Spirometry is a common, non-invasive pulmonary function test that measures how effectively a person can move air in and out of their lungs. The test is performed by having an individual inhale deeply and then forcefully exhale into a spirometer. The resulting report contains numerical data and visual graphs that provide medical professionals with a clear picture of lung mechanics. Understanding the fundamental measurements and how they are compared to expected values is the first step in translating the raw data into meaningful information about lung health.
Understanding the Key Measurements
The spirometry report hinges on three main measurements that quantify the volume and speed of air movement. The Forced Vital Capacity (FVC) represents the total volume of air a person can forcefully exhale after taking a maximal breath in. Measured in liters, FVC reflects the overall size and capacity of the lung and chest wall system.
The Forced Expiratory Volume in 1 Second (\(\text{FEV}_1\)) isolates the volume of air expelled in the first second of the forced exhalation maneuver. The \(\text{FEV}_1\) is a direct indicator of how quickly air can flow out of the lungs, making it sensitive to any narrowing or obstruction in the airways. Healthy airways allow a person to rapidly empty a large portion of their lung volume within this single second.
The most important derived value is the \(\text{FEV}_1/\text{FVC Ratio}\), calculated by dividing the \(\text{FEV}_1\) by the FVC and expressed as a percentage. This ratio reveals the proportion of the total air volume expelled during the first second. A high ratio suggests clear, unrestricted airflow, while a low ratio is the primary indicator of an obstructive lung disease.
Analyzing the Numerical Results
Spirometry results are judged by comparing the measured values to the Predicted Value expected for a healthy person with similar characteristics. Predicted values are calculated from large population studies and are tailored to the individual based on their age, biological sex, height, and ethnicity.
The report displays the measured result alongside the specific predicted value for FVC and \(\text{FEV}_1\). The “Percentage of Predicted” column expresses the measured value as a percentage of the predicted value. A result of 100% of predicted means the person’s lung function matches the average expectation, and a value above 80% of predicted is generally considered normal.
Modern interpretation increasingly relies on the Lower Limit of Normal (LLN) to define abnormality. The LLN is a statistically derived cutoff point, representing the lowest 5% of values found in the healthy population. If a measured value falls below its specific LLN, it is definitively classified as an abnormal result.
Identifying the Lung Disease Patterns
The combination of the \(\text{FEV}_1\), FVC, and their ratio allows for the identification of three distinct patterns of lung function: normal, obstructive, and restrictive. The \(\text{FEV}_1/\text{FVC}\) ratio is the initial parameter used to categorize the pattern. If this ratio falls below the LLN, an obstructive pattern is present, indicating difficulty with airflow leaving the lungs.
An Obstructive Pattern is characterized by a low ratio, caused by a disproportionately lower \(\text{FEV}_1\). This indicates air is moving slowly out of the lungs, typical of conditions like asthma and Chronic Obstructive Pulmonary Disease (COPD). Severity is graded based on the \(\text{FEV}_1\) percentage of predicted.
A Restrictive Pattern is suggested when the \(\text{FEV}_1/\text{FVC}\) ratio is normal or high, but the FVC itself is low (below the LLN). The total volume of air the lung can hold is reduced, suggesting a loss of lung volume. Causes include stiff lungs (e.g., pulmonary fibrosis) or issues outside the lungs, such as chest wall deformities.
Reversibility Testing involves repeating the spirometry test after the patient inhales a bronchodilator medication. A significant improvement in the \(\text{FEV}_1\) (an increase of at least 12% and 200 mL) suggests the airflow limitation is reversible. This finding is characteristic of asthma, helping distinguish it from conditions like COPD, where the limitation is generally fixed.
Interpreting the Visual Graphs
The spirometry report includes two primary visual representations that confirm the numerical findings and assess the quality of the test maneuver.
The Flow-Volume Loop plots the speed of airflow against the volume of air in the lungs during both inhalation and exhalation. A normal flow-volume loop has a distinctive shape: a rapid peak flow at the start of exhalation, followed by a straight or slightly convex downward curve as the lungs empty.
The shape of this loop changes dramatically with disease patterns. In an obstructive pattern, the expiratory curve becomes concave or “scooped out,” reflecting the decrease in airflow rate as the lung volume decreases. Conversely, in a restrictive pattern, the loop maintains its normal shape but is narrower and smaller, indicating a reduction in total lung capacity.
The second graph, the Volume-Time Curve, plots the total exhaled volume against the time taken for the maneuver. A normal curve shows a steep rise in volume during the first second, followed by a plateau where no more air can be exhaled, typically reaching this plateau within six seconds.
An obstructive pattern causes the curve to rise slowly without reaching a full plateau, showing that it takes much longer to exhale all the air. Both graphs are also used to check for a valid effort, ensuring the patient exhaled forcefully and completely for the required duration.