The Bronchodilator Response in FEF_25-75% Compared to FEV₁ in Assessing Asthma

Abstract

Background: Asthma is a common, chronic heterogeneous disease. Tests of large airways function (FEV₁) are commonly used to diagnose/monitor disease and bronchodilator responsiveness (BDR: measured by a change in FEV₁) is considered the hallmark of asthma. Small airways (<2 mm in diameter) dysfunction (SAD) has been described in asthma, but it is unclear how common BDR in the small airways is in asthma. The aim of this study was to compare BDR in the small airways compared to FEV₁.

Methods: A retrospective analysis of patients attending a secondary referral hospital in the Midlands, United Kingdom, including non-smoking adults (>18 y) referred to the pulmonary function laboratory as part of an asthma assessment between January 2016 and April 2021. BDR was assessed 20 min after the administration of 2.5 mg of salbutamol using a jet nebulizer. Changes in FEV₁ and FEF_25-75 were assessed. The study was approved by the Research Ethics Committee and the Health Research Authority of the NHS (20/WM/0024).

Results: 219 patients were identified for this initial analysis. The population consisted of 104 (47.5%) males and the mean age was 44.7 y ± 18.9 with a BMI of 28.2 ± 6.5. 28 (12.8%) demonstrated airflow limitation in pre-BDR tests. The mean difference in pre and post bronchodilator %predicted FEV₁was 9.7 (95% CI = 8.7-10.8) and 22.3 (95% CI = 19.7-24.9) %predicted FEF_25-75; (P < 0.01 for both). In total 73 (33.3%) demonstrated BDR in FEV₁, 101 (46.1%) in FEF_25-75. All of those with significant BDR response in FEV₁ had a significant response in FEF_25-75. The BDR in FEF_25-75 was noted to be the lowest in severe cases, with no distinctive differences in other patients' characteristics.

Conclusions: This study confirms the presence of BDR in more adults with asthma using measures of small airways function than FEV₁. It is unclear whether this represents a separate phenotype of asthma.