Early Small Airway Mechanics and Functional Correlates in Asymptomatic Smokers: An Impulse Oscillometry-Based Study

Early mechanical alterations in small airways may occur in asymptomatic smokers despite preserved spirometric lung function. This study aimed to evaluate respiratory system mechanics using impulse oscillometry (IOS) in asymptomatic smokers and to investigate their relationship with exercise capacity and muscle oxygenation.

In this cross-sectional study, university students who were current smokers and never-smokers were included. All participants underwent spirometry and IOS measurements before and after the six-minute walk test (6MWT). IOS parameters included resistance (R5-R25, R5-R20) and reactance (X5, X5VEX, X5VIN, AX, Fres). Respiratory muscle strength (MIP/MEP) and muscle oxygen saturation (SmO₂) measured by near-infrared spectroscopy at the vastus lateralis were also assessed. Associations between IOS parameters, smoking exposure, exercise capacity, and muscle oxygenation were analyzed.

Despite comparable FVC % predicted, FEV₁ % predicted, and FEV₁/FVC values, smokers demonstrated higher resistance indices (R5, R5-R20) as well as higher AX and Fres values, and lower reactance parameters (X5, X5VIN) compared with non-smokers (p < 0.001 for most comparisons). Smoking exposure correlated positively with resistance parameters and negatively with X5. Smokers exhibited reduced 6MWT distance and lower SmO₂ levels. In multivariable analysis, lower R5 and higher X5VIN were associated with longer 6MWT distance (p = 0.008 and p < 0.001, respectively).

IOS can detect early small airway mechanical alterations in asymptomatic smokers despite preserved spirometry. The associations of these changes with exercise capacity and muscle oxygenation suggest that early small airway involvement may have functional correlates. IOS may serve as a complementary tool for the multidimensional assessment of smoking-related airway effects.