Intrapulmonary gas distribution and mixing is an important functional property of the lungs and can be characterised by the multibreath washout (MBW) technique. By computing indices from the entire washout curve, ventilation inhomogeneities can be characterised.1^–5 Developed in the late 1980s for use in children,6^–8 MBW measurements are taken during tidal breathing and have been used in infants9^–12 as well as preschool children.7 12 13 This technique is especially advantageous in the latter as only a few tests exist for measuring pulmonary function without active subject cooperation.

The MBW allows us to measure functional residual capacity (FRC_MBW), which can be used in combination with the plethysmographic intrathoracic gas volume (FRC_pleth) to determine the amount of trapped gas in the lungs (V_TG).14^–17 In addition, using the washout curve, different measures of ventilation inhomogeneity can be obtained. The lung clearance index (LCI) is defined as the number of lung volume turnovers (cumulative expired volume divided by FRC).2 3 5 Other more sophisticated indices include the moment ratios (m₁:m₀, m₂:m₀),4^–8 10 18 19 mixing ratio (ratio between the actual and ideal breaths needed to lower the tracer gas to a defined end-tidal concentration in relation to the starting concentration)13 20 and the alveolar-based gas dilation number.19 21

At present, LCI is the most commonly used of these indices, partly because it is arguably the most intuitive and easiest to be communicated to patients. Of note, these indices of intrapulmonary gas mixing are independent of age. Moreover, LCI has been shown in patients with cystic fibrosis (CF) to be more sensitive than spirometry for detecting changes in small airways,13 to be the …