A 9-year-old boy presents with habitual snoring, witnessed apneas during sleep, and daytime hyperactivity. Polysomnography confirms moderate obstructive sleep apnea (OSA). Spirometry is performed and shows: FEV₁ 92% predicted, FVC 96% predicted, FEV₁/FVC ratio 80%, but the parameter marked **A** in the flow-volume loop is reduced at 58% predicted, with characteristic scooping of the descending expiratory limb. Which of the following best explains the spirometric pattern observed at **A**?

Question

Accepted Answer

C. Selective obstruction of airways <2 mm in diameter (small airway zone) due to chronic intermittent hypoxia and neurogenic bronchial hyperreactivity from recurrent upper airway collapse. ## Why option 1 is correct

The parameter marked **A** is the maximal mid-expiratory flow (MMEF or FEF₂₅₋₇₅), which represents the average flow between 25% and 75% of the forced vital capacity. FEF₂₅₋₇₅ is the most effort-independent and sensitive segment of the expiratory maneuver, making it highly selective for obstruction in small airways (<2 mm diameter)—the "quiet zone" of the lung. In pediatric OSA, recurrent upper airway collapse during sleep triggers chronic intermittent hypoxia and systemic inflammation, which propagates neurogenic bronchial hyperreactivity to the distal airways. This causes isolated small airway obstruction with characteristic scooping of the expiratory limb while FEV₁ and FEV₁/FVC remain normal. FEF₂₅₋₇₅ is the earliest spirometric marker of distal airway disease and is significantly reduced in children with moderate-to-severe OSA even in the absence of daytime asthma (Goldbart, Tauman studies). This pattern is called isolated small airway obstruction or dysanapsis-like physiology.

## Why each distractor is wrong

- **Option 2 (Flattened inspiratory limb)**: A flattened inspiratory limb on the flow-volume loop indicates variable extrathoracic obstruction, such as vocal cord dysfunction or laryngeal pathology. This is not the pattern described in the stem; the scooping is on the *expiratory* limb, not the inspiratory limb.

- **Option 3 (Reduced FVC with preserved ratio)**: The stem explicitly states that FVC is 96% predicted and FEV₁/FVC is 80% (normal), so there is no restrictive defect. A restrictive pattern would show reduced FVC with a preserved or elevated FEV₁/FVC ratio, which is not present here.

- **Option 4 (Reduced FEV₁/FVC <0.7 with COPD pattern)**: The stem shows FEV₁/FVC of 80% (>0.7), which is normal. A COPD pattern with concave expiratory limb and FEV₁/FVC <0.7 represents established obstructive airway disease, not the early, selective small airway obstruction seen in pediatric OSA.

**High-Yield:** FEF₂₅₋₇₅ (MMEF) is the earliest and most sensitive spirometric marker of small airway disease; isolated reduction with preserved FEV₁ is pathognomonic for distal airway obstruction in pediatric OSA and requires adenotonsillectomy ± nasal CPAP.

[cite: Nelson Textbook of Pediatrics 22e; ATS/ERS Pediatric Spirometry Guidelines 2025; Goldbart & Tauman studies on OSA-related small airway dysfunction]

Answer

A. Reduced FEV₁/FVC ratio <0.7 with concave expiratory limb indicating established obstructive airway disease (COPD pattern)

Answer

B. Reduced forced vital capacity with preserved FEV₁/FVC ratio indicating a restrictive ventilatory defect

Answer

D. Flattening of the inspiratory limb indicating variable extrathoracic obstruction from vocal cord dysfunction

Explanation

Why option 1 is correct

Why each distractor is wrong

Practice similar questions