Which of the following is the most specific and effective antidote for acute cyanide poisoning?

Explanation

## Antidotes for Cyanide Poisoning **Key Point:** Hydroxocobalamin is the most specific, effective, and safest antidote for acute cyanide poisoning. It directly binds cyanide ions and converts them to non-toxic cyanocobalamin (vitamin B12), which is excreted in urine. ### Mechanism of Hydroxocobalamin 1. **Direct binding**: Hydroxocobalamin has a cobalt atom with a high affinity for cyanide ions 2. **Conversion**: CN⁻ + Hydroxocobalamin → Cyanocobalamin (B12) 3. **Elimination**: Cyanocobalamin is water-soluble and readily excreted by the kidneys 4. **Speed**: Works rapidly, within minutes of IV administration **High-Yield:** Hydroxocobalamin is preferred over the traditional nitrite-thiosulfate combination because: - It does NOT induce methemoglobinemia (unlike sodium nitrite) - It is directly effective without a second agent - It is safer in patients with concurrent carbon monoxide poisoning - FDA-approved for acute cyanide poisoning ### Comparison of Antidote Regimens | Antidote | Mechanism | Advantages | Disadvantages | |----------|-----------|-----------|---------------| | **Hydroxocobalamin** | Binds CN⁻ directly | Specific, safe, no methemoglobinemia | Expensive, requires IV access | | **Sodium nitrite + thiosulfate** | Nitrite induces methemoglobin (binds CN⁻); thiosulfate converts CN⁻ to thiocyanate | Rapid, synergistic | Risk of methemoglobinemia, hypotension | | **Sodium thiosulfate alone** | Converts CN⁻ to thiocyanate via rhodanese | Slow, safer than nitrite | Requires high doses, slower onset | | **Activated charcoal** | Adsorbs cyanide in GI tract | Useful in ingestion | Only for oral poisoning, not systemic | **Clinical Pearl:** In modern practice, hydroxocobalamin has largely replaced the nitrite-thiosulfate regimen as first-line therapy because it is more specific and avoids the risk of inducing dangerous methemoglobinemia. **Mnemonic:** **COBRA** = **C**yanide + **O**xidized **B**12 (hydroxocobalamin) → **RA**pid antidote