A METHOD FOR ASSESSING SMALL AIRWAYS INDEPENDENT OF INSPIRATORY CAPACITY

Reduced forced vital capacity may confound assessment of small-airway function. In 17 healthy and 16 asthmatic volunteers, we validated a me thod for measuring mean expiratory flow during the middle half of the forced vital capacity, mean expiratory flow during the third quarter o f the forced vital capacity, instantaneous forced expiratory flow at 5 0% of forced vital capacity, and instantaneous expiratory flow at 75% of forced vital capacity. These measurements were conducted at the sam e absolute lung volume (isovolume) when forced vital capacity was redu ced voluntarily to 100%, 85%, and 75% of maximum, and the variances, e xpressed as the coefficients of variations, were compared. Absolute lu ng volumes above residual volume were determined with two reference sp irograms: 100% and 75% forced vital capacity. In normals, means of flo w rates at the same absolute lung volume did not differ with the three forced vital capacities, regardless of whether the 100% or 75% forced vital capacity served as the reference spirogram. Reduced forced vita l capacity among asthmatics was associated with modest increases in is ovolume flow rates, an effect that may underestimate airway narrowing. Intrasubject variability was least among volume-averaged flow rates ( e.g., mean expiratory flow during the middle half of the forced vital capacity). Volume-adjusted flow rates can be used to assess small-airw ays narrowing when forced vital capacity is reduced, and volume-averag ed rates provide the least variability.