Toxicology – Antidotes: Atropine & Pralidoxime (2-PAM)

Indications

Atropine and pralidoxime are used for cholinergic poisoning, especially from:

• Organophosphate insecticides
• Nerve agents
• Selected carbamate insecticides

These poisonings result from excessive acetylcholine activity caused by inhibition of acetylcholinesterase.

Mechanism of Action – Atropine

Atropine is a competitive muscarinic acetylcholine receptor antagonist.

It blocks the effects of excess acetylcholine at muscarinic receptors and is especially important for treating:

• Excess bronchial secretions
• Bronchospasm
• Bradycardia
• Other muscarinic manifestations

Atropine does not reverse the underlying acetylcholinesterase inhibition and has limited effect on skeletal muscle weakness.

Mechanism of Action – Pralidoxime (2-PAM)

Organophosphates bind to and inhibit acetylcholinesterase, causing acetylcholine to accumulate.

Pralidoxime can reactivate the inhibited enzyme by removing the organophosphate from acetylcholinesterase, provided this is given before the enzyme-toxin complex undergoes “aging.”

Aging refers to a chemical change that makes the organophosphate–acetylcholinesterase bond effectively irreversible.

Pralidoxime is particularly helpful for nicotinic and neuromuscular effects, including:

• Muscle weakness
• Fasciculations
• Respiratory muscle paralysis

Management

Treatment priorities include:

• Airway and respiratory support
• Rapid atropine administration for significant secretions and respiratory compromise
• Pralidoxime for suspected organophosphate toxicity
• Benzodiazepines for seizures, marked agitation, or severe muscle activity
• Appropriate decontamination while protecting healthcare personnel from secondary exposure

Atropine may need to be repeatedly administered and titrated to improvement in airway secretions and ventilation, rather than to a fixed total dose.

Carbamate Poisoning

Carbamates generally inhibit acetylcholinesterase reversibly and for a shorter duration than organophosphates.

• Atropine remains important
• The benefit of pralidoxime is less certain, but it may be considered in severe or unclear cholinergic poisoning when organophosphate exposure cannot be excluded

Key Points

• Atropine treats muscarinic symptoms, especially excessive secretions and bronchospasm.
• Pralidoxime reactivates acetylcholinesterase before aging occurs.
• Early pralidoxime is most important in significant organophosphate poisoning.
• Severe cases may require unusually large cumulative amounts of atropine.
• Benzodiazepines are used when seizures or severe CNS manifestations occur.