[Measuring lung volume in the infant--problems with the nitrogen washout method]

The open-circuit nitrogen washout technique, as developed by Gerhardt et al., seems to be ideally suited for assessing functional residual capacity (FRC) in infants. By performing this measurement in over 250 infants throughout the last three years, we gathered considerable practical experience of our own, but also encountered several methodological problems, which, so far, have received only little attention by the relevant literature, or have remained unresolved altogether. Our data illustrate the importance of reproducing the infant's own breathing rate and tidal volume when calibrating the system. The choice of the O2-background-flow should be based on the individual peak tidal inspiratory flow, as derived from the tidal flow-volume loop. The importance of maintaining this O2-background-flow unchanged between calibration and measurement is also demonstrated. The question, at which N2-concentration the measurement should be terminated, has remained unresolved. Diffusion of N2 from blood and tissue into the alveolar space is responsible for considerable noise. Our own measurements resulted in widely differing FRC-values for different N2-target-concentrations; these differences seem to increase with more severe bronchial obstruction. Finally, there remains the question, how long the minimal interval between two subsequent measurements should be. In conclusion, these unresolved questions have to be answered in relevant prospective studies, before recommending this technique for routine clinical application.