Antidotes in Emergency Medicine

Antidote Essentials - Toxin Terminators

• Definition: Agent nullifying poison effects.
• Mechanisms:
  • Inert complex formation (e.g., chelators + metals)
  • Receptor competition (e.g., naloxone vs. opioids)
  • Accelerated detoxification (e.g., N-acetylcysteine for paracetamol)
  • Blocked toxic metabolite formation (e.g., fomepizole for ethylene glycol)
  • Physiological antagonism (e.g., glucagon for beta-blockers)
• Use Principles: ABCs first! Timeliness crucial. Know indications & contraindications.

  ⭐ N-acetylcysteine (NAC) for paracetamol poisoning is most effective if given within 8-10 hours post-ingestion.

Opioids & Benzos - Wake‑Up Calls

• Opioids (Triad: Miosis, Resp Depression, CNS Depression):
  • Antidote: Naloxone (Narcan)
  • MOA: Competitive opioid receptor antagonist.
  • Dose: 0.4-2 mg IV/IM/SC; repeat q2-3min. Max 10 mg.
  • 📌 "NarCAN" can reverse opioid overdose.
• Benzodiazepines (Sedation, Ataxia, Slurred Speech):
  • Antidote: Flumazenil (Anexate)
  • MOA: Competitive GABA-A (BZD site) antagonist.
  • Dose: 0.2 mg IV, then 0.3-0.5 mg q1min. Max total 3 mg.
  • ⚠️ Risk of seizures in chronic BZD users or TCA/pro-convulsant co-ingestion.

⭐ Flumazenil: best for reversing procedural sedation; cautious in OD due to seizure risk.

Paracetamol & OPs - Damage Control

Paracetamol (PCM) Poisoning:

• Toxic metabolite: NAPQI. Antidote: N-acetylcysteine (NAC).
• NAC: Replenishes glutathione; best if given <8-10h.
• Indications: Level > Rumack-Matthew line; or ingestion >150 mg/kg (>7.5g) & levels delayed >8h.
• IV NAC: 150 mg/kg (1h) → 50 mg/kg (4h) → 100 mg/kg (16h). Total 21h.

  ⭐ King's College Criteria predict need for liver transplant in PCM-induced fulminant hepatic failure.

Organophosphate (OP) Poisoning:

• Mechanism: Irreversible AChE inhibition → ↑ACh.
• Symptoms: 📌 Muscarinic: DUMBELS. Nicotinic: MTWTF.
• Antidotes:
  • Atropine: Muscarinic antagonist. 1-2 mg IV, repeat q5-10min until atropinization (↓secretions, HR >80).
  • Pralidoxime (PAM): AChE reactivator. Give early. 30 mg/kg IV, then 8-10 mg/kg/hr.
• 💡 Intermediate syndrome: Muscle weakness 24-96h post-exposure, affects respiratory muscles.

Metabolic & Cardiac Tox - System Savers

• Methanol & Ethylene Glycol (MEG):
  • Antidotes: Fomepizole (inhibits alcohol dehydrogenase; 15 mg/kg load, 10 mg/kg q12h); Ethanol (competes ADH; target 100-150 mg/dL).
  • Clues: ↑AGMA, ↑Osmolal gap. Methanol → visual sx; EG → renal failure.
  • Hemodialysis: Severe acidosis, end-organ damage, high levels.
• Cyanide:
  • Antidotes: Hydroxocobalamin (5g IV adult, forms cyanocobalamin) - preferred; Na Thiosulfate (sulfur donor); Nitrites (induce MetHb ~10-20%) ⚠️MetHb, hypotension.
  • Clues: Rapid collapse; bitter almond odor (variable); severe lactic acidosis.
• Digoxin Toxicity:
  • Antidote: Digoxin-specific Ab fragments (DigiFab).
  • Indications: Life-threatening arrhythmias, K+ > 5.5 mEq/L, ingestion >10mg (adult)/>4mg (child), end-organ dysfunction.

  ⭐ Hyperkalemia (K+ > 5.5 mEq/L) is a key indication for DigiFab in digoxin toxicity.

• Beta-Blocker (BB) & Calcium Channel Blocker (CCB) Overdose:
  • Manifestations: Bradycardia, hypotension. BBs: bronchospasm. CCBs: hyperglycemia.
  • Key Interventions:
    • Glucagon: 5-10 mg IV, then 1-5 mg/hr (↑cAMP, bypasses β-receptors).
    • Calcium salts (Chloride/Gluconate): Esp. for CCB, improves inotropy/BP.
    • High-Dose Insulin Euglycemic Therapy (HIET): Insulin 1 U/kg bolus, then 0.5-1 U/kg/hr + Dextrose. Monitor glucose, K+. (Improves myocardial glucose uptake, inotropy).
    • Vasopressors (Norepinephrine); Lipid emulsion (refractory).

High‑Yield Points - ⚡ Biggest Takeaways

• Naloxone reverses opioid overdose; repeat dosing may be needed due to shorter half-life.
• Flumazenil for benzodiazepine overdose; use cautiously due to seizure risk in chronic users.
• N-acetylcysteine (NAC) is crucial for paracetamol poisoning, most effective within 8 hours.
• Atropine and Pralidoxime (PAM) are cornerstones for organophosphate poisoning.
• Sodium bicarbonate alkalinizes urine for salicylate poisoning and narrows QRS in TCA overdose.
• Fomepizole or ethanol inhibits alcohol dehydrogenase in methanol/ethylene glycol poisoning.
• Glucagon for beta-blocker overdose; calcium salts for calcium channel blocker overdose.