B78-09 Spirometry and Impulse Oscillometry Identify Distinct Bronchodilator Response Patterns in CPFE Versus COPD

Abstract Rationale Patients with combined pulmonary fibrosis and emphysema (CPFE) experience symptoms and have radiographic abnormalities, but spirometry and lung volumes tend to be normal. The spirometry pseudo-normalization is thought to result from increased airway traction due to fibrosis, which preserves FEV1. This study explores whether impulse oscillometry (IOS) can reveal bronchodilator (BD) responsiveness reflective of subclinical airways disease in CPFE. Methods Retrospective single-center cohort study of Veterans undergoing clinical pulmonary function testing. CPFE patients were defined as those with upper lobe-predominant emphysema and lower lobe-predominant fibrosis on CT scans. Each CPFE patient was matched to two COPD controls for age (±2 years), sex, and smoking status. Absolute BD change was defined as (post-BD value minus pre-BD value), while relative BD change was defined as (absolute BD change/prebronchodilator value) × 100. Categorically significant bronchodilator responses for spirometry were defined by ATS criteria and for IOS by ERS and Oostveen et al (see Table 1). Groups were compared using the Wilcoxon rank-sum (continuous variables) and Fisher’s exact test (categorical variables). Results COPD patients (n = 42) had significantly larger BD changes in FEV1 (relative change) and FVC (absolute, relative, and ATS-defined BD response) than CPFE patients (n = 21) (p 0.05 for all). Table 1 compares IOS BD responsiveness in the CPFE group and the COPD control group. Absolute BD changes in respiratory system resistance at 5Hz minus resistance at 20Hz (R5-R20) and in reactance (X5) were significantly larger in COPD than in CPFE. Categorical BD response analysis showed no significant differences by ERS criteria; however, a greater proportion of COPD patients demonstrated absolute improvements in X5 and reactance area (AX) based on the Oostveen criteria. Although respiratory system resistance at 5Hz decreased more in COPD, the difference did not reach statistical significance. Conclusion COPD patients demonstrated greater bronchodilator responses than those with CPFE across both spirometry and oscillometry metrics. COPD patients showed notable reductions in frequency dependence of resistance (R5-R20), improved reactance, and reduced reactance area following bronchodilator use, reflecting enhanced compliance and airway patency. FVC improvements in COPD controls reflect a reduction in air-trapping post-BD. In contrast, patients with CPFE exhibited minimal changes post-BD, consistent with fibrosis-driven mechanical fixation that limits both airway collapsibility and reversibility. These results suggest that IOS BD testing exposes the mechanical rigidity of CPFE lungs and distinguishes them from the partially reactive airways in COPD, providing a unique window into respiratory system physiology in these conditions. This abstract is funded by: None