A 32-year-old male underwent elective open cholecystectomy under general anesthesia with succinylcholine and isoflurane. Intraoperatively, at 45 minutes, the patient developed unexplained tachycardia (HR 140/min), muscle rigidity, and core temperature rose to 38.5°C. Capnography showed elevated end-tidal CO₂ (ETCO₂ 65 mmHg). The anesthesiologist immediately discontinued triggering agents and hyperventilated with 100% O₂. Which investigation is most appropriate to confirm the diagnosis of malignant hyperthermia at this critical moment?

Explanation

## Investigation of Choice in Acute Malignant Hyperthermia ### Acute Phase Investigations **Key Point:** During an acute MH crisis, the most critical immediate investigations are those that assess metabolic derangements and guide emergency management, not confirmatory tests. **High-Yield:** ABG and serum potassium are the investigations of choice *during* an acute MH episode because they: - Detect **hyperkalemia** (K⁺ can rise to 5.5–8 mEq/L due to muscle breakdown and succinylcholine-induced potassium release) - Identify **metabolic acidosis** (from anaerobic metabolism and rhabdomyolysis) - Reveal **respiratory acidosis** (from elevated ETCO₂) - Guide immediate treatment decisions (calcium gluconate, insulin-glucose, bicarbonate) - Help predict cardiac dysrhythmias ### Why ABG and Potassium Are Acute Investigations These results directly influence immediate perioperative management: 1. **Hyperkalemia** → risk of cardiac arrest → requires urgent treatment 2. **Acidosis** → worsens hyperkalemia effects → requires buffering 3. **Results available in minutes** → guide ICU admission and monitoring ### Confirmatory vs. Acute Investigations | Investigation | Timing | Purpose | Clinical Use | |---|---|---|---| | **ABG + K⁺** | Acute (during crisis) | Assess metabolic derangement | Guide emergency treatment | | **CK, myoglobin** | Acute–subacute (6–24 hrs) | Assess rhabdomyolysis severity | Predict acute kidney injury | | **Caffeine halothane contracture test** | Post-crisis (weeks–months) | Confirm MH susceptibility | Genetic counseling, family screening | | **EMG/NCS** | Not indicated | Assesses nerve function | Not relevant to MH diagnosis | **Clinical Pearl:** The caffeine halothane contracture test (CHCT) is the *gold standard* for confirming MH susceptibility, but it is performed **after** the acute episode has resolved (weeks to months later) in a specialized laboratory. It cannot be done acutely. **Mnemonic: CRASH-C** — Creatine kinase, Rhabdomyolysis, Acidosis, Succinylcholine hyperkalemia, Hyperkalemia, Contracture test (later) ### Why Other Options Are Wrong - **CK and myoglobin:** Important for assessing rhabdomyolysis severity and AKI risk, but not the *first* investigation to guide acute management. Results take hours; potassium is more urgent. - **EMG/NCS:** Assesses peripheral nerve and muscle electrical activity; not diagnostic for MH and not useful acutely. - **Muscle biopsy with CHCT:** The definitive diagnostic test, but performed weeks after recovery in a specialized center—not acutely during crisis. [cite:Miller's Anesthesia 8e Ch 27]