A Pulmonary Function Test (PFT) is a non-invasive procedure that measures lung capacity, volume, and airflow rates. PFTs are frequently performed to diagnose common respiratory conditions, such as chronic obstructive pulmonary disease (COPD), asthma, and pulmonary fibrosis. Physicians use this objective data to gauge the severity of lung impairment, monitor disease progression, and evaluate treatment effectiveness.
The Foundation: Key Spirometry Measurements
Spirometry is the initial and most common part of a PFT, focusing on forced breathing maneuvers using three primary values. Forced Vital Capacity (FVC) represents the total volume of air you can forcefully exhale after taking the deepest possible breath. A low FVC suggests a limitation in the total amount of air the lungs can hold or move.
Forced Expiratory Volume in 1 Second (FEV1) quantifies the volume of air forcefully expelled from the lungs during the first second of exhalation. This value reflects the speed and ease with which air leaves the airways, acting as a direct indicator of airflow obstruction.
The relationship between these two volumes is summarized by the FEV1/FVC Ratio, calculated by dividing the FEV1 by the FVC. This ratio is the most informative number on the spirometry report, revealing whether reduced lung function is due to narrowed airways or limited lung volume. In healthy adults, this ratio typically falls between 70% and 85%.
Establishing the Baseline: Understanding Predicted Values
PFT results must be interpreted relative to the “Predicted Value,” a statistical average calculated using demographic data. Reference equations account for your age, height, sex, and ethnicity, as these factors naturally influence lung size and function. Results are reported as a percentage of this predicted value, showing how your performance compares to the expected average for your demographic.
For example, an FEV1 of 100% predicted means your airflow aligns perfectly with the statistical norm. A result is considered abnormal only if it falls below the Lower Limit of Normal (LLN). The LLN is mathematically defined as the 5th percentile of results from a healthy population.
While 80% predicted is sometimes used as a simple guideline, the LLN is a more statistically accurate cutoff, especially for the FEV1/FVC ratio, because it accounts for the natural decline in lung function that occurs with age.
Interpreting Results: Identifying Obstructive and Restrictive Patterns
Synthesizing the FEV1, FVC, and their ratio allows for the identification of two primary patterns of lung dysfunction.
Obstructive Pattern
The Obstructive Pattern is characterized by a disproportionately low FEV1 compared to FVC, resulting in an FEV1/FVC ratio typically below the LLN. This pattern signals difficulty moving air out of the lungs quickly due to narrowed or blocked airways. Common causes include asthma, chronic bronchitis, and emphysema (components of COPD).
The inability to fully empty the lungs can lead to air trapping and hyperinflation. Severity is graded based on how low the FEV1 percentage of predicted is.
Restrictive Pattern
The Restrictive Pattern is suggested when the FVC is low, but the FEV1/FVC ratio remains normal or elevated. This indicates that the airways are open, but the total lung volume is reduced, making it difficult to get air in. Restriction can stem from issues that stiffen the lung tissue, such as pulmonary fibrosis, or external factors like chest wall deformities or severe obesity.
A normal pattern is confirmed when the FEV1, FVC, and the FEV1/FVC ratio are all above the lower limit of normal. A Mixed Pattern shows characteristics of both obstruction and restriction, typically a low FEV1/FVC ratio accompanied by a significantly low FVC. This complex presentation requires further investigation using additional lung volume measurements.
Beyond Flow Rates: Assessing Lung Volumes and Gas Exchange
While spirometry provides a strong initial assessment, a complete PFT often includes measurements of static lung volumes and gas exchange for a deeper diagnosis. Lung Volume Measurements, such as Total Lung Capacity (TLC) and Residual Volume (RV), confirm or rule out a restrictive pattern. TLC is the total amount of air the lungs can hold after a maximal inhalation.
A TLC value below the LLN definitively confirms a true restrictive defect. Conversely, an increased RV (the air remaining after maximal exhalation) points toward air trapping and hyperinflation, often seen in severe obstructive diseases like emphysema.
The Diffusing Capacity of the Lung for Carbon Monoxide (DLCO) measures how efficiently oxygen moves from the air sacs into the bloodstream. This test uses carbon monoxide to indirectly assess the integrity of the alveolar-capillary membrane. A low DLCO suggests damage to the lung tissue’s gas-exchange surface, a finding common in diseases like emphysema and pulmonary fibrosis.