Looking beyond LCI: Multiple breath washout phase III slope derived indices and their application in chronic respiratory disease in children

The multiple breath washout (MBW) test is widely reported in the context of Lung Clearance Index (LCI). LCI reflects global ventilation inhomogeneity but does not provide information regarding the localization of disease along the respiratory tree. The MBW‐derived normalized phase III slope (SnIII) indices (Scond and Sacin), instead, can distinguish between convective‐dependent and diffusion‐convection‐dependent ventilation inhomogeneity considered to occur within the conductive and acinar airways, respectively. In cystic fibrosis, Scond tends to become abnormal even earlier than LCI and spirometry. The value of Scond and Sacin in clinical practice has been recently explored in other respiratory conditions, including asthma, primary ciliary dyskinesia, bronchopulmonary dysplasia, bronchiolitis obliterans, and sickle cell disease. In this narrative review we offer an overview on the theoretical background, potentialities, and limitations of SnIII analysis in children, including challenges and feasibility aspects. Moreover, we summarize current evidence on the use of SnIII‐derived indices across different groups of pediatric chronic respiratory disease and we highlight the gaps in knowledge that need to be addressed in future studies.