Small airway resistance is best measured by:-

FEV timed: the percentage of total air that is exhaled in the given time. E.g: FEV1 : the air that is exhaled in the 1st second of forceful expiration. Reduced in airway obstruction (FEV, <70%). FEV1: 80 - 83 % FEV2: 90 - 93 % FEV3: 97 % In exhaled air: First 1/3rd - is air from large airways Next 1/3rd - from small airways Last 1/3rd - from alveoli: Air from small airways is the mid - 1/3rd faction of expired air i.e the 25- 75th percent of air Exhaled = FEV 0.25 - 0.75 FEV0.25 - 0.75 = MMFR (maximum mid expiratory flow rate) =reduction indicates small airway obstruction.