WS06.05 The volume of trapped air (VTA) from multiple breath washout (MBW) substantially underestimates the extent of trapped air (TA) demonstrated with spirometry-controlled computed tomography

Objectives: A limitation of the lung clearance index (LCI) from MBW is that it may ‘overlook’ areas of TA, a major component of CF lung disease pathophysiology. Previous methodology addressed this by performing 5 inspiratory capacity (5IC) breaths after the standard end of washout to mobilise TA. In calculating the VTA, a fundamental assumption is that the gas mobilized has a [N2] of 78%. We assessed this assumption and the relationship of VTA to TA scores from CT scans.