Background: The purpose of this study was to use CT scanning with computational fluid dynamics to evaluate the mechanisms by which Bronchial Thermoplasty (BT) improves asthmatic symptoms. Methods: The study was conducted in a university teaching hospital, experienced in performing BT. Imaging studies were performed before, and after, BT of the left lung, and prior to treatment of the right lung, which therefore acted as a control. On each occasion, two high-resolution CT scans were performed, one at full inspiration (TLC) and the other at Functional Residual Capacity (FRC). The study protocol was offered to 10 patients, all of whom met the definition of severe asthma, despite high dose inhaled corticosteroids and dual long acting bronchodilators. Results: Significant increases in airway luminal volume were observed on the treated side, compared with control, at both full inspiration (by 27%) and at FRC (by 17%). The ratio of distal airway volume to lung volume significantly increased on the treated side. The change in airway volume with inspiration from FRC increased by 48% on the treated side compared to 5% in the control lung, suggesting treatment increased airway distensibility. No effect was observed on airway wall thickness, nor air trapping. There was a trend towards correlation between the improvement in airway volume at TLC and improvement in symptoms. Conclusion: This study demonstrates that BT increases the luminal airway volume on the treated side compared to the control lung. We suggest that this is an important link between the airway smooth muscle atrophy demonstrated pathologically, and the improvement in symptoms observed clinically.

BackgroundIn randomized controlled trials, bronchial thermoplasty (BT) has been proven to reduce symptoms in severe asthma, but the mechanisms by which this is achieved are uncertain as most studies have shown no improvement in spirometry. We postulated that BT might improve lung mechanics by altering airway resistance in the small airways of the lung in ways not measured by FEV1. This study aimed to evaluate changes in measures of gas trapping by body plethysmography.MethodsA prospective cohort of 32 consecutive patients with severe asthma who were listed for BT at two Australian university hospitals were evaluated at three time points, namely baseline, and then 6 weeks and 6 months post completion of all procedures. At each evaluation, medication usage, symptom scores (Asthma Control Questionnaire, ACQ-5) and exacerbation history were obtained, and lung function was evaluated by (i) spirometry (ii) gas diffusion (KCO) and (iii) static lung volumes by body plethysmography.ResultsACQ-5 improved from 3.0 ± 0.8 at baseline to 1.5 ± 0.9 at 6 months (mean ± SD, p < 0.001, paired t-test). Daily salbutamol usage improved from 8.3 ± 5.6 to 3.5 ± 4.3 puffs per day (p < 0.001). Oral corticosteroid requiring exacerbations reduced from 2.5 ± 2.0 in the 6 months prior to BT, to 0.6 ± 1.3 in the 6 months after BT (p < 0.001). The mean baseline FEV1 was 57.8 ± 18.9%predicted, but no changes in any spirometric parameter were observed after BT. KCO was also unaltered by BT. A significant reduction in gas trapping was observed with Residual Volume (RV) falling from 146 ± 37% predicted at baseline to 136 ± 29%predicted 6 months after BT (p < 0.005). Significant improvements in TLC and FRC were also observed. These changes were evident at the 6 week time period and maintained at 6 months. The change in RV was inversely correlated with the baseline FEV1 (r = 0.572, p = 0.001), and in patients with a baseline FEV1 of < 60%predicted, the RV/TLC ratio fell by 6.5 ± 8.9%.ConclusionBronchial thermoplasty improves gas trapping and this effect is greatest in the most severely obstructed patients. The improvement may relate to changes in the mechanical properties of small airways that are not measured with spirometry.

Background: The mechanism for symptomatic improvement after bronchial thermoplasty (BT) is unclear, since spirometry reveals little or no change. In this study, the effects of BT on airway resistance were examined using two independent techniques. Methods: Eighteen consecutive patients, with severe asthma (57.6 ± 14.2 years) were evaluated by spirometry and plethysmography at three time points: (i) baseline, (ii) left lung treated but right lung untreated and (iii) 6 weeks after both lungs were treated with BT. At each assessment, total and specific airway resistance (Raw, sRaw) were measured. High resolution CT scans were undertaken at the first two assessments, and measurements of lobar volume, airway volume and airway resistance were made. The Asthma Control Questionnaire (ACQ) was administered at each assessment. Results: The baseline ACQ score was 3.5 ± 0.9, and improved progressively to 1.8 ± 1.2 (p < 0.01). At baseline, severe airflow obstruction was observed, FEV1 44.8 ± 13.7% predicted, together with gas trapping, and elevated Raw at 342 ± 173%predicted. Following BT, significant improvements in Raw and sRaw were observed, as well as a reduction in Residual Volume, increase in Vital Capacity and no change in FEV1. The change in Raw correlated with the change in ACQ (r = 0.56, p < 0.05). CT scans demonstrated reduced airway volume at baseline, which correlated with the increased Raw determined by plethysmography (p = − 0.536, p = < 0.05). Following BT, the airway volume increased in the treated lung, and this was accompanied by a significant reduction in CT-determined local airway resistance. Conclusion: Symptomatic improvement after BT is mediated by increased airway volume and reduced airway resistance.