Which of the following is the most characteristic finding in cyanide poisoning that helps differentiate it from other causes of hypoxia?

Explanation

## Distinguishing Feature of Cyanide Poisoning **Key Point:** Cyanide poisoning causes **histotoxic hypoxia**, characterized by normal or elevated venous oxygen saturation (SvO₂) despite tissue hypoxia and normal arterial oxygen saturation (SaO₂). ### Pathophysiology of Histotoxic Hypoxia | Feature | Cyanide Poisoning | Other Hypoxias | |---------|-------------------|----------------| | **SaO₂** | Normal (95–100%) | Decreased (in hypoxemic hypoxia) | | **SvO₂** | Normal or HIGH | Decreased (tissues extract more O₂) | | **A-V O₂ difference** | Narrow (cells cannot use O₂) | Wide (cells extract available O₂) | | **Tissue oxygenation** | Severely impaired | Impaired | | **PaO₂** | Normal | Decreased (in hypoxemic hypoxia) | ### Why This Occurs 1. Cyanide blocks cytochrome c oxidase → cells cannot use oxygen 2. Oxygen remains in venous blood because tissues cannot extract it 3. Venous blood returns to the heart still oxygen-rich 4. This creates the paradox: **normal oxygen levels but cellular hypoxia** **High-Yield:** The **narrow arterio-venous oxygen difference** (A-V O₂ < 3 mL/dL) is pathognomonic for histotoxic hypoxia and helps distinguish cyanide from: - Hypoxemic hypoxia (low SaO₂) - Anemic hypoxia (low hemoglobin) - Circulatory hypoxia (wide A-V O₂ difference) **Mnemonic: CHOP** — Cyanide causes Histotoxic hypoxia with normal Oxygen saturation in venous blood and normal Partial pressure of oxygen. **Clinical Pearl:** A patient with cyanide poisoning may have "pink" or "rosy" cheeks due to high venous oxygen saturation, contrasting sharply with their severe clinical deterioration.