Obesity in women with asthma: Baseline disadvantage plus greater small-airway responsiveness

Background Obesity is known to diminish lung volumes and worsen asthma. However, mechanistic understanding is lacking, especially as concerns small-airway responsiveness. The objective of this study was therefore to compare small-airway responsiveness, as represented by the change in expiratory:inspiratory mean lung density ratios (MLDe/i, as determined by computed tomography [CT]) throughout methacholine testing in obese versus non-obese women with asthma. Methods Thoracic CT was performed during methacholine bronchoconstriction challenges to produce standardized response curves (SRC: response parameter versus ln[1 + % PD20], where PD20 is the cumulative methacholine dose) for 31 asthma patients (n = 18 non-obese and n = 13 obese patients). Mixed models evaluated obesity effects and interactions on SRCs while adjusting for age and bronchial morphology. Small airway responsiveness as represented by SRC slope was calculated for each third of the MLDe/i response and compared between groups. Results Obesity-associated effects observed during experimental bronchoconstriction included: (i) a significant baseline effect for forced expiratory volume in 1 second with lower values for the obese (73.11 +/- 13.44) versus non-obese (82.19 +/- 8.78; p = 0.002) groups prior to methacholine testing and (ii) significantly higher responsiveness in small airways as estimated via differences in MLDe/i slopes (groupxln(1 + % PD20 interaction; p = 0.023). The latter were pinpointed to higher slopes in the obese group at the beginning 2/3 of SRCs (p = 0.004 and p = 0.021). Significant obesity effects (p = 0.035 and p = 0.008) indicating lower forced vital capacity and greater % change in MLDe/I (respectively) throughout methacholine testing, were also observed. Conclusion In addition to baseline differences, small-airway responsiveness (as represented by the change in MLDe/i) during methacholine challenge is greater in obese women with asthma as compared to the non-obese.