The biphasic spirogram: a clue to unilateral narrowing of a mainstem bronchus.

Two patients with narrowing of a mainstem bronchus each showed two unusual functional features that are likely to be characteristic of this condition. The maximum inspiratory flow-volume curve showed an end inspiratory "tail" and the forced expiratory spirogram had a biphasic shape with normal initial curvature but a "straight line" appearance in later expiration. In one patient relief of the bronchial stenosis by the insertion of a stent restored normal contours to the spirogram and flow-volume curves.