Bronchial challenge tests: usefulness, availability and limitations

Indirect challenges to assess bronchial hyperresponsiveness are being increasingly used both for research and for assessment in the routine pulmonary function laboratory. ‘‘Indirect challenges act by causing the release of endogenous mediators that cause the airway smooth muscle to contract with or without inducing microvascular leakage. Because the responses to these challenges are modified or completely inhibited by inhaled steroids, the airway response to the challenges may be a closer reflection of active airway inflammation’’ (fig. 1) [1]. In contrast, the direct tests, inhaled aerosols of histamine and methacholine act directly on receptors on the bronchial smooth muscle to cause contraction. While sensitive for detecting bronchial hyperresponsiveness a positive response to these agents is not specific for identifying asthma particularly at the higher concentrations. The best known of the indirect challenges is exercise. Other indirect challenges include eucapnic voluntary hyperpnoea, mannitol, adenosine monophosphate and hyperosmolar saline. All these indirect challenges are associated with release of mediators of bronchoconstriction. For this reason they have the potential to be used to identify presence of inflammatory cells particularly mast cells [2].