Fixed airflow obstruction is a clinical subtype or phenotype of Difficult-to-treat Asthma. Is there any specific spirometry criteria on which we can tell of having fixed air-flow limitation in Asthma?
Morphological changes in asthma are primarily localized in the small airways. Look at the peripheral flows (FEF75) In %pred it will be way down the FEV1 and FVC. Clinically it is silent (except for the typical dry asthma cough en reduced excursive tolerance)
In a person with childhood Asthma & who is a non smoker , fixed air flow obstruction is probably due to Asthma . In a person with childhood Asthma & history of smoking , the fixed air flow obstruction is due Asthma - COPD overlap syndrome . Fixed air flow obstruction is more common in elderly persons with Asthma , than younger persons who have reversible obstruction . I do not think there is any specific criteria for fixed air flow obstruction criteria for Asthma , as Asthma is considered a reversible obstructive disease . This subset of fixed air flow obstruction in Asthma should be treated as Asthma , as there is benefit in the long term . There is also the view that Asthma would evolve into COPD , if not treated properly in the younger age .
Reversible airflow obstruction in Bronchial Asthma is defined as increase in FEV1 by > 200 ml & FEV1 > 12% post bronchodilator challenge . COPD is defined as post bronchodilator FEV1 / FVC < 0.7 which confirms that airflow obstruction is not fully reversible . If there is no post bronchodilator reversibility in bronchial asthma , it should be considered as fixed air flow obstruction . The main problem is that if spirometric findings of Bronchial Asthma is suggestive of COPD in elderly patients , it is difficult to diagnose the Asthma component ,unless the history of childhood wheeze , allergy , dry cough & non smoking is available