A 28-year-old man is brought to the emergency department 15 minutes after ingesting cyanide salt in a suicide attempt. He is conscious but agitated, with a respiratory rate of 32/min and heart rate of 118/min. Arterial blood gas shows pH 7.28, PaCO₂ 32 mmHg, HCO₃⁻ 14 mEq/L, and PaO₂ 95 mmHg on room air. Venous oxygen saturation is 85%. What is the most likely mechanism of cyanide toxicity in this patient?

Explanation

## Mechanism of Cyanide Toxicity **Key Point:** Cyanide binds irreversibly to the ferric iron (Fe³⁺) of cytochrome c oxidase (Complex IV) in the electron transport chain, halting aerobic respiration and forcing cells into anaerobic metabolism. ### Pathophysiology Cyanide's lethal action occurs at the mitochondrial level: 1. **Binding site**: The CN⁻ ion binds to Fe³⁺ in the heme a₃ component of cytochrome c oxidase 2. **Effect**: Blocks electron transfer to oxygen, preventing ATP synthesis 3. **Consequence**: Cells shift to anaerobic glycolysis, producing lactate and causing metabolic acidosis ### Clinical Correlation with Lab Findings The patient's presentation demonstrates classic cyanide toxicity: | Finding | Explanation | |---------|-------------| | **pH 7.28, HCO₃⁻ 14** | Metabolic acidosis from anaerobic metabolism and lactate accumulation | | **PaO₂ 95 mmHg (normal)** | Oxygen is present but cannot be utilized — "histotoxic hypoxia" | | **Venous O₂ sat 85%** | Venous blood remains oxygen-rich because tissues cannot extract it | | **Tachypnea, tachycardia** | Compensatory response to tissue hypoxia and acidosis | **High-Yield:** The hallmark of cyanide poisoning is the **paradox of normal arterial oxygen with tissue hypoxia** — this is histotoxic (cellular) hypoxia, not hypoxemic hypoxia. ### Why Aerobic Respiration Fails ```mermaid flowchart TD A[Glucose metabolism]:::action --> B[Pyruvate enters mitochondria]:::action B --> C[TCA cycle produces NADH & FADH₂]:::action C --> D[Electrons enter ETC]:::action D --> E[Complex IV: Cytochrome c oxidase]:::outcome E --> F{Cyanide present?}:::decision F -->|No| G[Electrons transfer to O₂]:::action F -->|Yes| H[CN⁻ binds Fe³⁺]:::urgent H --> I[ETC blocked]:::urgent I --> J[ATP synthesis stops]:::urgent G --> K[ATP generated]:::action J --> L[Anaerobic glycolysis only]:::action L --> M[Lactate accumulation & acidosis]:::outcome ``` **Clinical Pearl:** In cyanide poisoning, the **mixed venous-arterial oxygen difference is minimal** because tissues cannot extract oxygen — this distinguishes it from cardiogenic or septic shock where tissue extraction is normal but delivery is impaired. ### Treatment Rationale Antidotes work by: - **Hydroxocobalamin**: Binds CN⁻ directly, forming cyanocobalamin (excreted in urine) - **Sodium thiosulfate**: Provides sulfur for rhodanese enzyme to convert CN⁻ to thiocyanate - **Sodium nitrite** (older): Induces methemoglobinemia to compete for CN⁻ (less favored now due to hypotension risk)