## Venous Blood Colour in Cyanide Poisoning ### The Paradox of Bright Red Venous Blood **Key Point:** Cyanide poisoning produces **bright red venous blood** — a pathognomonic finding that reflects the inability of tissues to extract oxygen despite its availability. ### Mechanism: Histotoxic Hypoxia In normal physiology: - Arterial blood is bright red (high O₂ saturation ~98%) - Venous blood is dark red/purple (low O₂ saturation ~75%) because tissues extract oxygen - Arteriovenous O₂ difference (A-V O₂) = ~5 mL/100 mL blood In cyanide poisoning: - Arterial blood remains bright red (normal oxygenation) - **Venous blood also remains bright red** (high O₂ saturation ~90%+) - A-V O₂ difference is **narrow** (only ~1–2 mL/100 mL blood) - Tissues are hypoxic despite adequate oxygen supply **Mnemonic: "Bright Red Venous Blood = Can't Use Oxygen"** — the blood is red because it's still loaded with oxygen that the poisoned cells cannot extract. ### Why This Occurs Cytochrome c oxidase inhibition blocks the electron transport chain. Cells cannot generate ATP aerobically and cannot use oxygen, so oxygen remains in the venous blood. This is **histotoxic hypoxia** (cellular poisoning), not hypoxemic hypoxia (low blood oxygen). **High-Yield:** Bright red venous blood is a diagnostic clue for cyanide poisoning in forensic and clinical settings. It distinguishes cyanide from other causes of shock or respiratory failure. **Clinical Pearl:** This finding is so characteristic that its absence should prompt reconsideration of the diagnosis.
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