Anti-cholinergics (aka antimuscarinics agents)
These are also usually inhaled, but can be given IV in hospital.
- They essentially bind to and block ACh receptors. ACh is the main neurotransmitter involved in muscle contraction thus by blocking its action you prevent muscle contraction.
- They are mainly used as an adjuvant to β2 agonists – they are not very effective on their own.
- They also reduce mucous secretion
- There are 3 types of muscarinic receptor, and these drugs act on them all – thus they are not selective.
- They are virtually never used in asthma, and far more commonly used in COPD. They may be used in asthma during severe exacerbations where β2 agonists are not having the desired effect.
- They have a slow onset of action (30-60m) compared to salbutamol, probably because they are poorly absorbed from the respiratory tract.
- The half-life is short (about 15 minutes) – and the half-life is due to the removal of the drug from its receptor, and not due to removal of the drug from the blood.
- Triatropium has a much longer half life of up to 3.5 hours.
- Ipratropium – this is the main drug that it used, and it is usually inhaled
- Tiotropium – this is longer acting
They are given again by inhalation. They are poorly absorbed into the blood stream and so have very little systemic effect.
- Dry mouth
- Urinary retention in men (triotropium – tends to occur in men who already have a large prostate)
- Can contribute to angle-closure glaucoma
However – generally there are few side effects due to the localised nature of administration – for example, the systemic drug atropine is associated with far greater side-effects.