For a long time, muscarinic receptor-blocking drugs occupied a major place in the therapy of asthma, but they have been largely displaced by the adrenergic drugs (see Chapter 41). The problems associated with the use of antimuscarinic alkaloids in respiratory disorders are low therapeutic index and impaired expectoration. The latter is a consequence of their inhibition of mucous secretion, ciliary activity, and mucous transport.
Ipratropium bromide (Atrovent), in contrast, is a synthetic muscarinic blocking drug that has gained widespread use in recent years for the treatment of respiratory disorders. The drug is a quaternary ammonium compound, and it is applied topically to the airways through the use of a metered-dose inhaler. A substantial portion of the dose is swallowed, but absorption from the airways and gastrointestinal tract is negligible and most of the drug is eliminated in the feces. Consequently, systemic antimuscarinic effects are not observed with ipratropium. Dryness of the mouth, cough, and a bad taste have been reported by some patients, but the drug appears to have no other significant adverse effects. Ipratropium does not affect mucociliary transport or the volume and viscosity of sputum.
Clinical studies have demonstrated the effectiveness of ipratropium in chronic obstructive lung disease, for which it is equal or better in effectiveness than p2-adren-ergic agonists. Maximum bronchodilator responses to ipratropium develop in 1.5 to 2 hours. Consequently, it would be less suitable than a rapidly acting (3-adrenergic agonist in emergencies. Ipratropium is less effective than the p2-receptor agonists in asthma, but it may be useful when combined with other bronchodilators.
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