Obstructive vs. Restrictive Lung Disorders: A Clinical Guide

Exam Tip: When you see "increased airway resistance" or "difficulty with expiration" in a clinical vignette, think Obstructive.

Restrictive Lung Disorders: The Problem of Compliance

Restrictive lung diseases are characterized by a decrease in lung compliance. Compliance refers to the "stretchability" of the lungs—how easily they can expand when pressure is applied.

Pathophysiology

In restrictive patterns, the lungs are "stiff." This stiffness can stem from the lung tissue itself (intrinsic) or from the chest wall and pleura (extrinsic). Because the lungs cannot expand fully, the total lung capacity (TLC) is significantly reduced. Unlike obstructive patients, restrictive patients typically have no trouble getting air out; rather, they have a profound struggle getting enough air in.

Common Examples

• Intrinsic (Lung Tissue): Pulmonary fibrosis (scarring of the lung parenchyma) or sarcoidosis.
• Extrinsic (Chest Wall/Neuromuscular): Obesity (excess weight restricting chest expansion), kyphoscoliosis (spinal curvature), or neuromuscular diseases like ALS or Myasthenia Gravis.

Comparing Clinical Presentations and Spirometry

To distinguish these two, clinicians rely heavily on Pulmonary Function Tests (PFTs), specifically spirometry. The most critical ratio to monitor is the FEV1/FVC ratio.

• FEV1 (Forced Expiratory Volume in 1 second): The amount of air exhaled during the first second of a forced breath.
• FVC (Forced Vital Capacity): The total amount of air that can be forcibly exhaled from the lungs after taking the deepest breath possible.

The Diagnostic Breakdown

Understanding the Ratio

In obstructive disease, the FEV1 drops much more significantly than the FVC because the airways are blocked during the exhale. Therefore, the ratio:

$\frac{FEV_1}{FVC} < \text{normal threshold}$

In restrictive disease, both FEV1 and FVC decrease because the lungs are simply smaller and cannot hold as much air. However, because the airways themselves are not blocked, the patient can still blow out the small amount of air they have very quickly. This results in a ratio that remains normal or even appears high:

$\frac{FEV_1}{FVC} \geq \text{normal threshold}$

Summary Case Study

Patient A presents with a chronic cough and wheezing. Spirometry shows a Total Lung Capacity (TLC) of 120% of predicted and an FEV1/FVC ratio of 0.55.

• Diagnosis: Obstructive (due to hyperinflation and low ratio).

Patient B presents with rapid, shallow breathing and progressive dyspnea. Spirometry shows a TLC of 60% of predicted and an FEV1/FVC ratio of 0.85.

• Diagnosis: Restrictive (due to low lung volumes and normal ratio).