U01.05.022 Atropine

Atropine is a muscarinic antagonist widely used in medicine for its effects on multiple organ systems. It is primarily used to treat bradycardia and in ophthalmic applications, such as pupil dilation.

Mechanism of Action

Atropine selectively blocks muscarinic acetylcholine receptors, thereby inhibiting parasympathetic nervous system effects. It does not block nicotinic receptors.

Effects by Organ System

Organ System	Action	Details
Eye	Pupil dilation (mydriasis), cycloplegia	Used in ophthalmology to prevent accommodation; blocks muscarinic effects.
Airway	Bronchodilation, ↓ secretions	Useful in reducing airway secretions during anesthesia.
Stomach	↓ Acid secretion	May reduce gastric acid production.
Gut	↓ Motility	Can result in delayed gastric emptying and reduced bowel movements.
Bladder	↓ Urgency in cystitis	Relaxes bladder smooth muscle, reducing urgency.

Adverse Effects

Common Side Effects

Hyperthermia: Elevated body temperature due to decreased sweating.
Increased Heart Rate: Tachycardia as parasympathetic inhibition occurs.
Dryness: Dry mouth and skin due to reduced secretions.
Vision Issues: Cycloplegia (loss of accommodation), mydriasis, and possible blurry vision.
Constipation: Reduced gut motility.
Disorientation: Cognitive effects, especially in elderly patients.

Severe Side Effects

Acute Angle-Closure Glaucoma: Triggered by mydriasis in predisposed individuals.
Urinary Retention: Especially problematic in men with prostatic Hyperplasia.
Hyperthermia in Infants: Requires cautious use in young children.

Mnemonic for Side Effects

To remember atropine’s side effects, use this mnemonic:

Hot as a Hare: Hyperthermia.
Fast as a Fiddle: Tachycardia.
Dry as a Bone: Dry mouth and skin.
Red as a Beet: Flushed skin.
Blind as a Bat: Mydriasis and cycloplegia.
Mad as a Hatter: Disorientation or confusion.
Full as a Flask: Urinary retention.

Additional Note

Exposure to Jimson weed (Datura), a plant containing atropine-like alkaloids, can lead to “gardener’s pupil” (mydriasis).

Points to Remember

Atropine is a key muscarinic antagonist used in emergency and ophthalmologic settings.
It blocks parasympathetic effects while sparing nicotinic receptors.
Use caution in elderly patients and those with certain predisposing conditions like glaucoma or prostatic Hyperplasia.
Side effects are dose-dependent and can be remembered with the mnemonic.

References

First Aid for the USMLE Step 1, Pharmacology Section.
Katzung’s Basic and Clinical Pharmacology.

USMLE Step 1 Review Series

Curriculum