Summary of Forced Spirometry: Diagnosis of Lung Disorders

## Introduction This material summarizes the key principles and clinical contexts of pulmonary function measurement using spirometry and related physiological concepts. The goal is to explain why we observe characteristic changes in FEV1 and FVC values in obstructive and restrictive lung diseases, and how to interpret these changes. ## Basic Concepts > **Definition:** FEV1 is the volume of air exhaled during the first second of a forced expiration. > **Definition:** FVC (Forced Vital Capacity) is the total volume of air an individual can forcibly exhale after a maximal inspiration. > **Definition:** The FEV1/FVC ratio is a percentage that shows how much of the FVC was exhaled during the first second. ## How FEV1 and FVC Change in Different Types of Lung Diseases Let's divide this into two main categories: obstructive disorders and restrictive disorders. ### Obstructive Disorders (e.g., emphysema) - Characteristics: narrowing of the airways or reduced ability to exhale air. - Changes in Lung Mechanics: - Lung elasticity (recoil) decreases, lung compliance increases. - The destruction of elastic tissue and alveolar septa (in emphysema) reduces the lung's ability for passive recoil. - Measurement Implications: - $FEV1$ significantly decreases. - $FVC$ may be normal or slightly reduced, but the decrease in $FEV1$ is greater. - The $FEV1/FVC$ ratio decreases. ### Restrictive Disorders (e.g., pulmonary fibrosis, interstitial lung disease) - Characteristics: reduced ability of the lungs to expand, increased stiffness (decreased compliance). - Changes in Lung Mechanics: - Elasticity may be relatively higher or normal, but overall stiffness increases due to fibrosis. - Tissue changes lead to a reduced ability of the lungs to fill with air. - Measurement Implications: - $FVC$ significantly decreases because lung volumes are restricted. - $FEV1$ may decrease, but relatively less than FVC. - The $FEV1/FVC$ ratio remains normal or is increased (often $>80\%$). ## Visualizing Changes (How to Read the Curve) - In obstruction: the forced expiratory curve will show a steeper initial decline and a reduced $FEV1$; the total area and shape of the graph indicate air trapping. - In restriction: the curve will be shifted left/down (lower total volume), the peak of the curve (FVC) is reduced, but the relative portion exhaled in 1 s remains high. ## Practical Example 1. Patient A (obstruction): $FEV1 = 1.0\;\text{l}$, $FVC = 4.0\;\text{l}$, the $FEV1/FVC$ ratio = $\frac{1.0}{4.0}=0.25$ or $25\%$ – significant obstruction. 2. Patient B (restriction): $FEV1 = 3.2\;\text{l}$, $FVC = 3.8\;\text{l}$, the $FEV1/FVC$ ratio = $\frac{3.2}{3.8}\approx0.84$ or $84\%$ – typical for restrictive impairment. ## Table: Comparison of Obstruction vs. Restriction | Property | Obstructive Disease | Restrictive Disease | |---|---:|---:| | $FEV1$ | Decreased | Slightly decreased or normal | | $FVC$ | Normal or slightly decreased | Significantly decreased | | $FEV1/FVC$ | Decreased | Normal or increased (often $>80\%$) | | Elasticity | Decreased | Increased or normal | | Compliance | Increased | Decreased | ## Clinical Applications - Diagnosis: The $FEV1/FVC$ ratio is used to differentiate obstructive and restrictive patterns. - Treatment: For obstructive conditions (e.g., COPD, emphysema), the goal is to improve airway patency; for restrictive conditions, the underlying cause is addressed, along with capacity rehabilitation. - Interpretation: Always compare against reference values based on age, sex, height, and ethnicity. ## Practical Tips for Interpreting Results - Pay attention to absolute values as well as the percentage relationship to expected norms. - An $FEV1/FVC$ value below the diagnostic threshold (typically ~70\% according to some guidelines) indicates obstruction, but always correlate with the clinical picture. - In cases of suspected combined conditions, further testing may be necessary (e.g., diffusing capacity of the lungs for