Comparison of indirect measurements of multiple breath nitrogen washout with mass spectrometry

Abstract

Background: Multiple breath nitrogen washouts (MBW) and derived lung clearance index (LCI) are increasingly used in clinical applications, due in part to the availability of commercial equipment. These devices are usually based on indirect measurements of N₂ concentration (C_N2), by measuring O₂ and CO₂ or molar mass and CO₂.

Aim: To compare N₂ washout tracings and LCI from a commercial MBW device using molar mass and CO₂ measurement, with direct C_N2 measurements by mass spectrometry (MS).

Methods: MBW were performed in triplicate in 12 healthy adult subjects (6M/6F; age: 50±13yr) with 1-liter tidal breathing using a commercial device (MBW_device). Simultaneous C_N2 measurements were obtained with a MS (MBW_MS) by inserting a sampling line in the mouthpiece.

Results: Mean expired alveolar C_N2 was significantly higher throughout the MBW_device compared to the MBW_MS, with an average difference of 0.94±0.35(SD)% (p<0.001). The LCI (mean±SD) calculated from the raw MBW_device trace was 7.8±0.7, and after offsetting mean expired C_N2 by 0.94%, LCI became 6.2±0.4. The latter LCI value fell within the normal range for a device with instrumental dead space of 15ml, whereas the former exceeded LCI upper limit of normal [1]. Corresponding syringe tests showed no significant difference in expired C_N2 between MBW_device and MBW_MS (0.03±0.03; P>0.1).

Conclusion: Indirect measurements of C_N2 were consistently higher than those by mass spectrometry, resulting in a considerable overestimation of LCI. Syringe MBW data suggest a potential role of humidity in exhaled gases on the overestimation of C_N2.

Reference:

1. Verbanck et al. Thorax 2012;67:689-695.

This work was funded by grant 5R01 HL135496-02 from NHLBI/NIH.