Interstitial Lung Disease and Pulmonary Fibrosis

Overview

Interstitial lung disease (ILD) is not a single diagnosis but a broad category of more than 200 distinct disorders that share a common pathologic thread: inflammation and/or fibrosis of the pulmonary interstitium—the connective-tissue scaffold surrounding the alveoli, capillaries, and airways. The end result is stiff, poorly compliant lungs that cannot expand normally and cannot transfer oxygen efficiently across the alveolar-capillary membrane.

Idiopathic pulmonary fibrosis (IPF) is the most studied and most feared form. It is a chronic, relentlessly progressive fibrosis of unknown cause with a median survival of roughly 3–5 years from diagnosis if left untreated. Understanding IPF serves as the clinical template for all fibrosing ILDs, even as the practitioner learns to distinguish the inflammatory forms—which respond to immunosuppression—from purely fibrotic ones, for which steroids cause harm rather than benefit.

For the respiratory therapist, ILD demands mastery of restrictive PFT interpretation, targeted oxygen titration including ambulatory and nocturnal prescriptions, and awareness of the pharmacologic agents that slow—but do not reverse—fibrotic progression.

Key Concepts

Restrictive physiology is the defining functional signature of ILD. Fibrosis and inflammation reduce lung compliance, shrinking all lung volumes. On spirometry and full PFTs this appears as a low FVC and a low TLC. Crucially, the FEV₁/FVC ratio is preserved or elevatedbecause both compartments are reduced proportionately—there is no airflow obstruction. This is the cardinal distinction from obstructive diseases such as COPD or asthma, where FEV₁/FVC falls below the lower limit of normal. The diffusing capacity for carbon monoxide (DⁿCO) is also reduced, reflecting the thickened, scarred alveolar-capillary membrane that impairs gas exchange even before resting hypoxemia becomes apparent.

Categories of ILD span a wide etiologic range:

• Idiopathic interstitial pneumonias: IPF (usual interstitial pneumonia pattern) and nonspecific interstitial pneumonia (NSIP), among others.
• Connective-tissue-disease-associated ILD (CTD-ILD): occurs in rheumatoid arthritis, systemic sclerosis, polymyositis/dermatomyositis, and Sjögren’s syndrome.
• Hypersensitivity pneumonitis (HP): an immune-mediated response to inhaled organic antigens (e.g., bird proteins, mold); exposure removal is essential.
• Sarcoidosis: granulomatous ILD, often involving lymph nodes and other organs.
• Pneumoconioses: dust-induced fibrosis, including asbestosis and silicosis from occupational exposures.
• Drug-induced and radiation-induced ILD: numerous agents (amiodarone, methotrexate, bleomycin) and thoracic radiation can injure the interstitium.

Gas exchange impairmentin ILD arises from two mechanisms working in concert: diffusion limitation across the thickened alveolar-capillary membrane and ventilation-perfusion (V/Q) mismatch in fibrotic lung units. The result is exertional hypoxemia—oxygen saturation that may be acceptable at rest plummets with activity because increased cardiac output demands more rapid gas exchange that the damaged membrane cannot provide.

Assessment & Findings

Symptoms develop insidiously and are frequently attributed to deconditioning or cardiac disease before the diagnosis is made. Progressive exertional dyspnea and a chronic dry, nonproductive cough are the hallmarks. Fatigue is nearly universal. Patients often present late, having adapted to a gradual decline over months to years.

Physical examinationyields findings that, once recognized, are nearly pathognomonic. Fine bibasilar inspiratory crackles with a dry, “Velcro-like” quality are heard in the posterior lung bases and do not clear with coughing. Digital clubbing—loss of the normal angle between the nail and the dorsum of the finger—is particularly common in IPF and signals chronic hypoxemia. Cyanosis is a late finding.

High-resolution CT (HRCT) is the single most important imaging tool in ILD. The pattern guides both diagnosis and management:

• UIP pattern (IPF): bilateral, basal- and subpleural-predominant reticulation, honeycombing (stacked cystic airspaces), and traction bronchiectasis from fibrotic distortion. Ground-glass opacity is absent or minor.
• NSIP / CTD-ILD: bilateral ground-glass opacity, often with subpleural sparing, and less honeycombing than UIP.
• Hypersensitivity pneumonitis: centrilobular nodules, mosaic attenuation, and ground-glass opacity—often upper-lobe predominant in chronic HP.
• Sarcoidosis: perilymphatic micronodules along bronchovascular bundles and subpleural regions, often with hilar lymphadenopathy.

Pulmonary function testsshow the restrictive pattern: low FVC and low TLC with a normal or elevated FEV₁/FVC ratio. The DⁿCO is reduced, often out of proportion to the volume loss, indicating diffusion impairment as an independent component. The 6-minute walk test (6MWT) is a critical functional assessment—exertional desaturation (SpO₂ dropping below 88–90%) during the walk identifies patients who need ambulatory oxygen and provides prognostic information.

Arterial blood gasanalysis typically reveals hypoxemia and a chronic respiratory alkalosis: PaO₂ is reduced, PaCO₂ is low-to-normal (hyperventilation compensates), and pH is mildly elevated. Hypercapnia is unusual until very advanced disease. Hypoxemia worsens predictably and significantly with exertion.

RT Priorities / Interventions

Oxygen therapyis the cornerstone of respiratory therapy management in ILD. The goal is to maintain SpO₂ ≥90% at rest, during ambulation, and during sleep. Because exertional desaturation often precedes resting hypoxemia, the 6MWT is used to identify patients who need ambulatory oxygen even when resting saturations are acceptable. Nocturnal oximetry may be required separately to detect sleep-related desaturation. Oxygen prescription should specify flow rates for rest, exertion, and sleep—the exertional requirement is almost always higher.

Pulmonary rehabilitation is recommended for patients with ILD regardless of disease subtype. A structured program improves exercise capacity, functional walk distance, and dyspnea scores, though it does not alter the underlying disease course. Supervised sessions allow safe titration of supplemental oxygen during exercise.

Antifibrotic agents (RT awareness): pirfenidone and nintedanib are FDA-approved for IPF. Both slow the annual rate of FVC decline by approximately 50% compared with placebo, reducing disease progression and acute exacerbations. They do not reverse established fibrosis, restore lost lung volume, or cure the disease. The RT should understand these limitations when counseling patients and should recognize that ongoing FVC decline despite antifibrotics is expected.

Treating the treatable ILDs: inflammatory ILDs—NSIP, CTD-ILD, sarcoidosis, and hypersensitivity pneumonitis—may respond to immunosuppressive therapy (corticosteroids, azathioprine, mycophenolate). In hypersensitivity pneumonitis and pneumoconioses, identifying and removing the causative antigen or dust exposure is the most important intervention, independent of any pharmacotherapy. Critically, corticosteroids are harmfulin IPF—they increase mortality—making accurate subtype diagnosis essential before any treatment decision.

Advanced disease and acute exacerbations:patients with progressive ILD should be referred early for lung-transplant evaluation, as wait times are substantial. Acute exacerbations of IPF—sudden, unexplained respiratory worsening with new bilateral opacities—carry very high short-term mortality and often require ICU-level care, including high-flow oxygen or noninvasive ventilation for comfort, though outcomes remain poor.

Common Pitfalls

• Misinterpreting a low FVC as obstruction.In restriction, both FVC and FEV₁ fall, keeping FEV₁/FVC normal or even elevated. Always check TLC and FEV₁/FVC together before labeling a pattern.
• Expecting antifibrotics to reverse fibrosis. Pirfenidone and nintedanib slow progression; they do not restore lung function. Counseling patients on realistic expectations reduces frustration and improves adherence.
• Checking SpO₂ only at rest. Resting saturation can be deceptively normal in early-to-moderate ILD. The 6-minute walk test or an exertional oximetry assessment is required to detect clinically significant desaturation and to guide ambulatory oxygen prescription.
• Labeling all ILD as IPF.Many ILDs are inflammatory and respond well to immunosuppression. Applying the IPF label—and withholding steroids—in a patient with CTD-ILD or HP can lead to preventable progression. Equally, prescribing steroids for true IPF causes harm.
• Underestimating nocturnal hypoxemia. Patients with ILD may desaturate significantly during sleep even if ambulatory saturations are borderline acceptable. Nocturnal oximetry should be considered separately from daytime and exertional assessments.

Board Exam Pearls

• Restrictive PFT pattern: low TLC and low FVC with a normal or elevated FEV₁/FVC ratio, plus a reduced DⁿCO. A low FEV₁/FVC rules in obstruction, not restriction.
• IPF radiology/histology = UIP: basal and subpleural honeycombing with traction bronchiectasis on HRCT. Knowing the distribution (basal, subpleural) is testable.
• “Velcro” crackles + clubbing → suspect pulmonary fibrosis. These findings together on a clinical vignette strongly point toward ILD, especially IPF.
• Antifibrotics slow IPF; corticosteroids are harmful in IPF. This bidirectional fact is a high-yield board distinction. Steroids help NSIP, CTD-ILD, HP, and sarcoidosis.
• 6-minute walk desaturation is the key functional marker for ambulatory oxygen prescription and prognosis in ILD.
• Exertional hypoxemia is early; resting hypoxemia is late. V/Q mismatch and diffusion limitation are unmasked by increased cardiac output during exercise.
• Remove the exposure in hypersensitivity pneumonitis and pneumoconioses—this is always a correct intervention regardless of other therapy.

FAQ

Related Resources

Sources

1. Kacmarek RM, Stoller JK, Heuer AJ. Egan's Fundamentals of Respiratory Care. 12th ed. Elsevier; 2021.
2. Raghu G, Remy-Jardin M, Myers JL, et al. Diagnosis of idiopathic pulmonary fibrosis. An official ATS/ERS/JRS/ALAT clinical practice guideline. Am J Respir Crit Care Med. 2018;198(5):e44-e68.
3. West JB, Luks AM. West's Respiratory Physiology: The Essentials. 11th ed. Wolters Kluwer; 2021.