A 28-year-old male factory worker is brought to the emergency department 15 minutes after suspected inhalation of hydrogen cyanide gas during an industrial accident. He is conscious but confused, with rapid shallow breathing (RR 32/min), heart rate 118/min, and blood pressure 140/88 mmHg. Skin appears flushed and warm. Arterial blood gas shows pH 7.28, PaCO₂ 32 mmHg, HCO₃⁻ 14 mEq/L, and normal PaO₂ of 95 mmHg on room air. What is the primary mechanism of toxicity in this patient?

Explanation

## Mechanism of Cyanide Toxicity **Key Point:** Cyanide's lethality stems from its ability to bind the ferric iron (Fe³⁺) in cytochrome c oxidase, the terminal enzyme of the electron transport chain (Complex IV). This blocks aerobic respiration at the mitochondrial level, causing **histotoxic hypoxia** — cells cannot utilize oxygen even when it is abundantly available. ### Why Normal PaO₂ Does Not Reassure This patient has a **normal arterial oxygen tension (95 mmHg)**, yet is in metabolic acidosis with lactic acidosis (low HCO₃⁻, low pH). This is the hallmark of cyanide poisoning: **oxygen delivery is preserved, but cellular utilization is blocked**. Tissues shift to anaerobic metabolism, generating lactate and causing metabolic acidosis. ### Clinical Features Explained | Finding | Explanation | |---------|-------------| | Flushed, warm skin | Venous blood remains oxygenated (cells cannot extract O₂); arteriovenous O₂ difference is minimal | | Tachycardia, tachypnea | Compensatory response to tissue hypoxia and acidosis | | Metabolic acidosis | Anaerobic metabolism → lactate accumulation | | Altered mental status | CNS is highly sensitive to mitochondrial dysfunction | ### Distinction from Other Hypoxia Types - **Hypoxemic hypoxia**: Low PaO₂ (not present here) - **Anemic hypoxia**: Low hemoglobin (not present here) - **Circulatory hypoxia**: Low cardiac output (not present here) - **Histotoxic hypoxia**: Normal PaO₂ and Hb, but cells cannot use O₂ ← **This is cyanide** **High-Yield:** The combination of **normal or high PaO₂ + metabolic acidosis + altered mental status** is pathognomonic for cyanide poisoning. **Clinical Pearl:** Cyanide poisoning is one of the few toxicologic emergencies where the patient may have a "healthy" oxygen saturation and blood gas oxygen level yet be dying from cellular asphyxia.