A 32-year-old woman is admitted 20 minutes after intentional ingestion of potassium cyanide (KCN) in a suicide attempt. She is unconscious, with fixed dilated pupils, generalized seizures, and profound bradycardia (HR 38/min). Skin is cyanotic despite supplemental oxygen at 15 L/min via non-rebreather mask. Blood pressure is 78/52 mmHg. Venous blood gas shows SvO₂ (mixed venous oxygen saturation) of 88% (normal 70–75%). Which antidote should be administered first as part of the immediate management?

Explanation

## Antidote Selection in Acute Cyanide Poisoning ### Why Hydroxocobalamin Is First-Line **Key Point:** Hydroxocobalamin (vitamin B₁₂ derivative) is the **preferred first-line antidote** for cyanide poisoning in most modern toxicology protocols, especially in severe acute poisoning with hemodynamic instability. **High-Yield:** Hydroxocobalamin directly binds cyanide ions, forming cyanocobalamin (which is non-toxic and renally excreted). It works rapidly, has minimal side effects, and does not cause hypotension. ### Mechanism of Action ``` CN⁻ + Hydroxocobalamin → Cyanocobalamin (excreted in urine) ``` - **Onset**: Immediate - **Dose**: 70 mg/kg IV (typically 5–10 g for adults) - **Advantage**: No risk of iatrogenic hypotension; can be used in hemodynamically unstable patients - **Limitation**: Requires large IV volumes; may turn urine and skin red-orange ### Why This Patient's Presentation Demands Hydroxocobalamin This patient has: - **Severe hemodynamic collapse** (BP 78/52, HR 38) - **Elevated SvO₂ (88%)** — hallmark of cyanide: venous blood remains oxygenated because cells cannot extract oxygen - **Seizures and fixed pupils** — profound CNS depression Sodium nitrite (the alternative in the cyanide antidote kit) **causes vasodilation and hypotension**, which would worsen her shock. Hydroxocobalamin is safer in this context. ### Comparison of Cyanide Antidotes | Antidote | Mechanism | Onset | Hemodynamic Effect | Use Case | |----------|-----------|-------|-------------------|----------| | **Hydroxocobalamin** | Binds CN⁻ directly | Immediate | No hypotension | **First-line, especially in shock** | | **Sodium nitrite** | Induces methemoglobinemia to bind CN⁻ | 30 min | Vasodilation → hypotension | When hydroxocobalamin unavailable | | **Sodium thiosulfate** | Provides sulfur for rhodanese enzyme | Slow (30–60 min) | None | Adjunct only; never monotherapy | | **Methylene blue** | Reduces methemoglobin | Moderate | Minimal | Not for cyanide; used for methemoglobinemia | **Clinical Pearl:** In the classic "cyanide kit," sodium nitrite + sodium thiosulfate was the standard. Modern practice favors hydroxocobalamin as monotherapy or as the first agent in combination regimens, because it avoids iatrogenic hypotension. ### Supportive Measures Concurrent with Antidote 1. **Airway management** — intubation and hyperventilation (to blow off CO₂ and reduce acidosis) 2. **IV fluids and vasopressors** — for shock (norepinephrine preferred) 3. **Seizure control** — benzodiazepines 4. **Activated charcoal** — if ingestion and airway protected **Mnemonic:** **HCB-First** = **H**ydroxocobalamin, **C**yanide, **B**est-first antidote.