(299) A Multicenter Experience with Extracorporeal Photopheresis As Treatment of Clad

measures respiratory system mechanics and may provide insights into CLAD phenotype physiology.We hypothesise that oscillometry parameters (Total resistance = R rs5 , Distal small airways resistance = R rs5-19 ) may further elucidate obstructive physiology between phenotypes.Methods: A cross-sectional study was performed at 2 Australian centres (Sydney and Melbourne) including bilateral LTx recipients presenting to clinic between 2020-2021.Stable LTx recipients without CLAD and those with CLAD were included.Participants performed concurrent airway oscillometry and spirometry.CLAD phenotypes were adjudicated as per ISHLT 2019 criteria for bronchiolitis obliterans syndrome (BOS), restrictive allograft syndrome (RAS) and mixed patterns.Undefined/unclassified CLAD phenotypes were excluded.Patients were matched for CLAD severity based on the ratio of concurrent FEV 1 /baseline FEV 1 .Oscillometry parameters were compared between CLAD phenotypes and to those without CLAD using the Kruskal-Wallis test.Results: A total of 167 patients were included in this study with No CLAD (n=90), RAS (n=9), Mixed (n=5), BOS (n=63).There were no significant differences in CLAD severity between patients in each CLAD phenotype (p=0.37).Median (IQR) R rs5 Z-Scores were significantly higher in recipients with BOS 1.00 (2.64) compared to those with RAS -0.28 (1.83) and those without CLAD 0.11 (1.56) p<0.001.Median R rs5-19 values were significantly higher in those with BOS 1.10 (1.35) compared to those without CLAD 0.25 (0.57), but no different to those with RAS 0.78 (0.76) or Mixed phenotypes 0.63 (1.68) p<0.001. Conclusion:In CLAD recipients with matched severity, R rs5 Z-Score is significantly more abnormal in BOS phenotype compared with RAS, reflecting greater airflow abnormalities in medium and large airways in this phenotype.R rs5-19 values were not significantly different between CLAD phenotypes, indicating that increased distal small airways resistance (reflective of bronchiolitis obliterans lesions) may be uniformly present.Oscillometry appears to provide additional physiologic insights into ISHLT 2019 CLAD phenotypes.