Inhomogeneity of ventilation in smokers and mild COPD assessed by the ratio of alveolar volume to total lung capacity

Background: Previous studies have shown that the ratio between alveolar volume (V_A) and total lung capacity (TLC) reflects ventilation heterogeneity in subjects with chronic obstructive pulmonary disease (COPD). However, the ratio and its correlation to respiratory symptoms had to our knowledge not previously been investigated in subjects with mild chronic airflow obstruction or without airflow obstruction (normal ratio FEV₁/VC). The purpose of this study was to investigate potential connection between V_A/TLC and respiratory symptoms, smoking habits and lung function parameters in subjects with normal spirometry and with mild chronic airflow obstruction. Methods: We examined 450 subjects (82 non-smokers with normal spirometry, 298 subjects who smoked or had smoked earlier but had a normal spirometry and 70 subjects with chronical airflow obstruction) with routine lung function tests and a questionnaire regarding respiratory symptoms. Results: This study showed 31 out of 54 subjects with a low V_A/TLC had a normal ratio FEV₁/VC. Of these subjects, 58.1 % had respiratory symptoms, compared to the group with normal ratio for both V_A/TLC and FEV_1/VC where 35.8 % had respiratory symptoms (p-value 0.02). Conclusion: This study has shown that within the group of subjects with a normal ratio FEV₁/VC, those with a decreased ratio V_A/TLC had a higher prevalence of respiratory symptoms compared to subjects with a normal V_A/TLC ratio. These findings indicate that including the ratio V_A/TLC in the evaluation of a pulmonary function test assessment might increase the possibility to identify subjects with early or at risk of lung disease.