Small airway function in obese individuals with self-reported asthma

Diagnosis of asthma in obese individuals frequently relies on clinical history, as airflow by spirometry may remain normal. This study hypothesised that obese subjects with self-reported asthma and normal spirometry will demonstrate distinct clinical characteristics, metabolic comorbidities and enhanced small airway dysfunction as compared with healthy obese subjects. Spirometry, plethysmography and oscillometry data pre/post-bronchodilator were obtained in 357 obese subjects in three groups as follows: no asthma group (n=180), self-reported asthma normal spirometry group (n=126), and asthma obstructed spirometry group (n=51). To assess the effects of obesity related to reduced lung volume, oscillometry measurements were repeated during a voluntary inflation to predicted functional residual capacity (FRC). Dyspnoea was equally prevalent in all groups. In contrast, cough, wheeze and metabolic comorbidities were more frequent in the asthma normal spirometry and asthma obstructed spirometry groups versus the no asthma group (p<0.05). Despite similar body size, oscillometry measurements demonstrated elevated R(5–20) (difference between resistance at 5 and 20 Hz) in the no asthma and asthma normal spirometry groups (0.19±0.12; 0.23±0.13 kPa/(L·s(−1)), p<0.05) but to a lesser degree than the asthma obstructed spirometry group (0.34±0.20 kPa/(L·s(−1)), p<0.05). Differences between groups persisted post-bronchodilator (p<0.05). Following voluntary inflation to predicted FRC, R(5–20) in the no asthma and asthma normal spirometry groups fell to similar values, indicating a reversible process (0.11±0.07; 0.12±0.08 kPa/(L·s(−1)), p=NS). Persistently elevated R(5–20) was seen in the asthma obstructed spirometry group, suggesting chronic inflammation and/or remodelling (0.17±0.11 kPa/(L·s(−1)), p<0.05). Thus, small airway abnormalities of greater magnitude than observations in healthy obese people may be an early marker of asthma in obese subjects with self-reported disease despite normal airflow. Increased metabolic comorbidities in these subjects may have provided a milieu that impacted airway function.