Acetaminophen Poisoning


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Acetylcysteine for Acetaminophen Poisoning N Engl J Med. July 17, 2008;359:285-92. CLINICAL PRACTICE

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The Clinical Problem According to poison centers in the US, acetaminophen poisoning was responsible for more than 70,000 visits to health care facilities and approximately 300 deaths in 2005. Acetaminophen poisoning can be due to ingestion of a single overdose (usually as an attempt at self-harm) or ingestion of excessive repeated doses or too-frequent doses, with therapeutic intent.

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4 Stages of Acetaminophen Poisoning Preclinical toxic effects (normal ALT) Hepatic injury (elevated ALT) Hepatic failure (hepatic injury with hepatic encephalopathy) Recovery

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N-Acetylcysteine and Acetaminophen Poisoning N-acetylcysteine prevents hepatic injury primarily by restoring hepatic glutathione. The primary pathways for acetaminophen metabolism are glucuronidation and sulfation to nontoxic metabolites. Approximately 5% of a therapeutic dose is metabolized by cytochrome P450 2E1 to the electrophile N-acetyl-p-benzoquinone imine (NAPQI). NAPQI is extremely toxic to the liver. Ordinarily, NAPQI is rapidly detoxified by interaction with glutathione to form cysteine and mercapturic acid conjugates. If glutathione is depleted, NAPQI interacts with various macromolecules, leading to hepatocyte injury and death. As long as sufficient glutathione is present, the liver is protected from injury.

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Pathophysiology and Effect of Therapy Overdoses of acetaminophen can deplete hepatic glutathione stores and allow liver injury to occur. Acetylcysteine prevents hepatic injury primarily by restoring hepatic glutathione.

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Clinical Use It is prudent to administer acetylcysteine to any patient who may have taken an overdose and who has a measurable acetaminophen concentration until risk stratification can be performed (i.e., until the time of ingestion is determined), until the patient meets the criteria for stopping, or until the course is completed. Treatment for any patient with an elevated ALT and a history of ingesting more than 4 g of acetaminophen per day.

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The Rumack–Matthew Nomogram

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Dosing of N-Acetylcysteine Current FDA-approved protocols for treatment of acute acetaminophen ingestion, oral acetylcysteine is loading dose 140 mg/kg, with maintenance doses 70 mg/kg Q4H for 17 doses. IV loading dose is 150 mg/kg over 15 to 60 minutes, followed by infusion 50 mg/kg over 4 hours, and finally infusion 100 mg/kg over 16 hours.

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Many toxicologists would recommend repeating the measurements of ALT and acetaminophen concentrations as the patient approaches the end of the 16-hour infusion period and continuing treatment if the ALT is elevated or if the acetaminophen concentration is measurable.

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OPD therapy may be considered for patients with a confirmed accidental, repeated supratherapeutic ingestion, supra-therapeutic acetaminophen concentration (threshold of <70 µg /ml), and low-grade elevation of the ALT (<3 times the upper limit of the reference). The patient can be discharged with 3 maintenance doses to be taken every 4 hours and be re-evaluated 12 hours after the loading dose. Treatment can be discontinued if the patient meets the criteria for stopping therapy (i.e., the ALT is decreasing, and acetaminophen concentration is undetectable).

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Patients receiving intravenous acetylcysteine for liver failure should be hospitalized in ICU. Treatment is continued until the hepatic encephalopathy resolves and the ALT and creatinine and INR have substantially improved or until the patient receives a liver transplant.

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What are the side effects of N-acetylcysteine? N-acetylcysteine has an unpleasant smell and taste, and vomiting is common with oral administration. The most commonly reported adverse effects of intravenous N-acetylcysteine are anaphylactoid reactions (15%), including rash, pruritus, angioedema, bronchospasm, tachycardia, and hypotension.

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Guidelines The United Kingdom National Health Service guideline recommends treating patients who have acute acetaminophen overdose and a acetaminophen concentration above the probable-toxicity line (the line that begins at 200 µg/ml at 4 hours after ingestion) and high-risk (alcoholic or malnourished) patients who have a concentration above high-risk line that begins at 100 µg/ml at 4 hours.

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Guidelines The American College of Emergency Physicians recommends acetylcysteine therapy for any patient with acute acetaminophen ingestion and a timed serum concentration above the line that begins at 150 µg/ml at 4 hours, as well as for any patient with liver injury or liver failure.

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Thanks for Your Attention

Summary: Acetylcysteine for Acetaminophen Poisoning N Engl J Med. July 17, 2008;359:285-92.

Tags: toxicology