Prospective longitudinal measurements of N ₂ multiple breath washout indices in primary ciliary dyskinesia

Background: Previous studies of multiple breath washout (MBW) indices (lung clearance index (LCI), Sacin*VT and Scond*VT) in primary ciliary dyskinesia (PCD) have demonstrated abnormal values in about 85%, 96% and 78%, respectively, - even with spirometric values within the normal range. Long term variability of N 2 MBW indices in PCD has not been reported, although important for the use and interpretation of N 2 MBW indices in the clinical management and as outcome measures in interventional studies. Aims and objectives: To assess the overall change in N 2 MBW indices during one year and estimate the within- and between-occasion variability in PCD. Methods: N 2 MBW (Exhalyzer D, Eco Medics AG) in triplets and standard spirometry were prospectively assessed on three occasions over one year. Occasions with less than two acceptable MBW measurements were excluded. Results: Twenty-six PCD patients, aged 6-18 years (median 12.8) performed 76 N 2 MBW test occasions of which 72 (95%) were included. Baseline mean (SD) FEV 1 % predicted: 84.8 (11.3); LCI: 9.8 (1.9); Sacin*VT: 0.15 (0.08); and Scond*VT: 0.06 (0.02). Within-occasion SD / CV% (95% CI) of LCI were 0.63 (0.55; 0.70) / 6.6% (6.0%; 7.2%). Scond*VT significantly improved over the study period.   Between-occasion variability: SD (95% CI) / CV% (95% CI) FEV1 5.21 (4.65; 5.71) / 6.3 (5.9; 6.8) LCI 1.08 (0.83; 1.28) / 11.4 (10.4; 12.5) Scond 0.016 (0.014; 0.017) / 33.1 (23.7; 42.6) Sacin 0.033 (0.030; 0.037) / 35.3 (12.3; 58.3) Conclusions: This study reports estimates of short- and long term variability of N 2 MBW indices in a cohort of paediatric PCD patients, which is essential for future studies in PCD with MBW indices as outcome measures.