Abstract
Purpose: Poor forced vital capacity (FVC) effort has been considered to be the main reason for FVC reduction by the ATS/ERS guideline; however, this has rarely been mentioned in previous studies. The present study aims to determine whether reduced FVC in asthmatic children is correlated to poor FVC effort.Methods: A total of 209 asthmatic children within 5–13 years old were included and divided into reduced FVC (“restricted,” n = 66) and typical obstruction group (“obstructed,” n = 143). Forced expiratory flows before and after bronchodilation were recorded in asthmatic children. The differences in clinical characteristics, spirometric results, FVC effort, and bronchodilator response were compared between two groups. Exhalation time (ET) was divided into effective ET (ETe) and plateau ET (ETp) by the start point of exhalation plateau on the time-volume curve. FVC effort was assessed by ET, ETp, and back extrapolated volume (EV)/FVC (%).Results: Asthmatic children in the restricted group had significantly higher slow vital capacity (SVC)/FVC (%), higher EV/FVC (%), shorter ET, shorter ETe, and longer ETp, when compared with those with obstructed. In the obstructed group, ET (r = 0.201, P = 0.016) and ETe (r = 0.496, P < 0.001) positively correlated with FVC, and ETp (r = −0.224, P = 0.007) negatively correlated with FVC. In the restricted group, FVC positively correlated with ETe (r = 0.350, P = 0.004) but not ET and ETp. FVC z-score significantly correlated with total IgE (n = 51, r = −0.349, P = 0.012) and with FEF25−75% z-score (n = 66, r = 0.531, P < 0.001) in the restricted group. The further logistic regression revealed that the risk of restricted increased by 1.12 (95% CI, 1.04–1.22, P = 0.005) with every 1% increase in %ΔFVC. In subjects with restricted and bronchodilation tests, %ΔFVC was significantly associated with FeNO (n = 29, r = 0.386, P = 0.039), FEF25−75% z-score (n = 29, r = −0.472, P = 0.010), and SVC/FVC (%) (n = 19, r = 0.477, P = 0.039) but not with EV/FVC (%), ET, ETe, or ETp (P > 0.05).Conclusion: These findings suggested that “poor FVC effort” does not account for the FVC reduction in asthmatic children. Short ET and high SVC/FVC (%) are characteristics of reduced FVC.
Highlights
Asthma is characterized by variable expiratory airflow limitation, which is caused by airway smooth muscle contraction, airway edema, airway thickening, or mucus hypersecretion
This airflow limitation is best documented by lung function tests, with a reduced ratio of forced expiratory volume in 1 s (FEV1) to forced vital capacity (FVC) (FEV1/FVC) from spirometry [1], especially the obstruction of the small airways, which can be revealed by the reduced mean forced expiratory flow between 25 and 75% of FVC (FEF25−75%) in spirometry [2, 3]
It has been suggested that FVC reduction might not be due to a restrictive pattern when total lung capacity (TLC) and FEV1/FVC are within the normal range and might be caused by an elevated residual volume (RV)/TLC ratio, which is thought to be a special obstructive defect [7,8,9,10]
Summary
Asthma is characterized by variable expiratory airflow limitation, which is caused by airway smooth muscle contraction, airway edema, airway thickening, or mucus hypersecretion. This airflow limitation is best documented by lung function tests, with a reduced ratio of forced expiratory volume in 1 s (FEV1) to forced vital capacity (FVC) (FEV1/FVC) from spirometry [1], especially the obstruction of the small airways, which can be revealed by the reduced mean forced expiratory flow between 25 and 75% of FVC (FEF25−75%) in spirometry [2, 3]. This special obstructive defect has been termed as pseudo-restrictive defect [7], small airway obstructive syndrome [11], or non-specific pattern (NSP) [8] in different studies
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