The airways in our lungs are like the branches of a tree. At their widest they resemble the sturdy tree trunk and at their most narrow, the fragile twigs. Due to their size, these twig-like structures are called the small airways and they are easily damaged by noxious particles or gases we inhale, for example cigarette smoke. When they are damaged, our small airways become inflamed, which stops air moving freely and leads to something called small airways obstruction. In conditions like asthma and chronic obstructive pulmonary disease (COPD), small airways obstruction is common. However, evidence also suggests that this condition can occur on its own and may be a sign of future and more serious lung disease. A test called spirometry is often used to investigate whether the small airways are obstructed or not, however, there is no agreement on how best to do this.
We conducted a systematic review of the scientific literature to identify and summarise studies that measured small airways obstruction in the general population using spirometry. We extracted information from suitable studies on the choice of spirometry parameter used to measure small airways obstruction, criteria used to diagnose this condition, the proportion of people affected by it, and any factors that could be increase the odds of having this type of obstruction.
We found that only 25 studies have measured small airways obstruction in general populations around the world. Across these studies, 16 different spirometry parameters were used to measure it, along with 8 different diagnostic criteria. The proportion of people with small airways obstruction ranged from 7.5% to 45.9% and varied depending on the choice of spirometry parameter and world region. Just two studies identified potential risk factors for this type of obstruction, with cigarette smoking, passive smoking, increasing age, being women, low education level, and exposure to high levels of air pollution being the most promising.
Small airways obstruction is a common but understudied condition in general populations around the world. Risk factors for this condition appear to be similar to more established lung diseases, such as COPD. However, there is no consensus on the best spirometry parameter or diagnostic criteria to use when measuring this type of obstruction. For this reason, more research is required to identify the best measure of small airways obstruction and to assess whether having it increases the risk of future lung disease.
The findings of this study were published in the peer-reviewed journal Respiratory Research. The article can be read here: https://doi.org/10.1186/s12931-022-01990-2