Flow volume loops in the evaluation of upper airway obstruction.

Patients with lesions that cause obstruction of the large airways are often misdiagnosed as having chronic lung disease or reactive airways disease. Close attention to the history and physical examination provides clues to the presence of a laryngeal or tracheal lesion. Obtaining a flow volume loop in the pulmonary function laboratory is a simple and effective method of noninvasively evaluating a patient for the presence of an upper airway obstruction. Fixed lesions cause plateaus in both the inspiratory and expiratory limbs of the flow volume loop. Variable intrathoracic lesions are characterized by expiratory slowing and flattening of the expiratory limb. An important caveat is that these changes may not be present in a patient with coexisting lower airway disease such as COPD or asthma. Variable extrathoracic lesions cause inspiratory slowing and a plateau on the inspiratory limb of the flow volume loop. Finally, the clinician should remember that the quality of the flow volume loop is totally dependent on the patient's effort and cooperation and, thus, that the tracings obtained in the pulmonary function laboratory may not have the classic shapes presented in this review.