There are several possible explanations that could account for the absence of change in the observed response postexercise in patients previously exposed to ozone. The results could imply factors related to the following: (1) the physiopathology of exercise-induced asthma bronchoconstriction and ozone exposure; (2) mediators; (3) airway epithelial permeability; and (4) the production of oxygen radicals. Each of these issues will be addressed to support the interpretation of our data. flovent inhalers
Exercise-induced bronchoconstriction in asthmatic patients is a well-documented and extensively studied phenomenon. Respiratory heat loss and respiratory water loss have been proposed as current theories to explain this typical response in asthmatics. However, the cellular events accounting for a thermal or osmotic stimulus producing bronchospasm are at present unclear.
For clinical purposes, a significant fall in FEVi after exercise can be used to diagnose the presence of exercise-induced asthma.20 The rationale for exposing asthmatics to a low concentration of ozone was to reproduce the same environmental conditions that might be encountered in a summertime urban atmosphere. Another important issue was how long this exposure must be to produce a measurable change in lung function. Changes in hyperresponsiveness to methacholine after ozone exposure at the concentration used in this study have been reported only after an exposure time of 6.6 h with moderate exercise during exposure. Since the asthmatic response to exercise challenge correlates with methacholine responsiveness,21 it is possible that longer ozone exposure with a concurrent increase in minute ventilation may be needed to show such a change in subsequent exercise challenge. However, this is not a practical experimental design since exercise during this exposure is likely to induce refractoriness to subsequent exercise challenge. The alternative would be to use high concentrations of ozone at rest but this does not address directly the effects of ambient concentrations.