Questions: Pulmonary Fibrosis and Fibrotic Lung Disease

Question 1

A 65-year-old patient presents with progressive exertional dyspnea and dry cough. Pulmonary function tests show a reduced FVC with a normal FEV1/FVC ratio and a reduced DLCO. What is the most likely underlying mechanism?

• A. Airway obstruction from mucus accumulation preventing normal exhalation
• B. Destruction of alveolar walls reducing the total surface area for gas exchange
• C. Excessive collagen deposition stiffening the lung and thickening the alveolar-capillary membrane
• D. Bronchospasm reducing airflow to peripheral alveoli during inspiration

Answer: (C)

The pattern — reduced FVC with preserved FEV1/FVC ratio — is the hallmark of a restrictive ventilatory defect: the lungs are stiff and hard to expand, but once inflated, airflow is unimpaired. Collagen deposition by myofibroblasts reduces lung compliance, shrinks total lung capacity, and thickens the alveolar-capillary membrane, impairing gas diffusion (reflected in the low DLCO). Obstruction (options A and D) would reduce the FEV1/FVC ratio. Alveolar wall destruction (option B) describes emphysema, which is obstructive, not restrictive.

Question 2

Why cannot current anti-fibrotic drugs like pirfenidone and nintedanib reverse established pulmonary fibrosis, even though they slow disease progression?

• A. These drugs target TGF-β signaling pathways that are no longer active once fibrosis is established
• B. Established fibrosis involves deposited collagen and architectural distortion of alveolar structure, and no biological mechanism exists to regenerate that structure
• C. The drugs are only approved for use in early-stage disease and cannot be administered once FVC decline exceeds 10%
• D. Anti-fibrotic drugs inhibit myofibroblast activity but cannot prevent ongoing collagen cross-linking

Answer: (B)

The fundamental problem is irreversibility of structural damage. Once myofibroblasts have deposited type I and III collagen and alveolar architecture has been distorted, there is no regenerative mechanism to rebuild functional gas-exchanging alveoli. Anti-fibrotic drugs reduce the rate of ongoing damage (slowing FVC decline), but they cannot un-lay collagen or restore destroyed tissue. This is why prevention and early detection matter more than any current pharmacologic intervention — a lesson specific to fibrotic diseases compared to inflammatory ones.

Question 3

Pulmonary fibrosis causes a restrictive ventilatory defect — the lungs are stiff and difficult to inflate, but once inflated, airflow through the airways is not significantly impaired.

• T. True
• F. False

Answer: True

This is the defining mechanical consequence of fibrosis. Collagen deposition reduces lung compliance (stiffness increases), so patients must work harder to inhale — inspiratory effort rises while total lung capacity falls. However, the airways themselves are not obstructed: the FEV1/FVC ratio is preserved or even elevated (the lung expels what little air it can hold efficiently). This distinguishes fibrosis (restrictive) from asthma or COPD (obstructive), where airflow is impaired during exhalation.

Question 4

Most forms of pulmonary fibrosis follow a UIP pattern and progressively worsen regardless of the underlying cause or treatment.

• T. True
• F. False

Answer: False

This is a key misconception. UIP (usual interstitial pneumonia) is the pattern seen in IPF and carries the worst prognosis with relentless progression. But other interstitial pneumonia patterns — nonspecific interstitial pneumonia (NSIP), organizing pneumonia — have substantially better prognoses and may stabilize or improve with immunosuppression. Pattern recognition (via HRCT) directly guides prognosis and treatment selection. Pulmonary fibrosis is a spectrum of conditions, not a single entity.

Question 5

Why does the DLCO (diffusing capacity for carbon monoxide) decline in pulmonary fibrosis, and why might it fall before obvious spirometric restriction appears?

Model answer: The DLCO measures how efficiently gas crosses the alveolar-capillary membrane. In pulmonary fibrosis, myofibroblast-deposited collagen thickens this membrane, increasing the diffusion distance for oxygen and CO. This impairs gas exchange even when total lung volume is not yet greatly reduced. Early fibrosis may thicken membranes before the overall scar burden is large enough to reduce TLC or FVC into the abnormal range, so diffusion impairment is a sensitive early indicator of fibrotic disease.

The DLCO is more sensitive to early alveolar membrane damage than spirometry because it directly measures the gas transfer process, not just lung volumes. As fibrosis progresses, both measures decline — but DLCO often leads because it reflects the functional integrity of individual alveoli rather than the aggregate volume available for ventilation.