Chronic obstructive pulmonary disease (COPD) is one of the leading causes of death. In early disease, inflammatory changes occur in the small airways. The measurement of small airways obstruction (SAO) using spirometry has shown promise as indicator of future disease in at-risk populations.
The aim of my thesis was to investigate the prevalence, risk factors, and clinical utility of SAO. The objectives included:
1. To systematically review all population-based studies where spirometry was used to measure SAO;
2. To estimate the prevalence and identify risk factors of SAO across several word regions;
3. To investigate the association of SAO with respiratory symptoms, comorbid conditions, and physical and mental quality of life (QoL);
4. To assess whether isolated SAO can predict future lung function decline and development of chronic airflow obstruction (CAO); and
5. To investigate whether SAO is associated with cardiorespiratory fitness;
I found that there is a lack of agreement on how best to measure SAO using spirometry. When estimating the prevalence of SAO using standardised criteria, I found that SAO is common (5% to 34%), but its prevalence varied according to world region and the parameter used. Risk factors for SAO were like those of COPD, with the strongest being tobacco smoking. Individuals with SAO were more likely to be symptomatic, have cardiovascular disease and a lower QoL. Using longitudinal data analysis, I found that those with isolated SAO were more likely to have lower lung function and develop CAO. In a small clinical population, I found that SAO is also associated with impaired ventilatory response to exercise.
As a whole, my research shows that SAO is common in the general population, that it is a clinically meaningful spirometric condition, and importantly, that it can be used to predict future morbidity, particularly when measured using FEF25-75.