Muscarinic receptor antagonists(Block of M3 receptors):
Synergistic effect with Beta2-Agonist - may be used if the patient presents with arrhythmias - in which selective muscarinic receptor antagonist is perferred
May be (A) non selective as atropine.
Atropine blocks the muscarinic receptors in airways leading to bronchodilatation through unopposed B2 actions
but has a lot of side effectse.g.:
- Dryness of the mouth.
- Skin flushing: children are more susceptible to develop coetaneous V.D which makes the child flushed.
- Retention of urine especially in patients with enlarged prostate.
- Acute attack of glaucoma in patients who have or susceptible to glaucoma.
- Constipation.
- Blurred vision is common adverse effects in all age groups.
- makes the sputum viscid and difficult to expel in asthmatic patients.
B) selective as (Ipratropium- Tiotropium)
Ipratropium is a quaternary antimuscarinic agent used by inhalation to reduce bronchoconstriction in asthma and chronic obstructive pulmonary disease (COPD). Although not as efficacious as beta-agonists, ipratropium is less likely to cause cardiac arrhythmias. It has very few antimuscarinic effects outside the lungs because it is( quaternary) i.e poorly absorbed and rapidly metabolized. Ipratropium is less effective than b2 adrenoceptor agonists so it is not administered alone.
In contrast to ipratropium, tiotropium has a longer bronchodilator action and can be given once daily (longer duration). Tiotropium reduces the incidence of COPD exacerbations.