What are ventilation defects in asthma?

Abstract

Background: Hyperpolarised 3He MRI provides a way to visualise regional pulmonary functional abnormalities that in asthma are thought to be related to airway morphological abnormalities. However, the exact aetiology of ventilation defects in asthma is not well understood. Objective: To better understand the determinants of ventilation defects in asthma, we evaluated well-established clinical as well as 3He MRI and X-ray CT airway measurements in healthy subjects and subjects with asthma. Methods: Thirty-four subjects (n=26 subjects with asthma, n=8 healthy volunteers) underwent MRI, spirometry, plethysmography, fraction of exhaled nitric oxide analysis, methacholine challenge and CT for a region-of-interest proximal to ventilation defects. For subjects who consented to CT (n=18 subjects with asthma, n=5 healthy volunteers), we evaluated 3rd to 5th generation airway wall area and wall thickness per cent and lumen area. Results: Seventeen subjects with asthma (17/26=65%) had visually obvious evidence of 3He ventilation defects prior to bronchoprovocation and nine subjects with asthma had no ventilation defects prior to bronchoprovocation (9/26=35%). Subjects with asthma with defects were older (p=0.01) with worse forced expiratory volume in 1 s (FEV 1)/forced vital capacity (p=0.0003), airways resistance (p=0.004), fraction of exhaled nitric oxide (p=0.03), greater bronchoprovocation concentration of methacholine that reduced FEV1 by 20% (p=0.008) and wall thickness per cent (p=0.02) compared with subjects with asthma without defects. There was a moderate correlation for wall area per cent with ventilation defect per cent (r=0.43, p=0.04). Conclusions: Subjects with asthma with 3He ventilation defects were older with significantly worse airway hyperresponsiveness, inflammation and airway remodelling but similar FEV1 as subjects with asthma without defects; hyperpolarised 3He ventilation abnormalities were spatially and quantitatively related to abnormally remodelled airways.