Upper Airway Obstruction

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Obstruction of airflow by abnormalities in the upper airway often goes undiagnosed or misdiagnosed because of improper interpretation of PFTs. Patients have obstructive physiology and often are misclassified as having asthma or COPD. The shape of the flow—volume loop, which includes inspiratory and expiratory flow—volume curves, and the ratio of forced expiratory and inspiratory flow at 50% of vital capacity (FEF50%/FIF50%) may be useful in the diagnosis of upper airway obstruction.8

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The shape of the flow—volume curve differs depending on whether the obstruction is fixed or variable (Fig. 32–5). Fixed lesions, as in strictures from previous intubations or tracheostomy, cause a uniform caliber of airway during inspiration and expiration. With variable lesions, the airway caliber changes with changes in intrathoracic pressure. Variable lesions are subclassified into variable intrathoracic and variable extrathoracic. If the lesion is intrathoracic, as with tumors of the trachea, the negative pressure generated during inspiration opens the obstruction, whereas the positive pressure during expiration worsens the obstruction. If the lesion is a variable extrathoracic obstruction, as with vocal cord dysfunction, the negative pressure within the airways will pull the vocal cord toward the midline and potentiate the obstruction. In this case, there will be a plateau on the inspiratory limb of the flow—volume loop, and FEF50%/FIF50% will be >1. Typical flow—volume curves from upper airway obstruction are shown in Figure 32–4. While 80% of subjects with vocal cord dysfunction demonstrate the classical variable extrathoracic pattern, 18% present with a pattern of variable intrathoracic obstruction, and 2% present with a pattern of fixed obstruction.

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Figure 32-5.
Graphic Jump Location

Maximum expiratory flow—volume curves from patients with fixed obstruction, variable extrathoracic obstruction, and variable intrathoracic obstruction. (RV, residual volume; TLC, total lung capacity.)

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Another test used to distinguish upper airway obstruction from COPD and asthma is FEV1/FEV0.5 (FEV at 0.5 second). This ratio usually is >1.5 in patients with upper airway obstruction.9 This is so because FEV0.5 is proportionately more reduced in upper airway obstruction because forced expiration measured at 0.5 second better reflects obstruction at high lung volumes. The abnormality seen on the flow—volume loop has been referred to as “straightening” of the curve during early expiration.

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