Antimuscarinics, cardiovascular and gastrointestinal uses

Key Examples

• Atropine
• Hyoscine butylbromide
• Glycopyrronium 

 

Common indications

1. Atropine is used first-line in the management of severe or symptomatic bradycardia to increase heart rate.
2. Antimuscarinics (particularly hyoscine butylbromide) are a first-line pharmacological treatment option for irritable bowel syndrome (IBS), where they are used for their antispasmodic effect.
3. In the care of the dying patient, antimuscarincs (e.g. hyoscine butylbromide) may have a role in reducing copious respiratory secretions.

 

Mechanisms of action

• Antimuscarinic drugs bind to the muscarinic receptor, where they act as a competitive inhibitor of acetylcholine. Stimulation of the muscarinic receptor brings about a wide range of parasympathetic ‘rest and digest’ effects. In blocking the receptor, antimuscarinics have the opposite effects: they increase heart rate and conduction; reduce smooth muscle tone and peristaltic contraction, including in the gut and urinary tract; and reduce secretions from glands in the respiratory tract and gut. In the eye they cause relaxation of the pupillary constrictor and ciliary muscles, causing pupillary dilatation and preventing accommodation, respectively.

 

Important adverse effects

• Predictably from their antagonism of parasympathetic ‘rest and digest’ effects, antimuscarinics can cause tachycardia, dry mouth and constipation. By reducing detrusor muscle activity, they can cause urinary retention in patients with benign prostatic hypertrophy. The ocular effects may cause blurred vision, especially for near objects. Some antimuscarinics (including atropine) have central effects, which may precipitate drowsiness and confusion, particularly in the elderly.

 

Warnings

• Antimuscarinics should be used with caution in patients susceptible to angle-closure glaucoma, in whom they can precipitate a dangerous rise in intraocular pressure. 
• They should generally be avoided in patients at risk of arrhythmias (e.g. those with significant cardiac disease), unless the indication for use is bradycardia.

 

Important interactions

• Adverse effects are more pronounced when they are combined with other drugs that have antimuscarinic effects, such as tricyclic antidepressants.