Paracetamol (Acetaminophen) Poisoning and ER Management — Presentation Notes

1. Introduction / Epidemiology

• Paracetamol is the most common drug taken in overdose worldwide and remains the leading cause of acute liver failure in the Western world (up to 72% of patients with severe hepatotoxicity in some series progress to encephalopathy and coagulopathy) - Harrison's Principles of Internal Medicine, 22E.
• Widely available over the counter, often combined with opioids or cold/flu preparations, which increases risk of unintentional staggered overdose.

2. Dosage Thresholds (Forensic Medicine reference values)

3. Mechanism of Toxicity

• Paracetamol is absorbed rapidly (peak plasma level 30-60 min); plasma half-life ~2 hours therapeutically.
• ~90% is safely conjugated hepatically to glucuronide and sulfate metabolites.
• A small fraction is oxidized by CYP2E1 to the toxic metabolite N-acetyl-p-benzoquinone-imine (NAPQI).
• NAPQI is normally detoxified by binding to hepatic glutathione, forming harmless mercapturic acid excreted renally.
• In overdose, glutathione stores are depleted; excess NAPQI binds covalently to hepatocyte macromolecules ("protein adducts"), causing centrilobular hepatic necrosis.
• Potentiating factors: chronic alcohol use, enzyme inducers (phenobarbital, isoniazid), malnutrition/fasting (reduced glutathione reserve) - Harrison's, 22E.

4. Clinical Stages (classic 4-stage course)

5. ER Assessment

• Serum paracetamol level drawn at or after 4 hours post-ingestion (levels before 4 h are unreliable as absorption/distribution is incomplete)
• LFTs (AST, ALT, bilirubin), PT/INR, renal function, electrolytes, venous/arterial blood gas and lactate, glucose
• Paracetamol-protein adduct assays are an emerging diagnostic marker but not yet routine

6. ER Management

A. Decontamination

• Activated charcoal (1 g/kg PO/NG) if the patient presents within 1-2 hours of ingestion and can protect their airway. In massive ingestions (≥40 g) it can reduce levels/hepatotoxicity up to 4 hours post-ingestion.
• Not routinely recommended once past this window or if the patient is vomiting/obtunded, given the availability of a highly effective antidote (NAC) - ROSEN's Emergency Medicine.

B. Antidote - N-Acetylcysteine (NAC)

• Mechanism: replenishes glutathione stores, provides an alternative substrate for NAPQI conjugation, and may improve microcirculation/oxygen delivery in established hepatotoxicity.
• Indications:
  • Serum level above the treatment line on the nomogram
  • Any elevated LFTs after supratherapeutic/chronic ingestion
  • Unknown time of ingestion with detectable paracetamol and abnormal LFTs
  • Massive ingestion or history concerning for a potentially toxic dose while awaiting levels
• Timing is critical: NAC is highly effective when started within 8 hours of ingestion (risk of AST >1000 IU/L is <1% and mortality approaches zero). Efficacy falls progressively if started between 8-24 hours, but it should still be given even late or once hepatotoxicity/liver failure has developed.
• Route: Oral and IV NAC are both effective; IV is generally preferred in the ER (vomiting, need for rapid protocol, altered mental status, pregnancy).
• Typical IV regimen (traditional 3-bag/21-hour "Prescott" protocol):
  • 150 mg/kg over 1 hour, then
  • 50 mg/kg over 4 hours, then
  • 100 mg/kg over 16 hours
  • Newer simplified 2-bag regimens (e.g., 200 mg/kg over 4 h then 100 mg/kg over 16 h) are increasingly used and appear comparably safe with fewer dosing errors and fewer anaphylactoid reactions - a 2023 systematic review (PMID 38165196) found two-bag regimens have similar efficacy with a possibly lower rate of adverse reactions compared with three-bag regimens, though further high-quality trials are still wanted.
• Adverse effects: anaphylactoid reactions (flushing, urticaria, bronchospasm, rarely hypotension), especially with the rapid loading dose - manage by slowing the infusion and giving antihistamines rather than stopping NAC.
• Duration is generally continued until INR normalizes, transaminases are falling, and the patient is clinically well; NAC infusion may need to be extended beyond the standard course in patients with ongoing hepatic injury.

C. Enhanced Elimination

• Hemodialysis is not routine but is indicated in massive ingestions per EXTRIP consensus criteria (Box 138.2, Rosen's Emergency Medicine):
  • Serum paracetamol >1000 mcg/mL and NAC not yet given
  • Altered mental status + metabolic acidosis + elevated lactate + level >700 mcg/mL without NAC
  • Same triad + level >900 mcg/mL even with NAC administered
  • NAC infusion should be continued during dialysis (dose should be increased since NAC itself is dialyzed).

D. Supportive Care

• IV fluids, correction of hypoglycemia and electrolyte/acid-base disturbances
• Monitor for coagulopathy, renal impairment (paracetamol can cause direct nephrotoxicity independent of liver failure), and cardiac injury in severe cases
• Serial LFTs, INR, renal function, glucose, and lactate
• Psychiatric evaluation for all intentional overdoses once medically stabilized

7. Special Situations

• Staggered/chronic supratherapeutic ingestion: nomogram not applicable; treat based on total 24-hour dose and any abnormal labs/detectable level - lower threshold for starting NAC.
• Pregnancy: NAC is safe and effective in pregnant patients with paracetamol overdose; no teratogenic effects reported - ROSEN's Emergency Medicine.
• Pediatric ingestion: activated charcoal may avoid the need for NAC in asymptomatic accidental ingestions if given early.
• Co-ingestants (opioids, alcohol, other analgesics) common - screen and manage accordingly.

8. Indications for Liver Transplant Referral - King's College Criteria (Paracetamol)

• Arterial pH <7.30, OR
• All three of: INR >6.5 (PT >100 s), serum creatinine ≥3.4 mg/dL (301 μmol/L), and grade III/IV hepatic encephalopathy
• Hyperlactatemia or hyperphosphatemia are additional strong predictors of poor prognosis without transplant

9. Disposition

• Asymptomatic patients with a non-toxic level (below treatment line) at 4 hours and normal LFTs can be medically cleared for psychiatric evaluation/discharge.
• Any patient started on NAC needs admission for serial monitoring until treatment endpoints are met.
• Rising INR, encephalopathy, worsening renal function, or acidosis warrants early transfer to a liver transplant center - don't wait for frank liver failure.

10. Suggested Slide Breakdown for Your Presentation

1. Title/Epidemiology
2. Pharmacokinetics & Mechanism of Hepatotoxicity (NAPQI pathway diagram)
3. Toxic/Lethal Dose Thresholds
4. Clinical Stages (4-stage timeline)
5. ER Assessment & Rumack-Matthew Nomogram
6. Decontamination
7. NAC - Indications, Regimens, Adverse Effects
8. Enhanced Elimination (Hemodialysis - EXTRIP criteria)
9. Special Populations (staggered dosing, pregnancy, pediatrics)
10. King's College Criteria & Transplant Referral
11. Disposition Algorithm / Summary Flowchart