Which of the following is the most characteristic finding on arterial blood gas analysis in acute cyanide poisoning?

Explanation

## Arterial Blood Gas Findings in Cyanide Poisoning **Key Point:** The pathognomonic ABG finding in cyanide poisoning is **metabolic acidosis with an elevated anion gap AND paradoxically high venous oxygen saturation** — reflecting histotoxic hypoxia. ### Mechanism of ABG Changes #### 1. Metabolic Acidosis (Elevated Anion Gap) - Cyanide blocks oxidative phosphorylation → cells shift to anaerobic metabolism - Anaerobic glycolysis produces lactate as the end product - Lactate accumulates → **lactic acidosis** (anion gap metabolic acidosis) - This occurs rapidly, often within minutes of exposure **High-Yield:** The anion gap is elevated because lactate is an unmeasured anion: $$\text{Anion Gap} = [Na^+] - ([Cl^-] + [HCO_3^-])$$ #### 2. High Venous Oxygen Saturation (Diagnostic Clue) - Cells cannot utilize oxygen despite its availability - Venous blood remains oxygenated because oxygen is not extracted - Normal venous saturation is 70–75%; in cyanide poisoning it may be **>90%** - Arterial saturation is also normal (SpO₂ 98–100%) **Clinical Pearl:** The combination of **normal or high oxygen saturation + profound metabolic acidosis + clinical toxicity** is virtually pathognomonic for cyanide poisoning. This distinguishes it from cardiogenic or septic shock, where venous saturation is low. ### Comparison with Other Poisonings | Poisoning | ABG Pattern | Venous O₂ Sat | Mechanism | |-----------|-------------|---------------|----------| | **Cyanide** | **Anion gap acidosis** | **High (>90%)** | **Histotoxic hypoxia** | | Carbon monoxide | Anion gap acidosis | Low | Carboxyhemoglobin | | Methanol/Ethylene glycol | Anion gap acidosis | Low | Toxic metabolites | | Septic shock | Anion gap acidosis | Low | Tissue hypoperfusion | | Cardiogenic shock | Anion gap acidosis | Low | Tissue hypoperfusion | **Mnemonic: CHAMP** — Cyanide, Hypoxia (histotoxic), Acidosis (lactic), Metabolic, Paradoxically normal oxygen saturation ### Timeline of ABG Changes 1. **Minutes 0–5:** Lactic acidosis begins; venous saturation rises 2. **Minutes 5–15:** Profound acidosis; pH may drop to 6.8–7.0 3. **Minutes 15+:** Respiratory compensation (hyperventilation) may partially correct pH, but metabolic component persists **Warning:** Do not be misled by normal SpO₂ or normal PaO₂ — these are expected in cyanide poisoning and do NOT rule out the diagnosis. The key is the **mismatch between normal oxygenation and severe metabolic acidosis**. [cite:Forensic Medicine & Toxicology by Reddy, Cyanide Poisoning Chapter]