The purpose of the study was to evaluate moment analysis (MA) of multibreath N2 washout (MBNW) for detecting gas mixing inefficiency in lungs of infants with mild BPD, and to test the sensitivity of MA relative to other conventional lung function tests. Fifteen preterm infants with mild BPD were studied at a postconceptional age of 35.1±2.4 weeks and body weight 2048±415g (mean ± SD) and compared to 15 age- and weight-matched healthy preterm infants. A new bedside computerized technique with very low dead space, suitable for small infants was constructed. Two moment ratios, M1/M0 and M2/M0, lung clearance index (LCI), and functional residual capacity (FRC) were derived from the washout data. Compliance (Crs) and resistance (Rrs) of the respiratory system were measured by the single occlusion method. Specific compliance (Crs/FRC) was calculated.

M1/M0 and M2/M0 were both significantly higher in BPD infants than in healthy infants, 2.29±0.23 vs. 1.98±0.16 and 9.73±1.34 vs. 7.46±0.97, (p<0.001), respectively, suggesting inefficient gas mixing in the lungs. M1/M0 and M2/M0 were strongly correlated to each other (r=.92) and also to LCI (r=.91 and r=.81, respectively). The most constant findings in the BPD group were low FRC and elevated moment ratios. These two measures together were abnormal in 14/15 BPD infants. Table

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Rrs as well as specific Crs were normal in all BPD infants. Consequently, moment ratios were capable to detect lung function abnormalities in BPD even in cases without abnormal lung elasticity or resistance. They were also more sensitive than the classical LCI.

We conclude that moment ratios of MBNW, as indices of mixing efficiency, provided a simple and sensitive tool to discriminate between infants with mild BPD and healthy infants, especially in combination with FRC, which is obtained in the same measuring session.