A 32-year-old industrial worker is brought to the emergency department after suspected exposure to cyanide gas at a chemical plant. His arterial blood gas shows a normal PaO₂ (95 mmHg) and normal oxygen saturation (98%), yet he is unconscious with severe metabolic acidosis. Which single feature best distinguishes acute cyanide poisoning from carbon monoxide poisoning in this clinical scenario?

Explanation

## Distinguishing Cyanide from Carbon Monoxide Poisoning ### Key Pathophysiologic Difference **Key Point:** Cyanide poisoning causes *histotoxic hypoxia* — the lungs oxygenate blood normally, but tissues cannot use oxygen because cytochrome c oxidase (Complex IV) is inhibited. Carbon monoxide causes *hypoxemic hypoxia* — carboxyhemoglobin reduces oxygen-carrying capacity. ### Comparison Table | Feature | Cyanide Poisoning | Carbon Monoxide Poisoning | |---------|-------------------|-------------------------| | **PaO₂** | Normal or high | Normal or high | | **SaO₂ (pulse oximetry)** | Normal (95–100%) | Normal or high (falsely reassuring) | | **Arterial oxygen content** | Low (tissue cannot extract) | Low (carboxyhemoglobin) | | **Venous oxygen saturation** | High (venous blood still oxygenated) | Low (tissues extract available O₂) | | **Metabolic acidosis** | Severe, rapid onset | Mild to moderate | | **Skin appearance** | Flushed, pink (high SvO₂) | Cherry-red (carboxyhemoglobin) | | **Carboxyhemoglobin** | Absent | Elevated (>10%) | ### Why This Discriminates **High-Yield:** In cyanide poisoning, the **arterial oxygen saturation remains normal** because hemoglobin is fully oxygenated — the problem is downstream at the mitochondrial level. This paradox (normal SaO₂ + severe tissue hypoxia + unconsciousness) is pathognomonic for cyanide. **Clinical Pearl:** The presence of a **high mixed venous oxygen saturation** (venous blood remains oxygenated because tissues cannot extract oxygen) is the smoking gun for cyanide. In carbon monoxide, venous saturation is low because tissues extract whatever oxygen is available. ### Mechanism 1. Cyanide binds to ferric iron (Fe³⁺) in cytochrome c oxidase. 2. Electron transport chain halts → no ATP production. 3. Lungs still oxygenate hemoglobin normally → normal PaO₂ and SaO₂. 4. But tissues cannot use that oxygen → severe lactic acidosis, rapid collapse. **Mnemonic:** **"Pink but Dead"** — cyanide victims may appear flushed and well-oxygenated (high SvO₂) yet are dying from cellular asphyxia. Carbon monoxide victims appear **cherry-red** (carboxyhemoglobin color). [cite:Parikh Textbook of Forensic Medicine Ch 18]