Pulmonary Function Tests (PFTs) are non-invasive breathing assessments that evaluate how well the lungs are performing. These tests measure the mechanical aspects of breathing, including lung volume, capacity, and the speed of airflow. PFTs also assess the efficiency of gas exchange, which is the process by which oxygen enters the bloodstream and carbon dioxide is removed. Physicians use these objective measurements to diagnose respiratory conditions, determine the severity of lung dysfunction, and monitor a patient’s response to treatment.
Key Measurements of Lung Function
PFTs rely on several quantifiable metrics to build a complete picture of pulmonary mechanics. The Forced Vital Capacity (FVC) measures the total volume of air a person can forcefully exhale after taking the deepest possible breath. The FVC represents the maximum amount of air that can be moved in and out of the lungs during a single breath. The Forced Expiratory Volume in 1 second (FEV1) captures the volume of air expelled during the first second of that forced exhalation.
The relationship between FEV1 and FVC is expressed as the FEV1/FVC ratio, a fundamental metric for identifying airflow limitation. PFTs also measure static volumes, such as Total Lung Capacity (TLC), which is the entire volume of air contained in the lungs after maximum inhalation. Residual Volume (RV) is the amount of air remaining in the lungs even after a forceful exhalation. The Diffusion Capacity of the Lung for Carbon Monoxide (DLCO) measures how efficiently oxygen moves from the air sacs (alveoli) into the red blood cells within the capillaries.
The Procedure and Testing Methods
The core component of a PFT is spirometry, which measures airflow and volume. To perform this, the patient wears a nose clip and seals their mouth around a mouthpiece connected to a spirometer. The technician instructs the patient to take a deep breath in and then blow out as hard and fast as possible until the lungs are empty. This maneuver is repeated several times to ensure the results are consistent and reflect the patient’s maximum effort.
To obtain true lung volumes like TLC and RV, a technique called body plethysmography is employed. For this test, the patient sits inside a sealed enclosure, often called a body box. The patient breathes against a mouthpiece, and the device measures pressure and volume changes inside the box and lungs to calculate air volumes based on Boyle’s Law.
The DLCO measurement is assessed by having the patient inhale a mixture of gases, including a trace amount of carbon monoxide, and holding their breath for about 10 seconds. The amount of the tracer gas absorbed into the blood is then measured. Before testing, patients are advised to avoid using bronchodilator medications and to refrain from heavy meals or smoking, as these factors can influence the results.
Clinical Conditions Identified by PFTs
The patterns revealed by PFTs distinguish between the two primary categories of lung disease. An Obstructive Ventilatory Defect is characterized by difficulty moving air out of the lungs due to narrowed or blocked airways. Conditions in this category include Chronic Obstructive Pulmonary Disease (COPD), such as emphysema and chronic bronchitis, and asthma. This pattern is confirmed by a disproportionately low FEV1 relative to the FVC, resulting in a low FEV1/FVC ratio.
A Restrictive Ventilatory Defect involves difficulty moving air in due to reduced lung expansion or stiffness. This issue can stem from problems with the lung tissue, such as pulmonary fibrosis or interstitial lung disease, or from external factors like chest wall deformities or obesity. The restrictive pattern is identified by a reduction in all lung volumes, most notably the TLC and FVC. The FEV1/FVC ratio remains normal or sometimes higher than predicted because, while total volume is decreased, the inhaled air can be exhaled quickly.
Understanding Your PFT Results
PFT results are presented as a percentage of a predicted normal value rather than simple absolute numbers. This predicted value is calculated using reference equations that account for a person’s age, height, sex, and ethnicity, as these factors influence lung size and function. An abnormal result is defined as a value that falls below the 5th percentile of this predicted range, often called the lower limit of normal.
If an obstructive pattern is confirmed by a low FEV1/FVC ratio, the severity is graded based on the FEV1 percentage of the predicted value. For example, a result between 70% and 80% of predicted might be considered mild, while a value below 30% indicates severe impairment. The DLCO measurement provides further detail, as a low value suggests damage to the air-blood barrier, often seen in emphysema or pulmonary fibrosis. PFTs are only one component of a comprehensive medical assessment and must be interpreted alongside a patient’s medical history and physical examination findings.