A 42-year-old woman with a history of chronic cyanide exposure (occupational hazard in a metal-plating factory) presents with progressive dyspnoea, fatigue, and cognitive decline over 6 months. Her arterial blood gas shows normal oxygen saturation (98%) and normal PaO₂ (95 mmHg), but venous oxygen saturation is abnormally high at 85% (normal 70–75%). Lactate is elevated at 5.2 mmol/L. Which electron transport chain complex is inhibited by chronic cyanide exposure, and why does the patient maintain normal arterial oxygenation despite cellular hypoxia?

Explanation

## Cyanide Toxicity and Histotoxic Hypoxia ### Mechanism of Cyanide Poisoning **Key Point:** Cyanide is a potent irreversible inhibitor of **Complex IV (cytochrome c oxidase)**, the final electron acceptor in the electron transport chain. It binds with extremely high affinity to the ferric iron (Fe³⁺) in the heme a₃ component of cytochrome oxidase. **High-Yield:** This creates a unique clinical scenario: **histotoxic hypoxia** — the tissues cannot utilize oxygen despite its availability in arterial blood and adequate delivery. ### Why Arterial Oxygenation Remains Normal ```mermaid flowchart TD A["Lungs: Normal O₂ uptake"] --> B["Arterial blood: Normal PaO₂ & SaO₂"] B --> C["Tissues receive adequate O₂"] C --> D{"Complex IV functional?"} D -->|Normal| E["O₂ reduced to H₂O"] D -->|Cyanide-inhibited| F["O₂ cannot be reduced"] E --> G["ATP synthesis proceeds"] F --> H["Electron transport halts"] H --> I["NADH accumulates"] I --> J["Lactate accumulation"] J --> K["Cellular hypoxia despite normal PaO₂"] style A fill:#ccffcc style B fill:#ccffcc style K fill:#ffcccc style F fill:#ffcccc ``` **Clinical Pearl:** The **arteriovenous oxygen difference (A-V O₂ diff) is abnormally small** because tissues cannot extract oxygen from venous blood — venous O₂ saturation remains elevated (85% instead of normal 70–75%). The blood returns to the lungs still carrying most of its oxygen. ### Biochemical Basis **Mnemonic:** **Cyanide Chokes Cytochrome** — Cyanide binds irreversibly to Fe³⁺ in cytochrome a₃, preventing the final step of electron transfer. $$\text{Cytochrome a}_3^{Fe^{3+}} + CN^- \rightarrow \text{Cytochrome a}_3^{Fe^{3+}-CN^-} \text{ (inactive)}$$ When Complex IV is blocked: 1. Electrons cannot be transferred to O₂ 2. The entire electron transport chain backs up and halts 3. NADH cannot be reoxidized → NAD^+^ becomes depleted 4. Glycolysis slows (requires NAD^+^) 5. Pyruvate is shunted to lactate → **lactic acidosis** 6. ATP synthesis via oxidative phosphorylation ceases → **cellular energy crisis** ### Three Types of Hypoxia | Type | Mechanism | PaO₂ | SaO₂ | A-V O₂ diff | Example | |------|-----------|------|------|-------------|----------| | **Hypoxaemic** | Lung disease | ↓ | ↓ | Normal | Pneumonia, ARDS | | **Anaemic** | Low Hb | Normal | Normal | ↑ | Severe anaemia | | **Histotoxic** | Impaired cellular utilization | Normal | Normal | ↓ | Cyanide, CO poisoning | | **Circulatory** | Reduced tissue perfusion | Normal | Normal | ↑ | Shock, heart failure | **High-Yield:** In cyanide poisoning, the A-V O₂ difference is **abnormally low** (tissues cannot extract oxygen), so venous O₂ saturation stays high. ### Why This Patient Has Elevated Lactate With Complex IV blocked and NADH accumulating, pyruvate is preferentially converted to lactate via lactate dehydrogenase: $$\text{Pyruvate} + NADH + H^+ \xrightarrow{LDH} \text{Lactate} + NAD^+$$ This regenerates NAD^+^ to sustain glycolysis, but at the cost of lactic acidosis and cellular energy depletion. [cite:Robbins 10e Ch 7; Harrison 21e Ch 397]