Atropine is a competitive antagonist of muscarinic acetylcholine receptors.

In cardiovascular practice, it is used to treat symptomatic bradycardia by blocking parasympathetic (vagal) tone at the SA and AV nodes.

It increases heart rate by removing inhibitory cholinergic signaling.

Primary Target:

• M2 muscarinic receptors in the SA and AV nodes (see Muscarinic Receptors)

Normal Physiology:

• Vagal stimulation → M2 activation → ↓ cAMP → ↓ calcium influx → slowed heart rate

Atropine Effect:

• Blocks M2 receptors
• Prevents vagal inhibition
• ↑ cAMP
• ↑ SA node firing
• ↑ AV nodal conduction

Net Effect:

• ↑ Heart rate
• Improved AV conduction

Cardiovascular:

• Symptomatic sinus bradycardia
• Vagal-mediated AV nodal block

ACLS:

• First-line medication for unstable bradycardia (ACLS)

Other Uses:

• Organophosphate poisoning
• Pre-anesthetic reduction of secretions
• Ophthalmic mydriasis/cycloplegia

Adult (ACLS bradycardia):

• 1 mg IV push
• Repeat every 3–5 minutes
• Max total dose: 3 mg

Important:

• Doses < 0.5 mg may paradoxically worsen bradycardia

If ineffective:

• Proceed to pacing / pressor support (see Bradycardia Algorithm)

Absolute:

• None in emergent unstable bradycardia situations

Relative / Cautions:

• Narrow-angle glaucoma
• Severe tachyarrhythmias (see Dysrhythmias)
• Obstructive uropathy
• Ileus

Common (anticholinergic effects):

• Tachycardia
• Dry mouth
• Urinary retention
• Constipation
• Blurred vision
• Confusion (especially elderly)

Serious:

• Arrhythmias
• Hyperthermia (impaired sweating)

Mechanism-based:

• “Hot as a hare, dry as a bone, blind as a bat, mad as a hatter”

Additive anticholinergic effects with:

• Other antimuscarinics
• Tricyclic antidepressants (TCAs)
• Antipsychotics
• First-generation antihistamines

Bradycardia context / AV node:

• Use caution if combined with AV nodal blockers:
  • Beta-Blockers
  • Non-DHP Calcium Channel Blockers
  • Digoxin

• Continuous ECG
• Heart rate response
• Blood pressure
• Mental status (elderly)
• Signs of anticholinergic toxicity

• Atropine removes vagal tone — it does NOT increase contractility directly.
• Ineffective in infranodal block (e.g., Mobitz II / complete heart block distal to AV node).
• Avoid low doses (< 0.5 mg IV) due to paradoxical bradycardia.
• If no response, escalate quickly to pacing and/or pressors.

Opposite AV nodal effects compared to:

• Adenosine (transient AV nodal block)
• Beta-Blockers (↓ sympathetic tone)
• Non-DHP CCBs (↓ AV nodal conduction)

Atropine:

• ↑ SA node rate
• ↑ AV conduction
• Used for bradycardia, not tachyarrhythmias

• Dysrhythmias Module
• Bradycardia
• Adenosine
• Beta-Blockers
• Calcium Channel Blockers
• Cardiovascular Pharmacology