Using the same cut-off for sulfur hexafluoride and nitrogen multiple-breath washout may not be appropriate.

Abstract

Recent evidence suggests that results from multiple-breath washout (MBW) using nitrogen (N 2 ) as washout gas are different to those obtained by using sulfur hexafluoride (SF 6 ). Explanations were mainly related to different MBW equipment and gas properties. We aim to mention a new aspect, namely using the same cut-off in both techniques to determine lung clearance index (usually 2.5% of initial tracer concentration) irrespective of additional contribution of tissue-nitrogen in N 2 -MBW. In a simple model calculation we subtracted 1% tissue-N 2 from the N 2 -MBW in real washout traces from two children, one with cystic fibrosis (CF) and one healthy. This "SF 6 -washout simulation" decreased LCI differently in the children. In the healthy subject 1/40 th (2.5%) was achieved two breaths earlier compared to the original signal and changed LCI from 6.7 to 5.9 (12%), while in CF 1/40 th was achieved 19 breaths earlier leading to a LCI decrease from 13.7 to 10.0 (27%). It would have been required to wash out SF6 until 1/66 th (1.5%) to be comparable to N 2 washout at 1/40th (2.5%), or to stop N2-MBW already at 3.5% of the starting concentration to make both techniques comparable. We show that a basic physiological-mathematical difference between both techniques additionally accounts for different sensitivities and poor agreement between SF 6 - and N 2 -MBW. The best way of adjusting for the contribution of tissue-N 2 needs to be examined in future studies. Thus, despite increasing use in different disease groups, the books on MBW technology are obviously not closed yet.