Between-visit variability of small airway obstruction markers in patients with asthma.

Clinical trials in patients with airway diseases often use forced expiratory volume in 1 s (FEV1) as the sole physiological outcome measure. However, FEV1 is thought to be insensitive to obstruction of the smaller airways, which may be particularly relevant in asthma [1]. Putative markers of small airway obstruction include measures of airway resistance using impulse oscillometry (IOS) [2] and indices of ventilation heterogeneity derived from multiple-breath inert gas washout (MBW) [3]. We recently showed that increased airway resistance at 5 Hz (R5) and 20 Hz (R20) is associated with worse asthma control and more frequent exacerbations [4], and FARAH et al. [5] demonstrated that MBW parameters may be responsive to asthma therapy.