Fig 1.
Tissue N2 excretion equations used for correction of MBWN2 measurements.
The three equations used to estimate the volume of N2 excreted from the body tissues are plotted over a 7 minute time period. The Cournand 1941 equation was adjusted for a constant excretion rate and plotted for a subject with the average body size of the subjects measured in the Lundin 1953 study. The ATS/ERS equation calculates the volume of tissue N2 excreted using Cournand’s 1941 equation standardized to a 7 minute washout for all subjects.
Table 1.
Summary of tissue nitrogen correction equations.
Table 2.
Characteristics of study participants.
Values are presented as mean (SD) unless otherwise indicated. P value indicates group difference between health and CF.
Fig 2.
Within-subject difference in FRC measured by multiple breath nitrogen washout (FRCN2) and body plethysmography (FRCpleth).
The mean within-subject difference FRCN2—FRCpleth decreased after the three tissue N2 excretion equations were applied to washout data from healthy and CF populations.
Table 3.
Estimates of tissue N2 contribution to MBW outcomes at the 2.5% washout cut-off.
Values are presented as the mean within-subject difference (95% CI) of the uncorrected–corrected MBWN2 outcome. Outcomes were corrected by applying either the Cournand or Lundin tissue N2 excretion equations.
Fig 3.
Relationship between the contribution of tissue N2 to FRCN2 and the difference between FRC as measured by MBWN2 and MBWSF6.
FRCN2 became disproportionately greater than FRCSF6 as the contribution of tissue N2 estimated by the within-subject difference FRCN2—FRCCournand increased.
Fig 4.
Representative examples of FRCN2 plotted against washout time in a) a healthy subject and b) a patient with CF. Application of the Cournand and Lundin tissue N2 excretion equations resulted in a less pronounced increase in the calculation of FRCN2 over the course of the washout, but a plateau was never achieved. Linear regressions of these curves over the second half of the washout are shown, demonstrating the slopes that are reported in Table 4.
Table 4.
Average slopes of the second half of all uncorrected, Lundin-corrected and Cournand-corrected FRCN2/breath number curves (depicted graphically in Fig 4) for healthy subjects and those with CF.
Average paired difference (uncorrected-corrected) in absolute and relative (percent of uncorrected slope) terms are shown. Data are expressed as mean 95% confidence interval) unless otherwise stated.
Fig 5.
Within-subject difference between uncorrected LCIN2 and LCIN2 corrected for tissue N2 excretion (LCICournand) at different washout cut-offs.
The difference was progressively smaller when calculated at earlier cut-offs of the washout in both health and CF.
Fig 6.
Relationship between the contribution of tissue N2 to LCIN2 and length of washout calculated at a) the traditional 2.5% washout cut-off and b) the 5% washout cut-off. The contribution of tissue N2 to LCIN2 calculated at the 2.5% cut-off (LCIuncorr−LCICournand) increased as washout time increased. However, this relationship was no longer observed at the earlier 5% cut-off.
Table 5.
Effect of applying Lundin and Cournand correction equations to previously published observational MBW data.
Data are shown as pre-treatment and post-treatment LCI with paired treatment effect. Values are presented as mean (SD) unless otherwise indicated.