To the best of our knowledge, no other study has directly compared outcomes measured by MBWN2 to those measured by both MBWSF6 and traditional lung function tests in healthy children and children with CF. LCIN2 and LCISF6 had similar discriminative power and intra-session repeatability but are not interchangeable as LCIN2 was on average higher than LCISF6. As such, interpretation of parameters measured by MBWN2 will require independent normative values to define an appropriate upper limit of normal.
The feasibility of using MBWN2
in a pediatric clinical setting has recently been described but this study did not include head to head comparison to other technologies
. Two studies have previously compared alternative MBW systems to mass spectrometry based MBWSF6
. However, both used SF6
as the tracer gas and neither performed between system comparisons in the same individual nor compared MBW based lung volume measurements to plethysmographic FRC measurements; therefore results are not directly comparable to our study.
Although the LCI and FRC were comparable between systems in health, albeit higher using N2, the bias observed in CF subjects clearly demonstrates that the two systems cannot be used interchangeably. These observed differences could potentially be explained by differing physiological properties of SF6 and N2. SF6 is a heavy gas and thus may behave differently in the periphery of the lung than a lighter gas (He or N2); however comparison of LCISF6 to LCIHe in CF did not demonstrate the same bias observed between LCISF6 and LCIN2. The endogenous nature of N2 results in the contribution of gas from very slowly ventilated lung units not captured by MBWSF6 as evidenced by the relationship between trapped gas, number of breaths and difference in LCI between systems. However, this will also increase washout time in subjects with uneven ventilation distribution as it will take longer to clear endogenous tracer gas from their lungs compared to SF6, which may not equilibrate in extremely slowly ventilated lung units.
FRC measured by MBW is subject to the same limitations as other gas dilution techniques in that only communicating lung units will contribute to measured volume, while FRC measured by body plethysmography includes all compressible gas volume. Thus, in subjects with significant peripheral airway obstruction we would expect differences between FRCpleth and FRCMBW, and indeed FRC measured by both MBW techniques was lower than that measured by plethysmography. However, we observed that FRCN2 more closely agreed with FRCpleth. These results suggest that the difference between FRCpleth and FRCSF6 may reflect trapped gas volume and that the volume contribution of slowly ventilated lung regions, not captured during MBWSF6, results in lower FRCSF6 values. Consequently, during MBWN2 subjects with CF required significantly more breaths to complete washout leading to the disproportionately higher CEVN2 compared to CEVSF6. Our data demonstrate that these differences are progressively more pronounced with worsening obstructive lung disease. LCIN2 was shown to increase disproportionately more than LCISF6 with greater disease severity (increased FRCpleth and lower FEV1) and as such may be able to more accurately reflect the degree of VI than LCISF6.
These interpretations are based on the assumption that the additional gas volume measured during MBWN2
can be attributed to measurement of gas in extremely slowly ventilated lung units. However, a further unquantifiable amount of tissue dissolved N2
will diffuse from the blood into the alveoli during MBWN2
, particularly during long washouts seen in subjects with significant VI. Most evidence would suggest unless lung disease is severe the tissue N2
contribution will be relatively low.
The close correspondence of FRCN2
observed in this study would support this hypothesis.
While it would appear that MBWN2
is better able to reflect the degree of peripheral airway disease than MBWSF6
, washout times will be substantially longer in subjects with significant VI. Long washout times may limit the feasibility of MBWN2
in the clinical setting. This limitation could potentially be overcome by choosing higher cut-off concentrations earlier in the washout. Preliminary evidence 
would suggest that this is possible without compromising the sensitivity of MBWN2
. Investigation into the minimal number of trials required to achieve reproducible results; another option to shorten the test duration, is ongoing.
In conclusion, MBWN2 offers a valid tool to investigate obstructive lung disease in CF. Furthermore, future studies in younger patients are required to better understand the sensitivity of multiple breath N2 washout in this age group. In addition, interventional studies similar to those performed with MBWSF6 are needed to further clarify the role of MBWN2 as an outcome measure in clinical trials in CF patients.