The Characteristics of the Concavity of Descending Limb of Maximal Expiratory Flow-Volume Curves Generated by Spirometry

Purpose: This study examined the concavity (angle β, central and peripheral concavity) of the descending limb of the maximal expiratory flow-volume (MEFV) curves to reflect various ventilatory defects, including obstructive, restrictive, or mixed patterns.

Methods: We conducted a cross-sectional study collecting spirometry data from a healthcare center and a tertiary hospital between 2017 and 2022, with additional raw flow-volume curve data from primary healthcare institutions in 2023. We analyzed differences in concavity between spirometric patterns. Receiver operating characteristic curves were used to assess the predictive power of concavity for spirometric patterns. The relationship among concavity indices was examined.

Results: This study included 18,938 cases, with 22% exhibiting an obstructive pattern. The dataset comprised 14,868 cases for training, 3716 cases for validation, and 354 cases for testing. In the training set, the mean angle β were 180.3 ± 12.4 and 148.5 ± 12.7 degrees in normal and obstruction patterns. The angle β had an AUC of 0.970 (95% CI 0.966-0.973) for identifying normal and obstructive patterns, with a cut-off value of 163.0 degrees. In the validation set, out of 2311 cases with a normal forced vital capacity (FVC), 3.1% cases exhibited a Z-score of forced expiratory volume in 1 s to FVC ratio (FEV₁/FVC) ≥ - 1.645 but an angle β < 163.0 degrees. In testing set, a correlation coefficient of - 0.96 and - 0.79 was found between the angle β and the central or peripheral concavity.

Conclusion: The concavity of the descending limb of MEFV curves may be crucial in identifying spirometric patterns.