Which parameter provides an estimate of small airway function?

The parameter that provides an estimate of small airway function is FEF25-75%. This measurement represents the forced expiratory flow during the middle half of the forced vital capacity (FVC) maneuver, specifically between the 25% and 75% points of the FVC. Because small airways (diameters less than 2 mm) are most affected by various pulmonary diseases, such as asthma and COPD, FEF25-75% is sensitive to detecting changes in airflow that occur in these smaller passages. It can often indicate early signs of small airway obstruction that may not be captured by other parameters such as FEV1 or PEF.

FEV1, while a critical parameter in assessing overall lung function, primarily reflects larger airway and lung function and may not detect subtle changes in the smaller airways. Peak Expiratory Flow (PEF) measures the maximum speed of expiration and can indicate large airway function but does not specifically target small airway involvement. Total Lung Capacity (TLC) reflects the total volume of air in the lungs and does not offer insight into airflow dynamics or small airway function. Thus, FEF25-75% serves as a more direct assessment of small airway performance in patients undergoing pulmonary function testing.