Spirometry is a non-invasive test that measures the volume of air you can inhale and exhale, and how quickly you can move air out of your lungs. This assessment helps healthcare providers diagnose and monitor various respiratory conditions. The test involves breathing into a device called a spirometer, which records these measurements for interpretation.
Essential Spirometry Measurements
To understand your results, it is necessary to know what the measurements represent. The Forced Vital Capacity (FVC) is the total amount of air you can forcefully exhale after taking your deepest possible breath. This value gives an indication of your total lung capacity. A low FVC might suggest that the lungs are restricted from fully expanding.
Another measurement is the Forced Expiratory Volume in one second (FEV1). This is the volume of air you can blow out in the first second of the FVC maneuver. FEV1 is informative for assessing how easily air can flow out of your airways; a lower FEV1 often points to an obstruction that is slowing down your exhalation, such as from mucus or narrowed airways.
The FEV1/FVC ratio is calculated by dividing your FEV1 by your FVC. This ratio, expressed as a percentage, is an indicator of airflow limitation. It helps distinguish between different categories of lung disease by showing what proportion of your total lung capacity can be exhaled in the first second. A low ratio signifies an obstructive issue, while a normal ratio with a low FVC can point toward a restrictive problem.
A final parameter is the Peak Expiratory Flow (PEF), which measures the fastest speed at which you can exhale air. While FEV1 measures volume over one second, PEF captures the maximum velocity of that airflow. This measurement is useful for monitoring day-to-day changes in conditions like asthma, as it can reflect acute narrowing of the airways.
Reading Your Spirometry Results
When you receive a spirometry report, the numbers are presented in a specific context for standardized interpretation. Your results will show “actual” or “measured” values, which are the raw numbers from your performance on the test. These numbers are not very informative without a point of comparison.
The report will include a set of “predicted” values. These are the results expected for a healthy individual of the same age, height, sex, and ethnicity. These predictions are generated from population studies and serve as a benchmark for what is considered normal.
To make the comparison straightforward, your results are often shown as a “percent predicted” (% Pred). This figure takes your actual value, divides it by the predicted value, and multiplies it by 100. For example, if your measured FEV1 was 2.0 liters and your predicted FEV1 was 4.0 liters, your FEV1 % predicted would be 50%. Some reports may also reference a Lower Limit of Normal (LLN), a more individualized cutoff for abnormality.
Identifying Lung Function Patterns
By analyzing the relationships between your FVC, FEV1, and FEV1/FVC ratio, a healthcare provider can identify specific patterns of lung function. A normal pattern is characterized by an FVC and FEV1 that are both above 80% of their predicted values, along with an FEV1/FVC ratio above 0.70 (70%). These figures indicate that your lung volume and airflow are within the expected healthy range.
An obstructive pattern is the hallmark of conditions where airflow out of the lungs is hindered, such as in asthma or Chronic Obstructive Pulmonary Disease (COPD). This pattern is identified by a reduced FEV1/FVC ratio, which falls below 0.70 or the LLN. In this scenario, the FEV1 is often more reduced than the FVC, because the issue is the speed of exhalation, not the total volume of the lungs.
Conversely, a restrictive pattern suggests that the lungs cannot fully expand, thus reducing total lung volume. This can be caused by conditions like pulmonary fibrosis or issues with the chest wall. In a restrictive pattern, the FVC is reduced (below 80% of predicted), and the FEV1 is also reduced proportionally. Because both values decrease together, the FEV1/FVC ratio remains normal or can be high.
A report may indicate a mixed pattern, meaning there is evidence of both obstructive and restrictive processes. A mixed pattern is identified by having both a low FEV1/FVC ratio (indicating obstruction) and a low FVC (indicating restriction).
Gauging Severity and Treatment Response
After a pattern is identified, its severity is determined. For obstructive defects like COPD, severity is graded based on the FEV1 percent predicted value, with lower percentages indicating more severe obstruction. For restrictive defects, severity is graded using the FVC percent predicted, as this value reflects the degree of volume loss.
A part of the diagnostic process, particularly when obstruction is found, is to assess for bronchodilator reversibility. This involves repeating the spirometry test 10-15 minutes after you inhale a bronchodilator medication designed to open the airways. A significant positive response, an increase of at least 12% and 200 milliliters in either your FEV1 or FVC, suggests the obstruction is reversible.
This reversibility is an indicator of asthma, as its airway narrowing is often temporary and responsive to medication. In contrast, the airflow limitation in COPD is fixed and shows little to no improvement after a bronchodilator. This follow-up test provides information for confirming a diagnosis and guiding the treatment strategy.
Contextualizing Results and Further Actions
Spirometry results do not exist in a vacuum and must be interpreted by a healthcare professional. Your doctor will consider your spirometry numbers alongside your specific symptoms, personal and family medical history, and findings from a physical examination. This complete clinical picture is necessary for an accurate diagnosis.
Depending on the spirometry findings, your doctor may recommend further investigations. For example, if a restrictive pattern is found, you may be sent for a chest X-ray or CT scan to look for structural changes in the lungs. More detailed pulmonary function tests (PFTs) might be ordered to measure lung volumes more directly or assess how well oxygen moves from your lungs into your bloodstream.
Ultimately, the goal of interpreting these results is to guide decisions about your health. This could involve starting a new treatment, adjusting an existing one, or monitoring your lung function over time. The information in this article is for educational purposes, and you should always discuss your specific results and health concerns with your healthcare provider.