Antimuscarinics, bronchodilators

Key Examples

• Ipratropium
• Tiotropium
• Glycopyrronium 

 

Common indications

1. In chronic obstructive pulmonary disease (COPD), short-acting antimuscarinics are used to relieve breathlessness, e.g. brought on by exercise or during exacerbations. Long-acting antimuscarinics (LAMAs) are used to prevent breathlessness and exacerbations.
2. In asthma, short-acting antimuscarinics are used as adjuvant treatment for relief of breathlessness during acute exacerbations (added to a short-acting β2 agonist, e.g. salbutamol). Long-acting antimuscarinics are added to high-dose inhaled corticosteroids and long-acting β2 agonists at ‘step 4’ in the treatment of chronic asthma.

 

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, including in the respiratory tract; and reduce secretions from glands in the respiratory and gastrointestinal tracts. In the eye they cause relaxation of the pupillary constrictor and ciliary muscles, causing pupillary dilatation and preventing accommodation, respectively.

 

Important adverse effects

• When antimuscarinic bronchodilators are taken by inhalation, there is relatively little systemic absorption. 
• Adverse effects, apart from dry mouth, are uncommon.

 

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 be used with caution in patients with or at risk of arrhythmias. 
• However, in practice, most patients can take these drugs by inhalation without major problems.

 

Important interactions

• Interactions are not generally a problem due to low systemic absorption.