Which of the following is the antidote of choice for acute cyanide poisoning in India?

Explanation

## Antidote Management in Cyanide Poisoning **Key Point:** The combination of **sodium nitrite** (induces methemoglobinemia) followed by **sodium thiosulfate** (enhances thiocyanate formation) is the standard antidote regimen in India and most resource-limited settings. ### Mechanism of Antidote Action #### Sodium Nitrite - Oxidizes hemoglobin to **methemoglobin** (Fe²⁺ → Fe³⁺) - Methemoglobin has high affinity for cyanide, binding it to form **cyanmethemoglobin** - This removes free cyanide from cytochrome c oxidase - **Dose**: 10 mL of 3% solution IV over 5–10 minutes (0.3 g) #### Sodium Thiosulfate - Acts as a sulfur donor for the enzyme **rhodanese** (sulfurtransferase) - Converts cyanide to less toxic **thiocyanate**, which is renally excreted - **Dose**: 50 mL of 25% solution IV (12.5 g) - Can be repeated if necessary ### Comparison of Antidotes | Antidote | Mechanism | Availability | Cost | Use in India | |----------|-----------|--------------|------|---------------| | **Sodium nitrite + thiosulfate** | Methemoglobin formation + rhodanese activation | Widely available | Low | **Standard** | | **Hydroxocobalamin** | Binds CN⁻ to form cyanocobalamin | Limited availability | High | Emerging alternative | | **Methylene blue** | Reduces methemoglobin (not for acute CN⁻) | Available | Low | Contraindicated | **High-Yield:** Hydroxocobalamin is the **first-line antidote in developed countries** (Europe, USA) but is expensive and not widely available in India. Sodium nitrite + thiosulfate remains the **gold standard in Indian practice**. ### Caution with Methylene Blue **Warning:** Methylene blue reduces methemoglobin back to hemoglobin, **reversing the antidote effect** of sodium nitrite. It is contraindicated in acute cyanide poisoning. ### Treatment Algorithm ```mermaid flowchart TD A[Suspected Cyanide Poisoning]:::outcome --> B{Confirmed or High Suspicion?}:::decision B -->|Yes| C[Administer Sodium Nitrite IV]:::action C --> D[Induces Methemoglobinemia]:::outcome D --> E[Cyanide binds to Methemoglobin]:::outcome E --> F[Give Sodium Thiosulfate IV]:::action F --> G[Rhodanese converts CN⁻ to Thiocyanate]:::outcome G --> H[Thiocyanate excreted in urine]:::outcome H --> I[Clinical recovery]:::outcome B -->|No| J[Supportive care]:::action ``` [cite:Forensic Medicine & Toxicology, Reddy 34e Ch 19]