Learning Objective
By the end of this section, the student will be able to describe the pharmacologic actions, clinical uses, and adverse effects of atropine, a prototypical muscarinic antagonist, and apply these concepts to USMLE Step 1–style questions.
Atropine
Atropine is a competitive muscarinic receptor antagonist that inhibits parasympathetic (cholinergic) activity at muscarinic receptors. It is commonly used in the treatment of bradycardia and for ophthalmic applications.
Organ System Effects of Atropine
| Organ System | Action |
|---|---|
| Eye | ↑ Pupillary dilation (mydriasis) and cycloplegia |
| Airway | Bronchodilation; ↓ bronchial secretions |
| Heart | ↑ Heart rate (vagolytic effect) |
| Stomach | ↓ Gastric acid secretion |
| Gut | ↓ Gastrointestinal motility |
| Bladder | ↓ Urinary urgency (useful in cystitis) |
Key Point:
Atropine blocks muscarinic effects of anticholinesterase poisoning (DUMBBELSS) but does NOT reverse nicotinic effects (e.g., neuromuscular blockade).
Adverse Effects of Atropine
Atropine toxicity reflects excessive antimuscarinic activity:
- ↑ Body temperature (due to ↓ sweating)
- Tachycardia
- Dry mouth
- Dry, flushed skin
- Cycloplegia and blurred vision
- Constipation
- Urinary retention
- CNS effects: confusion, agitation, disorientation
High-risk populations:
- Older adults: may precipitate acute angle-closure glaucoma (due to mydriasis)
- Men with BPH: urinary retention
- Infants: risk of hyperthermia
Classic Antimuscarinic Toxicity Mnemonic
- Hot as a hare (hyperthermia)
- Fast as a fiddle (tachycardia)
- Dry as a bone (anhidrosis, dry mouth)
- Red as a beet (flushed skin)
- Blind as a bat (mydriasis, cycloplegia)
- Mad as a hatter (delirium)
- Full as a flask (urinary retention)
Clinical Correlation
Jimson weed (Datura) exposure → antimuscarinic toxicity with marked mydriasis (“gardener’s pupil”)
