A 48-year-old man with severe community-acquired pneumonia is intubated in the ICU 36 hours after admission. Despite FiO₂ 0.8 and PEEP 12 cm H₂O, his PaO₂ is 92 mmHg (P/F ratio = 115). Chest X-ray shows bilateral diffuse infiltrates and bedside echocardiography reveals a normal left ventricle with no elevated filling pressures. ARDS is diagnosed by Berlin Criteria. Three weeks after extubation, pulmonary function testing is performed. The spirometric pattern marked **A** in the diagram is observed. Which of the following best explains the physiological basis for this finding in post-ARDS recovery?

Question

Accepted Answer

C. Proportional reduction in both FVC and TLC due to diffuse alveolar damage and pulmonary fibrosis, with preserved FEV₁/FVC ratio. ## Why option 1 is right

In ARDS, diffuse alveolar damage with hyaline membrane formation, surfactant loss, and protein-rich pulmonary edema cause widespread alveolar-capillary injury. During recovery and healing, this progresses to pulmonary fibrosis with loss of functioning alveolar units. On post-extubation spirometry, both the forced vital capacity (FVC) and total lung capacity (TLC) fall proportionally because the restrictive process reduces the total volume of air the lungs can hold. Critically, the FEV₁/FVC ratio remains normal (>0.7) because both the numerator (FEV₁) and denominator (FVC) decline in parallel — the hallmark of restrictive physiology. This pattern, marked **A** in the diagram, is the expected spirometric signature of post-ARDS fibrosis and is consistent with the Berlin Criteria definition of ARDS, which identifies bilateral opacities and impaired oxygenation (P/F 115 = severe) in the context of non-cardiogenic respiratory failure.

[cite: ARDS Definition Task Force (Berlin), JAMA 2012; ARDSnet (NEJM 2000); Harrison's Principles of Internal Medicine 21e]

## Why each distractor is wrong

- **Option 2**: Selective reduction in FEV₁ with preserved FVC describes an obstructive pattern (marked **B**), not the restrictive pattern seen in post-ARDS fibrosis. Surfactant depletion causes acute oxygenation failure in ARDS, not chronic airway obstruction.

- **Option 3**: Isolated DLCO reduction with normal spirometry (marked **C**) suggests early pulmonary vascular disease or interstitial lung disease without parenchymal restriction. While some ARDS survivors develop pulmonary hypertension, the spirometric finding here is clearly restrictive, not isolated gas exchange impairment.

- **Option 4**: A reduced FEV₁/FVC ratio (<0.7) with air trapping (marked **D**) is an obstructive or mixed pattern typical of COPD or bronchiectasis, not the proportional reduction seen in restrictive post-ARDS fibrosis. Post-ARDS does not typically cause chronic airway obstruction.

**High-Yield:** In restrictive lung disease (including post-ARDS fibrosis), both FVC and TLC fall proportionally, so FEV₁/FVC remains normal; in obstructive disease, FEV₁ falls more than FVC, so FEV₁/FVC drops below 0.7.

[cite: ARDS Definition Task Force (Berlin), JAMA 2012; ARDSnet (NEJM 2000); PROSEVA (NEJM 2013); Harrison's Principles of Internal Medicine 21e]

Answer

A. Reduced FEV₁/FVC ratio (<0.7) with air trapping, suggesting post-ARDS bronchiectasis and chronic airway disease

Answer

B. Selective reduction in FEV₁ with preserved FVC, reflecting small airway obstruction from surfactant depletion

Answer

D. Isolated reduction in DLCO with normal spirometry, indicating early pulmonary vascular remodeling

Explanation

Why option 1 is right

Why each distractor is wrong

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