Nitrogen-based lung clearance index: a valid physiological biomarker for the clinic

Multiple breath washout (MBW) testing is increasingly used as a physiological measurement in the clinic, due in part to the availability of commercial equipment and reference values for MBW indices. Commercial N₂ washout devices are usually based on indirect measurement of N₂ concentration (C_N2), by directly measuring either molar mass and O₂ and CO₂, or molar mass and CO₂. We aim to elucidate the role of two potential pitfalls associated with N₂-MBW testing that could override its physiological content: indirect N₂ measurement and blood-solubility of N₂. We performed MBW in 12 healthy adult subjects using a commercial device (MBW_indirect) with simultaneous direct gas concentration measurements by mass spectrometry (MBW_direct) and compared C_N2 between MBW_direct and MBW_indirect. We also measured argon concentration during the same washouts to verify the maximal effect gas solubility can have on N₂-based functional residual capacity (FRC) and lung clearance index (LCI). Continuous N₂ concentration traces were very similar for MBW_indirect and MBW_direct, resulting in comparable breath-by-breath washout plots of expired concentration and in no significant differences in FRC_N2, LCI_N2, S_cond, and S_acin between the two methods. Argon washouts were slightly slower than N₂ washouts, as expected for a less diffusive and more soluble gas. Finally, comparison between LCI_N2 and LCI_Ar indicates that the maximum impact from blood-tissue represents less than half a LCI unit in normal subjects. In conclusion, we have demonstrated by direct measurement of N₂ and twice as soluble argon, that indirect N₂ measurement can be safely used as a meaningful physiological measurement.NEW & NOTEWORTHY The physiological content of N₂ multibreath washout testing has been questioned due to N₂ indirect measurement accuracy and N₂ blood solubility. With direct measurement of N₂ and twice as soluble argon, we show that these effects are largely outweighed by ease of use.