Abstract
Background: Functional residual capacity (FRC) measured by sulfur hexafluoride (SF6) multiple breath washout (MBW) is lower than FRC measured by either nitrogen (N2) MBW or body plethysmography. A lower FRC should result in a higher lung clearance index (LCI); however we found SF6LCI to be consistently lower than N2LCI (PloS One 2013). In health, the difference was most pronounced in younger children. We designed a series of experiments to investigate the potential effects of equipment dead space (Vd), which is higher with N2MBW, on LCI measurements.
Methods: MBW was measured in triplicate in 10 healthy adults (median age 22 years) by both SF6MBW (Amis 2000, Innovision, Odense, Denmark) and N2MBW (Exhalyzer D®, EcoMedics AG, Durnten, Switzerland) using standard Vd conditions. SF6MBW Vd was then increased to match that of the N2MBW. In 11 healthy adults we also increased N2MBW Vd in steps approximating 1ml/kg to mimic the spectrum of Vd to weight ratios observed in preschool children.
Results: As previously described, N2LCI was higher than SF6LCI (mean difference ± SD: 0.137 ± 0.58). When SF6MBW Vd was increased to be equivalent to N2MBW Vd, SF6LCI increased to be higher than standard N2LCI (0.273 ± 0.54). When Vd was added to the N2MBW system, N2LCI increased linearly by 0.4 units for each unit increase in weight adjusted Vd.
Conclusion: Differences in equipment Vd explain differences in LCI observed between SF6MBW and N2MBW. Furthermore, added Vd in the N2MBW system increases N2LCI to the same degree observed in preschool children where N2LCI is disproportionately higher. These findings have important implications for the interpretation of MBW data in young children.
Supported by NHLBI and The Irwin Family Foundation.
- © 2014 ERS