What are ventilation defects in asthma?

BACKGROUND

Hyperpolarised (3)He 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 (3)He 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 3(rd) 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 (3)He 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 (FEV1)/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 (3)He ventilation defects were older with significantly worse airway hyper-responsiveness, inflammation and airway remodelling but similar FEV1 as subjects with asthma without defects; hyperpolarised (3)He ventilation abnormalities were spatially and quantitatively related to abnormally remodelled airways.