Which of the following is used to measure the resistance in small airways

Since compression of small airways by the raised intrathoracic pressure during expiration makes the airway resistance higher than during inspiration, expiratory flow rates are more informative than inspiratory flow rates. The measurement can be done from the same record as for FEV1. The expired volume is divided into four equal pas. The flow rate is calculated for the middle two pas, i.e. the middle half. This rate is termed the maximum mid-expiratory flow rate (MMFR). The normal value for MMFR is greater than 3.5 L.s-1. A decrease in MMFR indicates an increase in airway resistance. . The contribution of small airways to airway resistance increases at low lung volumes. Hence, in small airway obstruction, the FEV1 remains normal, but the FEV3 and forced expiratory flow rate from 25 to 75% (FEFR25-75%) are abnormally low..