Simultaneous SF6and N2gas multiple breath washout (MBW); understanding the difference between test gases

Abstract

Background: MBW data in cystic fibrosis (CF) have mainly been performed using SF 6 , but recent efforts have focussed on N 2 washout (WO). These different tracer gases give different results for unclear reasons. We performed MBW with both SF 6 and N 2 at the same time to investigate whether these differences were intrinsic to the gases or a feature of the different measurement technologies. Method: 8 healthy controls (HC) & 5 CF subjects completed at least 2 MBW trials on the Exhalyzer D (ExD/N 2 ) and Innocor (SF 6 ) attached in parallel. SF 6 was washed in to equilibrium (0.2% SF 6 in air) then SF 6 and resident N 2 were washed out (100% O 2 ) to 1/40 th of the starting concentration for each gas. Results were analysed using in house software for SF 6 (Simple WO, IGOR Pro) and Spiroware on ExD. Results: Both devices similarly discriminated in HC and CF. SF 6 WO decayed in a single exponential in HC, but 2 in CF. N 2 WO displayed 2 exponentials in HC, a longer tail to the WO curve in all cases. The first half of the WO was similar for both gases, separating beyond this. Gas concentration reached 1/40 th sooner in SF 6 than N 2 (HC 32.98±12.58 breaths in SF 6 vs. 47.89±13.97 in N 2 p=0.002 & CF 24.8 ± 8.72 vs. 59.5 ± 28.34 p=0.04). WO time (minutes) was lower in SF 6 (HC 2.5±0.59 vs. 3.34±0.78 p= Conclusions: There is a clear difference in the progression of N 2 and SF 6 throughout the WO, with this difference enhanced in disease. Results are being explored further to investigate whether the difference is due to innate nitrogen contribution or relates to offset in the derived gas signals within the devices.