Utility of single versus multiple breath washout in adult asthma

Nitrogen multiple breath washout (N₂ MBW) is a sensitive method to identify peripheral airway involvement in asthma, but is a time-consuming test. The N₂ vital capacity single breath (VC SBW) test offers greater time efficiency, but concordance with N₂ MBW is poorly understood. The prevalence of peripheral airway abnormality was determined by N₂ MBW and N₂ SBW tests in 194 asthmatic subjects aged 18-1 years. N₂ MBW data were related to findings in 400 healthy controls, aged 17-71 years, while N₂ SBW data were compared to findings in 224 healthy controls, aged 15-65 years, to derive equipment-specific reference values. Amongst asthmatic subjects, relationships between N₂ SBW and N₂ MBW outcomes were studied. N₂ SBW relationship with clinical history, spirometry, blood eosinophils and fraction exhaled nitric oxide (FENO) data was also explored. The prevalence of peripheral airway involvement (i.e. abnormal ventilation distribution) determined by N₂ SBW-derived phase III slope (N₂ S_III ) was 24·7%, compared to 44% determined by N₂ MBW-derived lung clearance index (LCI) (P<0·001). Predictors of abnormal N₂ S_III were older age, smoking history and lower FEV_1. N₂ SBW offers lower sensitivity than N₂ MBW to detect small airway dysfunction in adult asthma, but may be a marker of more severe disease.