Physiologic differentiation of upper and lower airway obstruction.

Usual lower airway obstruction and fixed upper airway obstruction can be differentiated physiologically by means of the flow-volume curve. Normally, maximal flow decreases nearly linearly as lung volume decreases during expiration. In lower airway obstruction, this decrease is greatest at the beginning of expiration resulting in a curve that is concave upward. In fixed obstruction (stenosis) flow is constant throughout the initial part of forced maximal expiration and throughout virtually all of inspiration. This results in a plateau or flat curve which is characteristic and different from the curve in obstruction of lower airways. Cases in which this differentiation is clinically important are discussed.