Pharm 101: Atropine

Class

Antimuscarinic agent


Pharmacodynamics
  • Atropine is a tertiary amine alkaloid ester of tropic acid
  • Competitive, reversible muscarinic ACh receptor antagonist
    • Binding prevents release of IP3 (inositol triphosphate), DAG (diacyglycerol), and inhibition of adenylyl cyclase, which are caused by muscarinic agonists
  • Equipotent at M1, M2, M3 receptors
  • No significant nicotinic effect

Organ system effects
  • CNS:
    • Sedation
    • Delirium
    • Decreased tremor in Parkinson’s disease
  • Eye:
    • Mydriasis (pupillary constrictor muscle depends on muscarinic cholinoceptor activation)
    • Cyclopegia (weakened contraction of ciliary muscle)
  • CVS:
    • Initial bradycardia at lower doses (blocks prejunctional M1 receptors)
    • Tachycardia
  • Resp: bronchodilation and decreased secretions
  • GIT:
    • Decreased secretions
    • Decreased gastric emptying
    • Decreased gut motility
    • Increased intestinal transit time
  • GUT:
    • Relaxation of ureteric and bladder wall smooth muscle
    • Urinary retention
  • Skin: decreased sweating

Pharmacokinetics
  • Route of administration: IV, PO, nebulised, topical
  • Well absorbed orally
  • Significant levels in CNS within 30-60 minutes, wide Vd
  • Two phases of elimination:
    • Rapid: half-life 2 hours
    • Slow: half-life 13 hours
  • Hepatic metabolism and subsequent renal excretion:
    • 40% phase I and phase II metabolism
    • 60% unchanged

Clinical uses
  • Symptomatic bradyarrhythmias / bradycardia
  • Opthalmological: mydriatic and cyclopegic
  • Drying of secretions e.g. cholinergic nerve agonist, OP poisoning, palliative care
  • Traveller’s diarrhoea

Adverse effects
  • Extension of organ system effects, typical antimuscarinic effects

Precautions
  • Glaucoma
  • Prostate hyperplasia
  • Gastric ulcers (slow gastric emptying thus worsen symptoms)

Further reading

References

Pharm 101 700

Pharmacology 101

Top 200 drugs

MBBS (UWA) CCPU Emergency Medicine Advanced Trainee based in Melbourne, Australia. Co-author of the LITFL ECG Library. Likes Ultrasound, Echo, ECGs, and anything and everything with caffeine. Part of the 2021 ANZCEN Clinician Educator Incubator Programme | @rob_buttner | ECG Library |

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