Spirometry is a common test that measures how much air a person can breathe in and out, and how quickly they can exhale that air. It is a type of pulmonary function test that helps healthcare providers assess lung health and diagnose various respiratory conditions. It can help determine if symptoms like wheezing, shortness of breath, or a persistent cough are related to a lung issue.
Understanding Key Spirometry Measurements
Forced Vital Capacity (FVC) represents the total amount of air you can forcefully exhale after taking the deepest possible breath. A lower than typical FVC can suggest restricted breathing, meaning your lungs may not be able to expand fully or hold as much air as expected.
Forced Expiratory Volume in 1 second (FEV1) measures how much air you can force out of your lungs in one second after a full inhalation. This value helps identify airway obstructions. A reduced FEV1 often points to blockages within the bronchial tubes.
The FEV1/FVC Ratio is a calculated value that compares the amount of air exhaled in the first second (FEV1) to the total forced exhalation (FVC). This ratio is expressed as a percentage. A lower ratio, specifically below 70%, often suggests an obstructive pattern where airflow is limited.
Peak Expiratory Flow (PEF) represents the maximum speed at which you can exhale air. While FEV1 provides insight into sustained airflow, PEF captures the instantaneous force of exhalation. It indicates how open the airways are and is often used by individuals with asthma to monitor their condition at home.
Interpreting Your Spirometry Report
Interpreting a spirometry report involves comparing your measured values to “predicted” values. These predicted values are established based on factors such as your age, sex, height, and ethnicity. Laboratories use large databases to generate these expected ranges, allowing for accurate comparison.
Your results are often presented as percentages of these predicted values. For instance, an FEV1 of 80% predicted means your measured FEV1 is 80% of what is expected for someone with your demographic characteristics. Values within 80-120% of predicted are considered within the normal range, though specific thresholds can vary.
An obstructive pattern is indicated when there is a reduction in the FEV1/FVC ratio, below 70%, while FVC might be near normal or only slightly reduced. This pattern suggests that air is having difficulty flowing out of the lungs, which is characteristic of conditions like asthma or chronic obstructive pulmonary disease (COPD). In asthma, the obstruction may improve after using a bronchodilator.
A restrictive pattern is identified when both FVC and FEV1 are reduced proportionally, but the FEV1/FVC ratio remains normal. This indicates that the lungs are not able to fully expand, limiting the total amount of air they can hold. Conditions such as pulmonary fibrosis, where lung tissue becomes stiff, present with a restrictive pattern.
Normal lung function is indicated when all measured values, including FVC, FEV1, and the FEV1/FVC ratio, fall within their respective predicted ranges. This suggests that your lungs are functioning as expected without airflow limitations or reductions in lung volume. However, even with normal values, a healthcare provider will consider your symptoms and medical history for a complete assessment.
Visualizing Results and Ensuring Test Accuracy
Spirometry reports include graphical representations of your breathing patterns, which offer additional insights beyond the numerical data. The flow-volume loop plots the rate of airflow against the volume of air exhaled and inhaled. A healthy loop has a characteristic shape, with a rapid peak flow followed by a steady decline.
In obstructive lung diseases, the flow-volume loop shows a scooped-out appearance on the expiratory limb, indicating reduced airflow. For restrictive conditions, the loop appears narrower but maintains its normal shape, reflecting a reduction in overall lung volume. Analyzing these visual cues helps confirm the patterns identified by the numerical values.
The volume-time curve plots the exhaled volume of air against time. In a healthy individual, this curve shows a rapid initial rise, reaching a plateau as all air is exhaled within 6 seconds.
Poor patient effort during the test can impact the accuracy of the spirometry report. If the patient does not take a maximal inspiration or exhale forcefully and completely, the measured values will be artificially low, potentially leading to misinterpretation.
Proper technique, including sealing the lips tightly around the mouthpiece and maintaining a consistent effort, is important. A technician provides instructions and encourages the patient throughout the maneuver to ensure the best effort. Multiple attempts are performed to obtain reproducible efforts, with the best values used for the report.