How is central airway obstructions assessed by spirometry testing?

Updated: May 14, 2020
  • Author: Kevin McCarthy, RPFT; Chief Editor: Nader Kamangar, MD, FACP, FCCP, FCCM  more...
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The configuration of the flow-volume curve of a properly performed spirometry test can be used to demonstrate various abnormalities of the larger central airways (larynx, trachea, right and left mainstem bronchi). Three patterns of flow-volume abnormalities can be detected: (1) variable intrathoracic obstructions, (2) variable extrathoracic obstructions, and (3) fixed upper airway obstructions. Reproducing these findings on every effort is important because spurious nonreproducible reductions in inspiratory flow are not uncommon after completion of forced expirations in subjects without upper airway obstruction. Examples of variable intrathoracic obstruction include localized tumors of the lower trachea or mainstem bronchus, tracheomalacia, and airway changes associated with polychondritis.

Variable upper airway obstructions demonstrate flow reductions that vary with the phase of forced respirations. Variable intrathoracic obstructions demonstrate reduction of airflow during forced expirations with preservation of a normal inspiratory flow configuration. This is observed as a plateau across a broad volume range on the expired flow limb of the flow-volume curve. The reduction in airflow results from a narrowing of the airway inside the thorax, in part because of a narrowing or collapse of the airway secondary to extraluminal pressures exceeding intraluminal pressures during expiration.

Variable extrathoracic obstructions demonstrate reduction of inspired flows during forced inspirations with preservation of expiratory flows. Again, the major cause of the reduced flow during inspiration is airway narrowing secondary to extraluminal pressures exceeding intraluminal pressures during inspiration. Causes of this type of upper airway obstruction include unilateral and bilateral vocal cord paralysis, vocal cord adhesions, vocal cord constriction, laryngeal edema, and upper airway narrowing associated with obstructive sleep apnea.

Fixed upper airway obstructions demonstrate plateaus of flow during both forced inspiration and forced expiration. Causes of fixed upper airway obstruction include goiters, endotracheal neoplasms, stenosis of both main bronchi, postintubation stenosis, and performance of the test through a tracheostomy tube or other fixed orifice device. (See the image below.)

Flow reduction must be consistent on every effort Flow reduction must be consistent on every effort to be considered actual flow limitation. Fixed upper airway obstruction may be caused by postintubation stenosis, goiter, endotracheal neoplasms, and bronchial stenosis. Variable intrathoracic obstruction may be caused by tracheomalacia, polychondritis, and tumors of the lower trachea or main bronchus. Variable extrathoracic obstruction may be caused by bilateral and unilateral vocal cord paralysis, vocal cord constriction, reduced pharyngeal cross-sectional area, and airway burns.

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