Purpose: To compare the influence of underlying airway inflammation and lung hyperinflation on dyspnea during induced bronchoconstriction in subjects with mild asthma (or asymptomatic airway hyperresponsiveness (AAHR).
Methods: Fourteen mild asthmatic and 14 AAHR subjects had methacholine and 5'-adenosine monophosphate (AMP) challenges, and induced sputum analysis. Changes in inspiratory capacity (IC) and respiratory symptom scores were measured after challenges. Perception of respiratory symptoms was recorded on a modified Borg scale.
Results: The mean baseline FEV1, IC, mean provocative concentration of methacholine inducing a 20% decrease in FEV1 (PC20), the mean PC20 AMP and median inflammatory cell counts were similar in both groups. After methacholine, mean (+/-SD) reductions in FEV1 were 24.7+/-10.3% in mild asthma and 35.6+/-19.1% in AAHR (P>0.05); reductions in IC were, respectively, 10+/-12% and 24+/-20% (P>0.05); mean breathlessness scores at PC20 were 1.1 in mild asthma and 0 in AAHR P=0.003), and mean chest tightness scores were 1.2 in mild asthma and 0.8 in AAHR (P>0.05). Maximum chest tightness scores following MC correlated with the maximum decrease in IC in mild asthma (rs=0.75,P=0.009) and with the maximum decrease in FEV1 in AAHR (rs=0.60,P=0.04). After AMP, symptom scores were not significantly correlated with decreases in FEV1 or IC. The number of inflammatory cells was not correlated with decreases in IC after methacholine, AMP or with their PC20s, although inflammation was minimal in both groups.
Conclusion: Lower breathlessness scores in AAHR compared to mild asthma were not explained by differences in lung hyperinflation nor in airway inflammation.