Abstract
Background: Tissue nitrogen impacts the calculation of multiple breath nitrogen washout (MBWN2) outcomes. The aim of this study was to apply correction equations for tissue N2 excretion to the functional residual capacity (FRC) and lung clearance index (LCI) derived from MBWN2.
Methods: MBWN2 measurements from 35 subjects with CF and 43 healthy controls were collected using an Exhalyzer D device (EcoMedics, Switzerland) and corrected breath-by-breath for the contribution of tissue N2 using equations proposed by Cournand, A et al. 1941. Turnover value was calculated at sequential normalized end-tidal N2 concentrations of 2.5%, 5%, 9%, 12%, and 18%. FRC was also measured by plethysmography (FRCpleth).
Results: Subjects were on average 16.4 (SD 6.8) years old with LCI ranging from 5.9 to 19.3. FRCMBW was greater than FRCpleth in health (mean difference 0.2 L (95% CI 0.1 to 0.3, p<0.001)); this difference increased with increasing height. Correcting for tissue nitrogen significantly decreased FRCMBW values in healthy subjects closer to FRCpleth (mean difference 0.1 L (95% CI 0 to 0.2, p=0.02)). The uncorrected LCI was significantly greater than the corrected LCI in both health (mean difference 0.4 (95% CI 0.3 to 0.4, p<0.001)) and CF (mean difference 0.9 (95% CI 0.6 to 1.2, p<0.001)). This difference was less evident in both groups at the 5% cut-off and no longer significant at the 9% cut-off. The effect of tissue N2 excretion on both FRC and LCI increased significantly with longer washouts.
Conclusions: Application of a tissue N2 correction or use of earlier washout cut-offs could reduce the influence of tissue N2 on MBWN2 outcomes.
Funding: Seller's Chair for Cystic Fibrosis, Irwin Family Foundation.
- Copyright ©the authors 2016
