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 method for measuring mean expiratory flow during the middle half of the forced vital capacity, mean expiratory flow during the third quarter of the forced vital capacity, instantaneous forced expiratory flow at 50% of forced vital capacity , and instantaneous expiratory flow at 75% of forced vital capacity. These measurements were conducted at the same absolute lung volume (isovolume) when forced vital capacity was reduced voluntarily to 100%, 85%, and 75% of maximum, and the variances, expressed as the coefficients of variations, were compared. Absolute lung volumes above residual volume were determined with two reference spirograms: 100% and 75% forced vital capacity. In normals, means of flow 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 vital capacity among asthmatics was associated with modest increases in isovolume 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-airways narrowing when forced vital capacity is reduced, and volume-averaged rates provide the least variability.