Multiple Breath Washout and Lung Clearance Index in Paediatric Lung Disease

Abstract

Multiple breath washout (MBW) is a non-invasive tidal breathing test that quantifies ventilation inhomogeneity by measuring how efficiently the lungs are able to clear an inert tracer gas such as nitrogen (N ₂ ) or sulphur hexafluoride (SF ₆ ), and holds the key advantage of being applicable to all age groups. Though first described over 60 years ago, MBW has only garnered increased interest in the past 20 years. This has primarily been driven by the development and availability of commercial equipment, which has seen momentous advances in rapid gas analysers, highly sensitive flow systems, and user-friendly software enhancements. The principal MBW outcome, Lung Clearance Index (LCI), reflects global gas mixing efficiency and is regarded as a more sensitive method to identify the subtle changes seen in early lung disease when compared to traditional lung function measures such as spirometry. These properties have seen MBW begin to transition from paediatric respiratory research in specialised laboratories towards becoming a routine clinical assessment. Although nitrogen-based systems are currently more common, the tracer gas selected can introduce important methodological differences, and SF ₆ -based MBW remains the preferred method in infants. This review examines the physiological principles underpinning MBW, outlines key methodological aspects of testing, and discusses the interpretation and application of LCI in paediatric clinical practice.