The unreliability of the maximal midexpiratory flow as an index of acute airway changes.

This study evaluates the reliability of the maximal midexpiratory flow (MMF) as an index of acute airway changes. In a group of asthmatics, intravenous histamine induced a significant decrease in the forced vital capacity, expiratory volume in the first second, and peak flow, while increasing the respiratory system resistance and the closing volume. The studies showed narrowing of the airways, while the MMF significantly increased in all subjects, implying dilation of the airways. The increased closing volume suggested narrowing of the small airways. The data, however, suggest that the paradoxic increase of the MMF is a result of histamine-induced early small airway closure, eliminating the lung units with longer time constants. Unless the MMF is related to absolute lung volumes (as presently measured the MMF is not related to absolute lung volumes) it may be misleading, and is thus an unreliable index of acute airway changes.