ISOFLOW-VOLUME CURVES IN DIAGNOSIS OF UPPER AIRWAY-OBSTRUCTION

Abstract

The ability of air and He-O2 maximal expiratory flow-volume curves to distinguish upper airway obstruction from the diffuse, peripheral airway obstruction of chronic obstructive pulmonary disease was evaluated. The increase in expiratory flows at peak, 75, 50 and 25% of the vital capacity during He-O2 breathing compared to air breathing was determined in 5 normal subjects and 3 patients with chronic obstructive pulmonary disease while breathing through fixed resistances and in 6 patients with documented tracheal obstruction. In the normal subjects, the He response at all 4 points remained normal and was unchanged from baseline until the simulated obstruction was severe (6 mm orifice), at which point all of the He responses increased by 50%. The patients with chronic obstructive pulmonary disease maintained their low baseline He responses until the obstruction was severe (6 mm orifice), when only the expiratory flows at peak, 75 and 50% of the vital capacity increased by at least 50%. Five of the 6 patients with upper airway obstruction had He responses very similar to those of the normal subjects with similar degrees of simulated obstruction, but the 1 patient with concomitant airway obstruction extending well below the carina had very small He responses at each point. Upper airway obstruction can usually be identified by high He responses. Upper airway obstruction, if severe, can be identified even in the presence of more peripheral airway obstruction by a normal He response at high lung volumes.