The dependence of maximal expiratory flow on vital capacity: a theoretical analysis.

The decrease of maximal expiratory flow rates (Vmax) at the 50 or 25 per cent level of vital capacity (V50, V25) in idiopathic pulmonary fibrosis (IPF) has been reported by several investigators, and most of them simply concluded that small airway obstruction was associated with IPF. However, Jayamanne et al. (1978) stressed that the reduced airflow in this disease was due to the reduction of lung volume than to abnormally elevated resistance to airflow in small airways. Theoretical analysis on the influence of lung volume on Vmax using a mathematical model supported the opinion of Jayamanne et al.