A 52-year-old man with a 10-year history of COPD presents with progressive dyspnea on exertion and expiratory stridor over the past 3 months. His symptoms have NOT improved with high-dose inhaled bronchodilators and corticosteroids. Spirometry shows a flow-volume loop with a characteristic flattened expiratory limb while the inspiratory limb remains preserved. The pattern marked **C** in the diagram represents the underlying pathophysiology. Which of the following is the MOST LIKELY diagnosis?

Explanation

## Why Tracheomalacia with dynamic airway collapse is right Variable intrathoracic obstruction (marked **C**) is characterized by a flattened expiratory limb with a preserved inspiratory limb on the flow-volume loop. This occurs because during forced expiration, positive pleural pressure compresses the obstructed intrathoracic segment, worsening the obstruction (compounded by Bernoulli effect). During inspiration, negative pleural pressure holds the segment open, preserving the inspiratory limb. Tracheomalacia is the classic cause of variable intrathoracic obstruction, presenting with expiratory stridor and dyspnea on exertion that fails to respond to bronchodilators. The clinical context (progressive dyspnea, expiratory stridor, lack of response to standard COPD therapy) and the specific flow-volume loop morphology are pathognomonic for this diagnosis (Harrison 21e Ch 287; Pellegrino R et al. ERJ 2005). ## Why each distractor is wrong - **Bilateral vocal cord paralysis**: This causes variable extrathoracic obstruction, which produces a FLATTENED INSPIRATORY LIMB (not expiratory), because negative intratracheal pressure during forced inspiration narrows the extrathoracic segment. The preserved expiratory limb in this case rules out vocal cord pathology. - **Post-intubation tracheal stenosis**: Tracheal stenosis below the sternal notch causes FIXED obstruction with a "box-shaped" flow-volume loop (both inspiratory AND expiratory limbs flattened), not the variable pattern with preserved inspiratory limb seen here. - **Extrathoracic laryngeal edema**: This causes variable extrathoracic obstruction with inspiratory limb flattening and preserved expiratory limb — the opposite of the pattern shown. The location of obstruction (above the sternal notch) means negative intratracheal pressure during inspiration narrows the lesion. **High-Yield:** Variable intrathoracic obstruction = flattened expiratory limb + preserved inspiratory limb = think tracheomalacia or intrathoracic tumor; expiratory stridor + bronchodilator failure = UAO, not asthma. [cite: Harrison 21e Ch 287; Pellegrino R et al. ERJ 2005]