A spirometer is a diagnostic tool used to measure how a person moves air into and out of the lungs. The procedure, called spirometry, requires the patient to take a deep breath and then exhale as forcefully and completely as possible into a mouthpiece connected to the device. The primary purpose of this test is to assess lung function and determine if there is any limitation to airflow or a reduction in lung volume. Interpreting the results involves analyzing numerical measurements and visual graphs that provide a complete picture of respiratory health.
Essential Spirometry Measurements
The foundation of a spirometry report rests on core numerical values that quantify the amount of air moved and the speed at which it is expelled. These values are standardized measurements compared against predicted values based on a person’s age, sex, height, and ethnicity. The first measurement is the Forced Vital Capacity (FVC), which represents the total volume of air a person can forcefully exhale after taking a maximal inhalation. A low FVC suggests that the total lung volume is reduced, pointing toward conditions that restrict the lungs from fully expanding.
The second measure is the Forced Expiratory Volume in 1 Second (FEV1), which is the specific volume of air expelled during the first second of that forceful exhalation. FEV1 measures the mechanical efficiency and speed of the airways, indicating how easily air can flow out of the lungs. A reduction in FEV1 suggests that the airways are narrowed or obstructed, slowing the rate of air exit.
While FEV1 and FVC are important individually, their relationship is the most telling diagnostic measure. The FEV1/FVC Ratio is calculated by dividing the FEV1 value by the FVC value and is expressed as a percentage. This ratio is the most important parameter for detecting airflow limitation, showing the proportion of total air capacity expelled in the first second. For a healthy individual, approximately 75% to 85% of the FVC should be exhaled within that first second.
Understanding the Flow-Volume and Volume-Time Graphs
Spirometry reports include two graphical representations that offer visual confirmation of the measurements and the quality of the patient’s effort. The Flow-Volume Loop plots the flow rate of air against the total lung volume. A normal loop begins with a rapid, steep incline to a sharp peak, known as the Peak Expiratory Flow, followed by a linear descent as the lungs empty. In a healthy lung, the expiratory curve typically resembles a tall triangle, demonstrating a rapid expulsion of air.
The initial part of the curve is effort-dependent, determined by the patient’s maximal effort. The latter portion of the curve is effort-independent, controlled primarily by the structural integrity and resistance of the smaller airways.
The second graph is the Volume-Time Curve, which plots the total volume of air exhaled against time. This graph assesses the completeness of the maneuver and the speed at which the FVC is achieved. In a typical test, the volume curve rises steeply, reflecting the rapid FEV1, and then quickly plateaus.
For a valid result, the curve must reach a definite plateau, indicating that no more air is being exhaled, usually within six seconds. If the curve does not plateau or the patient stops exhaling too soon, the test may be considered technically unacceptable. Both graphs must be visually checked to ensure the forced maneuver was performed correctly.
Connecting Results to Lung Conditions
The numerical data and visual graphs categorize lung function into three patterns: normal, obstructive, or restrictive. A normal result is indicated by an FEV1/FVC ratio within the expected range (typically above 70%) and an FVC near the predicted value. The flow-volume loop appears full and triangular, and the volume-time curve plateaus quickly.
An obstructive pattern is characterized by a disproportionately low FEV1 compared to FVC, resulting in a low FEV1/FVC ratio. This suggests difficulty in expelling air due to narrowed airways, common in conditions like Chronic Obstructive Pulmonary Disease (COPD) or asthma. Visually, the flow-volume loop shows a characteristic “scooping” or concave shape in the descending curve, reflecting reduced flow.
A restrictive pattern is indicated by a low FVC, often with a reduced FEV1, but the FEV1/FVC ratio remains normal or elevated. This occurs because the total lung volume is limited, but the airways are not obstructed, so the air that is inhaled is expelled quickly. This pattern is associated with conditions that limit lung expansion, such as pulmonary fibrosis or chest wall deformities.
The restrictive flow-volume loop is typically tall and narrow, maintaining its normal shape but showing a smaller overall volume. While spirometry suggests these patterns, a formal diagnosis of restrictive lung disease often requires additional testing to measure total lung capacity. Only a medical professional can synthesize these findings with a patient’s history to establish a definitive diagnosis and treatment plan.