A 32-year-old male factory worker is brought to the emergency department 20 minutes after suspected inhalation of hydrogen cyanide gas during an industrial accident. He is conscious, anxious, with a respiratory rate of 28/min, heart rate 118/min, and blood pressure 156/92 mmHg. Arterial blood gas shows pH 7.28, PaCO₂ 32 mmHg, HCO₃⁻ 14 mEq/L, and elevated lactate of 8.2 mmol/L. What is the most appropriate immediate next step in management?

Explanation

## Clinical Scenario Analysis This patient presents with acute cyanide poisoning from inhalation with classic signs of cellular hypoxia despite adequate oxygenation: - Metabolic acidosis with elevated lactate (8.2 mmol/L) - Tachycardia and hypertension (sympathetic activation) - Tachypnea (respiratory compensation for acidosis) ## Mechanism of Cyanide Toxicity **Key Point:** Cyanide binds irreversibly to the ferric iron (Fe³⁺) in cytochrome c oxidase, blocking the electron transport chain and causing histotoxic hypoxia—cells cannot utilize oxygen despite adequate oxygenation. ## Management Algorithm for Acute Cyanide Poisoning ```mermaid flowchart TD A[Suspected acute cyanide poisoning]:::outcome --> B[High suspicion or symptomatic?]:::decision B -->|Yes| C[Immediate: 100% oxygen]:::action C --> D[Administer antidote]:::action D --> E{Route of exposure?}:::decision E -->|Inhalation/IV| F[Sodium nitrite first]:::action E -->|Ingestion| G[Sodium thiosulfate first]:::action F --> H[Sodium nitrite induces methemoglobinemia]:::outcome H --> I[CN⁻ binds to MetHb instead of cytochrome]:::outcome I --> J[Follow with sodium thiosulfate]:::action J --> K[Thiosulfate converts CN⁻ to thiocyanate]:::outcome K --> L[Thiocyanate excreted renally]:::outcome ``` ## Antidote Protocol: Sodium Nitrite + Sodium Thiosulfate | Step | Agent | Dose | Mechanism | Timing | |------|-------|------|-----------|--------| | 1 | Sodium nitrite 3% IV | 10 mL over 2–3 min | Converts Hb to MetHb; CN⁻ preferentially binds MetHb | Immediate | | 2 | Sodium thiosulfate 25% IV | 50 mL (12.5 g) over 10 min | Rhodanese enzyme converts CN⁻ to thiocyanate (less toxic) | After nitrite | | 3 | Repeat doses | If symptoms persist | May repeat nitrite at half dose; thiosulfate at full dose | Every 10–15 min | **High-Yield:** Sodium nitrite is the FIRST-line antidote for inhalation or parenteral cyanide because it rapidly creates a cyanide sink (methemoglobin). Sodium thiosulfate follows to detoxify the cyanide. ## Why Oxygen Alone Is Insufficient **Clinical Pearl:** Giving 100% oxygen without an antidote does NOT reverse cyanide toxicity because the block is at the mitochondrial level, not the lungs. The patient has histotoxic hypoxia, not hypoxemic hypoxia. ## Additional Supportive Measures - Maintain airway, breathing, circulation (ABCs) - Correct metabolic acidosis with sodium bicarbonate if severe - Monitor for methemoglobinemia (goal: 20–30% for cyanide binding) - Supportive care: vasopressors if hypotensive, anticonvulsants if seizures **Mnemonic:** **NITRITE-THIO** = Nitrite first (inhalation), THIOsulfate second (detoxification). [cite:Park 26e Ch 12; Poisoning & Toxicology]