A101-24 Impulse Oscillometry Complements Spirometry and Quantitative CT in Detecting Small-Airway Changes in COPD and Asthma

While spirometry remains the standard for diagnosis and follow-up in COPD and asthma, it is relatively insensitive to peripheral airway changes. Impulse oscillometry (IOS) provides effort-independent assessment of airway resistance and reactance, and quantitative computed tomography (QCT) offers complementary structural information such as functional small-airway disease (fSAD), airway-wall thickness, and vascular pruning. This study evaluated the longitudinal responsiveness of IOS and spirometry and explored their associations with CT-based structural indices in routine clinical practice. We retrospectively analyzed 133 patients (mean age 60.5 ± 14.3 years; 51.1% male) who underwent at least two IOS and spirometry assessments, including a subset with paired chest CTs. IOS indices (R5, R20, R5-R20, X5, Fres, AX) and spirometric indices (FEV1, FVC, FEV1/FVC, FEF25–75) were compared between visits using paired tests. Responsiveness was quantified by standardized response mean (SRM = mean Δ / SD Δ), and correlations were assessed between physiological (ΔIOS, Δspirometry) and CT parameters [PRM-derived fSAD %, emphysema index (LAA %), airway-wall thickness (Awt-Pi10), and vascular pruning (BV5/TBV)]. Group means improved significantly in IOS (R5 −0.074 kPa·s/L, R5-R20 −0.059 kPa·s/L, X5 +0.033 kPa/L, AX − 0.688 kPa/L; all p < 0.01) and spirometry (FEV1 +0.125 L, FEV1/FVC +3.0 points, FEF25–75 +0.257 L/s; all p < 0.001). SRMs were moderate for IOS (|SRM| = 0.27-0.36) and largest for spirometric small-airway indices (FEF25–75 = 0.88, FEV1/FVC = 0.85). In CT analysis, higher baseline fSAD % correlated with more negative X5 (r = −0.42) and higher Fres (r = 0.37), linking structural obstruction to impaired reactance. Patients classified as IOS responders (ΔR5-R20 ≤ −0.06 or ΔAX ≤ −0.65) or spirometry responders (ΔFEV1 ≥ 100 mL) had greater baseline fSAD % and Awt-Pi10 than non-responders, suggesting that structural small-airway pathology underlies functional improvement. In conclusion, both IOS and spirometry demonstrated significant physiological improvement during follow-up, but IOS provided complementary insights into small-airway mechanics not captured by spirometry. The observed correlations between IOS indices and CT-based fSAD % highlight the structural-functional continuum of small-airway disease, supporting the integrated use of IOS and QCT with spirometry for longitudinal monitoring in COPD and asthma.