A new asthma severity index: a predictor of near-fatal asthma?

Bronchial hyperresponsiveness (BHR), measured as the provocative dose of in haled histamine or methacholine required to produce a 20% fall in forced ex piratory volume in one second (FEV1) (PD20), is widely used as one of the i ndices of asthma severity. Excessive broncho constriction, reflected by the maximal percentage fall in forced vital capacity (FVC) at PD20 (Delta FVC %) during BHR testing, is considered to be the most important pathophysiolo gical determinant in fatal asthma. The present study hypothesized that an i ndex which combines both the ease of airway narrowing and excessive broncho constriction, Delta FVC %/log(PD20), may be better in assessing asthma seve rity, especially in those at risk of near-fatal attacks. The dose-response curves of 46 asthmatics who underwent methacholine challe nge testing were studied. Group 1 (n=14) patients had mild disease, Group 2 (n=21) had moderate disease and Group 3 (n=11) had severe disease, as clas sified according to the Global Initiative for Asthma. Nine patients had pri or intubation for near-fatal asthma. Delta FVC %/log (PD20) was better than Delta FVC % and PD20 in categorizing patients into the three severity groups (p <0.0001), but more importantly, it was able to discriminate patients with previous intubation from those w ithout (p=0.04). It also correlated better with FEV1 (% predicted), frequen cy of symptoms and inhaled steroid requirement than either index alone. It is concluded that the percentage fall in forced vital capacity/log of th e provocative dose causing a 20% fall in forced expiratory volume in one se cond combines information on the ease and excessive degrees of airway narro wing in asthma. This new index may be better at assessing asthma severity a nd in discriminating those at risk of near-fatal attacks.