Oscillometry defined small airways dysfunction in patients with severe uncontrolled asthma and preserved spirometry

Small airways dysfunction (SAD) is an important treatable trait in asthma that is often under-recognised. It can be assessed through spirometry as forced expiratory flow between 25% and 75% of vital capacity however this is limited due to being both effort and volume dependent. Forced oscillometry technique is effort independent and showing to be more sensitive for SAD. We reviewed patients with severe uncontrolled asthma prior to commencing biologic therapy to identify the incidence of abnormal SAD defining values. SAD was identified in 63% of patients. Of the 31 patients with preserved spirometry (normal FEV1 and FEV1/FVC), only 6% had an impaired FEF25-75 while 29% had impaired oscillometry. This result highlights SAD is a common finding in severe uncontrolled asthma and that oscillometry is more sensitive than spirometry at identifying SAD, particularly when spirometry is preserved.