COMPARISON OF NITROGEN WASHOUT AND PLETHYSMOGRAPHIC MEASUREMENTS OF LUNG-VOLUME IN HEALTHY INFANTS

Functional residual capacity (FRC), the only lung volume to be assesse d routinely in infants, can be measured using plethysmography or gas d ilution. Although it is well recognized that both methods yield simila r FRC values in healthy adults, gas dilution techniques have consisten tly produced lower values in healthy infants when compared with plethy smography. However, interpretation of this difference is difficult sin ce data comparing the different techniques within the same infants hav e rarely been reported. We performed paired measurements of FRC using an automated open-circuit nitrogen washout technique (FRC(N2)) and who le-body plethysmography (FRC(pleth)) in 11 healthy infants with a medi an age of 12 months (range, 2 to 18 months). The mean (SD) FRC was 21. 7 (4.0) ml/kg for the N-2 washout and 25.6 (4.9) ml/kg for plethysmogr aphy. The mean within-subject difference between FRC(N2) and FRC(pleth ) was 3.9 (range, -0.3 to 7.2) ml/kg (p = 0.001). Both N-2 washout and plethysmography yielded reproducible results, with the mean of the co efficients of variation (CV) being 3.6 and 3.9%, respectively. The res ults from these paired measurements support previously reported data f rom separate populations of infants which suggest that gas dilution te chniques consistently yield smaller values for FRC than do those measu red by plethysmography.