A 52-year-old man undergoing elective abdominal surgery develops sudden hypoxia (SpO₂ 88%) and hypotension (BP 90/55 mmHg) 45 minutes after induction of general anaesthesia with propofol and succinylcholine. The anaesthetist notes bilateral breath sounds are present, but end-tidal CO₂ has dropped sharply. What is the most common cause of this acute perioperative complication?

Explanation

## Most Common Cause of Acute Perioperative Hypoxia and Hypotension with Sharp ETCO₂ Drop **Key Point:** Neuromuscular blocking agents (NMBAs), particularly succinylcholine and rocuronium, are the **most common triggers of perioperative anaphylaxis**, accounting for 50–70% of all intraoperative anaphylactic reactions (Mertes et al., French GERAP registry; Harper's Practical Anaesthesia). ## Why Anaphylaxis Fits This Presentation ### Clinical Clues in the Stem - **Sudden onset** at 45 minutes — anaphylaxis can occur at any point after NMBA administration, including delayed reactions - **SpO₂ 88%** — bronchospasm and distributive shock reduce oxygenation - **BP 90/55 mmHg** — massive histamine-mediated vasodilation causes profound hypotension - **Bilateral breath sounds present** — rules out tension pneumothorax (unilateral absent BS) and major airway obstruction - **Sharp drop in end-tidal CO₂** — hallmark of sudden reduction in pulmonary perfusion due to distributive (anaphylactic) shock, not hypoventilation ### Pathophysiology of NMBA-Induced Anaphylaxis - IgE-mediated (or direct mast cell degranulation) release of histamine, tryptase, and leukotrienes - Causes: peripheral vasodilation → ↓ SVR → hypotension; bronchospasm → ↓ V/Q → hypoxia; ↓ cardiac output → sharp ↓ ETCO₂ - Succinylcholine is a well-known direct histamine releaser even without prior sensitization ## Why Other Options Are Less Likely | Cause | Why Less Likely Here | |-------|----------------------| | **Hypovolemia + anaesthetic depression (B)** | Causes *gradual* hypotension; does NOT typically cause a *sharp* ETCO₂ drop with preserved bilateral breath sounds; propofol effect peaks at induction, not 45 min later | | **Tension pneumothorax (C)** | Would show *unilateral absent* breath sounds, tracheal deviation, JVD — none described | | **Pulmonary embolism (D)** | Uncommon acutely at 45 min; ETCO₂ drop with hypoxia is possible but PE is far less common than anaphylaxis intraoperatively | ## Differential Diagnosis of Intraoperative Hypotension + Hypoxia | Cause | Frequency | Key Features | ETCO₂ | |-------|-----------|--------------|--------| | **Anaphylaxis (NMBA)** | Most common acute cause | Sudden onset, flushing, urticaria, bronchospasm | Sharp ↓ | | Hypovolemia + anaesthetic depression | Common (gradual) | Gradual onset, responds to fluids | Mild ↓ | | Tension pneumothorax | Rare | Unilateral absent BS, JVD | ↓ | | Pulmonary embolism | Uncommon | Tachycardia, hypoxia, hypocapnia | ↓ | **High-Yield:** The combination of **sudden hypotension + hypoxia + sharp ETCO₂ drop + bilateral breath sounds** in the context of NMBA administration = **anaphylaxis until proven otherwise**. NMBAs are the #1 cause of perioperative anaphylaxis (Mertes PM, Anesthesiology 2011). ## Management of Suspected Intraoperative Anaphylaxis 1. **Epinephrine** 0.01 mg/kg IV (first-line) — reverses bronchospasm and vasodilation 2. Stop the offending agent 3. 100% O₂, aggressive IV fluid resuscitation 4. Antihistamines (H1 + H2) and corticosteroids as adjuncts 5. Send serum tryptase (within 1–2 hours) to confirm diagnosis **Clinical Pearl:** Always have epinephrine drawn up before NMBA administration in high-risk patients. Skin testing post-recovery identifies the causative agent and guides future anaesthetic planning. **Reference:** Mertes PM et al. "Anaphylaxis during anesthesia." *Anesthesiology* 2011; KD Tripathi, *Essentials of Medical Pharmacology*, 8th ed.