A 42-year-old male with a history of hypertension (on amlodipine) undergoes elective abdominal aortic aneurysm (AAA) repair under general anesthesia with endotracheal intubation. Intraoperatively, he receives succinylcholine 1.5 mg/kg IV for rapid sequence intubation. Within 2 minutes of drug administration, the anesthesiologist notes severe muscle rigidity, a core temperature of 38.5°C, and dark-colored urine in the catheter. Arterial blood gas shows pH 7.28, pCO₂ 52 mmHg, pO₂ 95 mmHg, and serum potassium 6.8 mEq/L. Which of the following is the MOST likely complication, and what is the immediate next step in management?

Explanation

## Malignant Hyperthermia (MH) — Clinical Recognition and Acute Management ### Clinical Presentation in This Case: The constellation of findings is pathognomonic for **malignant hyperthermia**: - **Muscle rigidity** (masseter muscle rigidity or generalized) within minutes of succinylcholine - **Hypercarbia** (pCO₂ 52 mmHg) — earliest and most sensitive sign - **Hyperthermia** (38.5°C core temperature) — late sign, not always present early - **Hyperkalemia** (K⁺ 6.8 mEq/L) — from sustained muscle contraction and rhabdomyolysis - **Myoglobinuria** (dark urine) — evidence of muscle breakdown - **Metabolic acidosis** (pH 7.28) — from hypermetabolism and anaerobic metabolism ### Key Pathophysiology: MH is an autosomal dominant pharmacogenetic disorder affecting calcium regulation in skeletal muscle. Triggering agents (succinylcholine, volatile anesthetics) cause uncontrolled calcium release from the sarcoplasmic reticulum, leading to sustained muscle contraction, hypermetabolism, and rhabdomyolysis. ### Immediate Management Protocol: 1. **Discontinue all triggering agents** — stop succinylcholine and volatile anesthetic immediately 2. **Hyperventilate with 100% oxygen** — reduce CO₂, improve oxygenation 3. **Administer dantrolene sodium 2.5 mg/kg IV** — inhibits calcium release from SR; repeat every 5 minutes up to 10 mg/kg if signs persist 4. **Activate MH crisis protocol** — notify ICU, prepare for aggressive cooling, continuous monitoring 5. **Supportive care** — aggressive fluid resuscitation, maintain urine output >200 mL/h, monitor electrolytes, treat hyperkalemia (calcium gluconate, insulin-dextrose, bicarbonate) 6. **Investigate CK, myoglobin, coagulation profile** — assess severity of rhabdomyolysis and DIC risk ### Why Dantrolene Is Essential: - Only specific treatment for MH - Blocks ryanodine receptor, preventing calcium release - Must be given early — mortality increases significantly if delayed ### High-Yield Clinical Pearl: **Hypercarbia (end-tidal CO₂ >55 mmHg) is the earliest and most reliable intraoperative sign of MH**, often preceding fever by 30–60 minutes. Unexplained hypercarbia during anesthesia should trigger immediate suspicion. --- ## Why Other Options Are Incorrect: **Option 1 (Correct):** Addresses MH with appropriate immediate management. **Option 2 (Succinylcholine-induced hyperkalemia):** While succinylcholine-induced hyperkalemia is real and this patient does have elevated K⁺, this diagnosis does NOT explain the acute muscle rigidity, hypercarbia, or hyperthermia. Succinylcholine hyperkalemia is typically mild (K⁺ 0.5–1 mEq/L increase) unless there is pre-existing muscle disease or massive tissue damage. The clinical picture is overwhelmingly consistent with MH, not simple hyperkalemia. **Option 3 (Anaphylaxis):** Anaphylaxis to succinylcholine would present with urticaria, bronchospasm, hypotension, and tachycardia — not muscle rigidity, hypercarbia, or hyperthermia. The absence of cardiovascular collapse and presence of hypermetabolic signs rule this out. **Option 4 (Neuroleptic malignant syndrome):** NMS is a rare complication of antipsychotic drugs (dopamine antagonists), not anesthetics. It does not occur with volatile anesthetics in the acute perioperative period and is not triggered by succinylcholine. Bromocriptine is not indicated in the operating room for anesthetic complications.