Expiratory time constant patterns reveal obstructive lung mechanics in non-obstructive spirometry

Based on the principle that increased airway resistance prolongs the expiratory time constant (TC), we investigated whether TC patterns could serve as a signature for obstruction. In this cross-sectional multicenter study of 19,937 validated spirometry records, we divided the descending expiratory flow-volume curve into five equal volume segments and derived consecutive TCs for each. A K-means algorithm trained on the normal subset defined data-driven patterns, which were then used to classify the entire cohort. We quantified pattern prevalence and used propensity score matching to assess spirometric differences between patterns within each spirometric category. Two distinct patterns were identified: a 'normal' pattern (stable consecutive TCs) and an 'obstructive' pattern (progressively prolonging TCs). The obstructive pattern was present in 7% of normal spirometry, 13% of possible restriction, and 33% of nonspecific FEV1 reduction, while 9% of patients meeting obstruction criteria had a normal TC pattern. In normal spirometry group, the obstructive pattern group showed lower forced expiratory volume in 1 second (FEV1) (males: 2.7 vs 3.0 L; females: 1.8 vs 2.0 L), reduced FEV1/forced vital capacity (FVC) (males: 67% vs 76%; females: 68% vs 78%), and impaired small airway metrics, including maximal mid-expiratory flow (MMEF25-75) (males: 1.6 vs 2.6 L/s; females: 1.0 vs 1.8 L/s) and FEV3/FEV6 (males: 91% vs 94%; females: 91% vs 95%), all p < 0.001. Other spirometric groups mirrored these findings. The obstructive TC pattern represents a distinct physiological phenotype suggestive of obstructive lung mechanics and is highly prevalent across common spirometric categories.