | Prevents further degranulation | **High-Yield:** Adrenaline is the ONLY agent that addresses ALL pathophysiological components of anaphylaxis simultaneously — hypotension, bronchospasm, angioedema, and ongoing mast cell degranulation. ### Route & Dosing **Intramuscular (IM) is preferred over IV:** - **IM 0.3–0.5 mg** (1:1000 solution) = fastest, safest onset - Onset: 5–15 minutes - Repeat every 5–15 minutes if needed - **IV route** reserved for refractory shock or cardiac arrest (risk of arrhythmias, myocardial ischaemia) **Mnemonic:** **IM-ADRENALINE** = **I**njectable **M**uscle is the **A**pproach for **D**irect **R**elief in **E**mergent **N**eed **A**llergic **L**ife-threat **I**njury **N**ow **E**ssential. ### Clinical Pearl Adrenaline should be given **immediately** — do not wait for IV access or investigations. Even mild anaphylaxis (urticaria + pruritus) warrants IM adrenaline if there is any concern for progression. Delay increases mortality risk. ### Adjunctive Therapy (After Adrenaline) 1. **IV fluids** — crystalloid bolus for hypotension 2. **Antihistamines** — H₁ blocker (e.g., promethazine 25–50 mg IM/IV) 3. **Corticosteroids** — hydrocortisone 200 mg IV or methylprednisolone 125 mg IV (prevents biphasic reaction) 4. **Salbutamol** — inhaled if persistent bronchospasm 5. **Observation** — minimum 4–6 hours (risk of biphasic anaphylaxis) ### Why Other Agents Fail in Anaphylaxis ```mermaid flowchart TD A[Anaphylaxis]:::urgent --> B{Agent choice?}:::decision B -->|Adrenaline IM|C[α₁ + β₁ + β₂ effects]:::action C --> D[Hypotension ↓, Bronchospasm ↓, Angioedema ↓]:::outcome B -->|Noradrenaline IV|E[α₁ + β₁ only]:::action E --> F[No β₂ bronchodilation]:::urgent B -->|Dopamine IV|G[β₁ + β₂ dominant]:::action G --> H[Weak vasoconstriction, tachycardia]:::urgent B -->|Salbutamol inhaled|I[β₂ only]:::action I --> J[No vasoconstriction, no mast cell stabilization]:::urgent ``` **Warning:** ~~Antihistamines alone~~ or ~~corticosteroids alone~~ are NOT sufficient for anaphylaxis — they are adjuncts only. Adrenaline must be given first. [cite:Harrison 21e Ch 297; Park 26e Ch 3]