A 28-year-old woman from Delhi presents with acute onset dyspnea, chest pain, and syncope following ingestion of cyanide-laced food at a restaurant. On arrival, she is unconscious with severe metabolic acidosis (pH 7.1, HCO~3~^−^ 8 mmol/L), elevated lactate 12 mmol/L, and normal arterial oxygen saturation (SaO~2~ 98% on room air). Venous blood appears bright red. Which electron transport chain complex is irreversibly inhibited by cyanide, and why does oxygen saturation remain normal despite cellular hypoxia?

Explanation

## Cyanide Poisoning: Mechanism of Cellular Hypoxia ### The Paradox: Normal SaO~2~ with Cellular Hypoxia **Key Point:** Cyanide poisoning causes **histotoxic hypoxia** — cells cannot utilize oxygen despite normal arterial oxygenation. Pulse oximetry and blood gas SaO~2~ remain normal because they measure oxygen *dissolved* in blood and haemoglobin saturation, NOT cellular oxygen utilization. **High-Yield:** This is the pathognomonic presentation of cyanide poisoning: **bright red venous blood + metabolic acidosis + normal SaO~2~**. The venous blood is red (not blue) because oxygen is not being extracted by tissues. ### Complex IV (Cytochrome c Oxidase) — The Target **Mnemonic:** **CCOX** — **C**ytochrome **C** **O**xidase = **C**omplex **IV** 1. **Structure and Function** - Complex IV catalyzes the final step of the ETC: electrons from cytochrome c → oxygen - Contains two haem groups (cytochrome a and cytochrome a~3~) and copper centres (Cu~A~ and Cu~B~) - Cytochrome a~3~ iron is the terminal electron acceptor before O~2~ 2. **Cyanide Binding Mechanism** - Cyanide (CN^−^) binds with **extremely high affinity** (K~d~ ~10^−14^ M) to the **ferric iron (Fe³⁺)** of cytochrome a~3~ - This binding is **irreversible** under physiological conditions - Electrons cannot be transferred to O~2~; the entire ETC backs up and halts - ATP synthesis ceases → cellular energy crisis 3. **Why Oxygen Saturation Stays Normal** - Pulse oximetry measures **haemoglobin saturation** (% of Hb bound to O~2~) - Arterial blood oxygen content is normal; oxygen reaches tissues - The problem is **not oxygen delivery** but **oxygen utilization** - Tissues cannot accept electrons → O~2~ remains unused → venous blood retains O~2~ (bright red) ### Biochemical Cascade of Cyanide Poisoning ```mermaid flowchart TD A["Cyanide ingestion"]:::urgent --> B["CN⁻ binds cytochrome a₃ Fe³⁺"]:::urgent B --> C["Complex IV blocked"]:::urgent C --> D["Electron transport halts"]:::outcome D --> E["NADH + H⁺ cannot be reoxidized"]:::outcome E --> F["Glycolysis blocked<br/>NAD⁺ depleted"]:::outcome F --> G["Pyruvate → Lactate<br/>Anaerobic metabolism"]:::outcome G --> H["Severe metabolic acidosis"]:::urgent I["Oxygen delivery normal"]:::action --> J["But oxygen NOT utilized"]:::urgent J --> K["Venous O₂ high<br/>SaO₂ normal"]:::outcome K --> L["Histotoxic hypoxia"]:::urgent L --> M["Cellular ATP depletion"]:::urgent M --> N["Seizures, loss of consciousness"]:::urgent ``` ### Metabolic Consequences | Parameter | Mechanism | Clinical Finding | |-----------|-----------|------------------| | **Lactate ↑↑** | Pyruvate shunted to lactate (anaerobic glycolysis) | 12 mmol/L (severe) | | **pH ↓** | Lactic acid accumulation | pH 7.1 (severe acidosis) | | **HCO~3~^−^ ↓** | Buffering of lactate | 8 mmol/L | | **SaO~2~ normal** | Oxygen NOT consumed; remains in venous blood | 98% | | **Venous blood bright red** | High O~2~ content in venous blood | Pathognomonic sign | | **ATP depletion** | No oxidative phosphorylation | Seizures, coma, death | **Clinical Pearl:** The **bright red venous blood** is the clinical hallmark that distinguishes cyanide poisoning from other causes of hypoxia (respiratory failure, anaemia, cardiac failure all cause dark/cyanotic venous blood). ### Why Other Complexes Are Wrong - **Complex I or II:** Cyanide does not bind to NADH dehydrogenase or succinate dehydrogenase; these complexes are upstream and would not cause the acute, irreversible block seen here. - **Complex III:** Cyanide has minimal affinity for Complex III; it is not the primary target. ### Treatment Implication **High-Yield:** Cyanide antidotes work by: 1. **Hydroxocobalamin:** Binds CN^−^ directly, forming cyanocobalamin (excreted in urine) 2. **Sodium thiosulfate:** Provides sulphur for rhodanese enzyme to convert CN^−^ → thiocyanate 3. **Sodium nitrite (older):** Induces methaemoglobin, which binds CN^−^ (less preferred due to methaemoglobin toxicity) The key is to **remove free cyanide** before it reaches Complex IV or to bind it once bound (hydroxocobalamin is most effective).