Effect of equipment dead space on multiple breath washout measures

Background and objective Multiple breath inert gas washout ( MBW ) systems are designed to minimize equipment dead space volume ( V d). Animal and infant studies have demonstrated the impact of increased V d on MBW measurements. In this study, we investigate the effect of V d of a nitrogen ( N 2 ) MBW system on MBW measurements in preschool children. Methods N 2 MBW measurements were performed in healthy adults under standard conditions; V d was added to match the relationship between V d and lung volumes observed in preschool children. Subsequently, subjects were measured on a sulfur hexafluoride ( SF 6 ) MBW system under standard conditions and with V d added to match that of the N 2 MBW system. Healthy preschool children and children with cystic fibrosis were tested on both the N 2 MBW and SF 6 MBW in random order on the same day. A correction equation was derived based on the adult experiments and tested on the preschool data. Results Increasing the V d of the N 2 MBW system resulted in a higher lung clearance index ( LCI ). A strong non‐linear relationship between N 2 LCI and the V d/tidal volume was observed. When the V d was equivalent between systems, LCI measured by the SF 6 MBW system was similar to that measured by the N 2 MBW . LCI was higher on the N 2 MBW than the SF 6 MBW in preschool children. Correcting for the equipment V d of the N 2 MBW resulted in better agreement. Conclusions Equipment V d affects LCI measurements especially in young children where V d is large relative to lung volumes.