Methacholine-induced fall in forced vital capacity as a marker of asthma severity

The dose of methacholine causing a 20% fall in forced expiratory volume in 1 sec (FEV1) from baseline (PD20) has been used as an index of asthma sever ity. The aim of this study was to determine if the percentage fall in force d vital capacity (FVC) from baseline at the PD20 (dFVC%) is an independent marker of asthma severity. We first retrospectively studied the dFVC% and PD20 obtained from 149 conse cutive newly diagnosed asthmatics with a positive methacholine-challenge te st (MCT). We then performed MCT on 20 normal subjects and 35 stable asthmat ics. The 'milder' asthmatics (n=20) and 'more severe' asthmatics (n=15) wer e on regular inhaled corticosteroids: 200 mu g or less and 800 mu g or more daily, respectively. A dosimeter technique was used, and normal subjects w ere given a cumulative dose of 2400 mu g. The PD20 and dFVC% were calculate d using log-linear interpolation of the last two points. Student's unpaired t-tests and linear regression analyses were used for comparison and correl ation of results. There was no significant correlation between dFVC% and PD20 among the 149 n ewly diagnosed asthmatics(r=0.1), or among the 35 known stable asthmatics ( r=0.008). The more severe asthmatics had a larger dFVC% compared with the m ilder asthmatics (15.8% vs. 9.6%; P=0.0005). In addition, inhaled corticost eroid usage correlated better with dFVC% (r=0.56) than with PD20 (r=0.36). The normal subjects had a mean fall in FVC of only 4.8%. The percentage fall in FVC at PD20 (dFVC%) may be a useful index of asthma severity which is independent of PD20. This index is potentially complement ary to the PD20 in the assessment of asthma severity.