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RationaleNitrogen multiple-breath washout (N2MBW) is an established technique to assess functional residual capacity (FRC) and ventilation inhomogeneity in the lung. Accurate measurement of gas concentrations is essential for the appropriate calculation of clinical outcomes.ObjectivesWe investigated the accuracy of oxygen and carbon dioxide measurements used for the indirect calculation of nitrogen concentration in a commercial MBW device (Exhalyzer D, Eco Medics AG, Duernten, Switzerland) and its impact on FRC and lung clearance index (LCI).MethodsHigh precision calibration gas mixtures and mass spectrometry were used to evaluate sensor output. We assessed the impact of corrected signal processing on FRC and LCI in a dataset of healthy children and children with cystic fibrosis using custom analysis software.ResultsWe found inadequate correction for the cross sensitivity of the oxygen and carbon dioxide sensors in the Exhalyzer D device. This results in an overestimation of expired nitrogen concentration, and consequently FRC and LCI outcomes. Breath-by-breath correction of this error reduced mean (SD) FRC by 8.9 (2.2)% and LCI by 11.9 (4.0)%. It also resulted in almost complete disappearance of the tissue nitrogen signal at the end of measurements.ConclusionsInadequate correction for cross sensitivity between the oxygen and carbon dioxide gas sensors of the Exhalyzer D device leads to an overestimation of FRC and LCI. Correction of this error is possible and could be applied by re-analysing the measurements breath-by-breath in an updated software version.Grants, Gifts, Equipment, DrugsEco Medics AG (Duernten, Switzerland) provided a research version of their commercial software Spiroware 3.2.1 including insight on signal processing algorithms and helped with the acquisition of mass spectrometry measurements. This project was funded by the Swiss National Science Foundation, Grant Nr. 182719 (P. Latzin) and 168173 (K. Ramsey)

Trapped gas (TG), defined as the difference between thoracic gas volume at end expiration (TGV) and functional residual capacity (FRC), has been reported to be of considerable magnitude in the neonatal period. In a previous investigation we found TG to average 20 ml in 26 full term infants 2 hours after birth, and 17 ml 24 hours later. The study to be reported was performed to assess how much of the TG that could be mobilized from the lungs by hyperventilation. Fifteen fullterm, healthy newborn infants were studied 1 to 4 days after birth by a modified nitrogen wash-out method. A face mask with its outlet connected to an opening in the side of a wide tube was gently applied to the sleeping baby. Air at high flow rate was continuously flushed through the tube which had a pneumotachygraph mounted at the outlet. The probe of a fast nitrogen analyzer was inserted into the mask. During an expiration the gas flow was instantanously shifted to 100% oxygen. After the expired nitrogen concentration reached below 2% the baby was stimulated to cry with the face mask still in place. Flow and nitrogen concentration signals were sampled at high rate by a computer and the lung nitrogen content before the wash-out was calculated. No rise in nitrogen output was detected during hyperventilation after the wash-out in any infant. We conclude, that the difference between TGV and FRC found by other investigators and ourselves in newborn infants most likely does not represent gas trapped in the lungs but is either an artifact due to methodological inaccuracy, or is due to gas localized in other oroans.