Small airways in asthma: their independent contribution to the severity of hyperresponsiveness

To the Editor: Bronchial hyperresponsiveness (BHR), i.e. increased narrowing of the airways after exposure to non-allergic stimuli, is a hallmark of asthma. BHR is a risk factor for asthma development and, additionally, a marker of worse disease outcome in asthma [1, 2]. It is generally acknowledged that obstruction of the large airways due to inflammation and remodelling contributes to more severe BHR. This is plausible, given that BHR is expressed as the concentration or dose of a stimulus that induces a 20% fall in forced expiratory volume in 1 s (FEV1). Recent studies suggest that asthmatics with BHR have more severe small airways obstruction [3, 4]. However, little is known about the converse, i.e. the association between small airways obstruction and the severity of BHR. Our aim was to assess: 1) whether asthma patients with small airways obstruction express more severe BHR than those without small airways obstruction; and 2) whether small airways obstruction is associated with more severe BHR independently of FEV1. We analysed data from patients with mild-to-moderate asthma who were included in a previously published study on inhaled corticosteroids (ICS) in primary care [5]. All subjects underwent spirometry before and after 1 mg terbutaline, measuring FEV1, forced vital capacity (FVC) and mean expiratory flow at 50% of FVC (MEF50). BHR was assessed using a histamine challenge test, measuring the provocative dose causing a 20% fall in FEV1 (PD20) (histamine). All patients were hyperresponsive to histamine (PD …