A 45-year-old woman operated on for thyroid carcinoma two years ago now has stridor and exertional dyspnoea. Spirometry shows a flow-volume loop in which both the inspiratory and expiratory limbs are flattened, producing a box-like configuration. The most likely diagnosis is:

• A Variable intrathoracic upper airway obstruction
• B Fixed upper airway obstruction ✓
• C Variable extrathoracic upper airway obstruction
• D Generalised small airway obstruction

Explanation

When the calibre of a major airway is rigidly narrowed, as with tracheal stenosis or a compressing goitre, airflow is limited in both directions regardless of transmural pressure gradients, flattening both limbs of the loop. Variable extrathoracic lesions flatten mainly the inspiratory limb because negative airway pressure during inspiration collapses the segment, while variable intrathoracic lesions flatten mainly the expiratory limb. Only a fixed lesion produces the symmetric box pattern described here.

Reference: West's Respiratory Physiology: The Essentials, 11th ed.

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Written and medically reviewed by the StethoPrep medical team.