Figure 1.

Classic flow–volume patterns. Flow–volume loop patterns suggestive of upper airway obstruction as the source of airflow obstruction on spirometry. (A) Variable intrathoracic obstruction, marked by flattening of the expiratory limb of the flow–volume loop. (B) Variable extrathoracic obstruction marked by flattening of the inspiratory limb of the flow–volume loop. (C) Fixed upper airway obstruction with flattening of the expiratory and inspiratory limbs of the flow–volume loop. (D) Unilateral mainstem obstruction marked by a rapid decline in airflow in the initial portion of exhalation and a flattening of the curve over the remainder of that phase.