The major limitation in the use of the nonselective muscarinic antagonists is the failure to obtain desired therapeutic responses without concomitant side effects. Some selectivity and reduction in side effects have been achieved by local administration and by the use of minimally absorbed quaternary compounds. Subtype-selective muscarinic receptor antagonists hold the most promise for treating specific clinical symptoms, but few show absolute selectivity.
RESPIRATORY TRACT These drugs reduce secretion in both the upper and lower respiratory tracts. This effect in the nasopharynx may provide symptomatic relief of acute rhinitis associated with coryza or hay fever. The contribution of antihistamines employed in "cold" mixtures is likely due to their antimuscarinic properties, except in conditions with an allergic basis.
Systemic administration of belladonna alkaloids or their derivatives for bronchial asthma or COPD carries the disadvantage of reducing bronchial secretions and inspissation of the residual secretions. This viscid material is difficult to remove from the respiratory tree, and its presence can dangerously obstruct airflow and predispose to infection. By contrast, ipratropium and tiotropium, administered by inhalation, do not produce adverse effects on mucociliary clearance, and can be used safely in the treatment of airway disease (see Chapter 27).
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If you suffer with asthma, you will no doubt be familiar with the uncomfortable sensations as your bronchial tubes begin to narrow and your muscles around them start to tighten. A sticky mucus known as phlegm begins to produce and increase within your bronchial tubes and you begin to wheeze, cough and struggle to breathe.