Spirometry is a common, non-invasive pulmonary function test designed to measure how well a person moves air in and out of the lungs. The test requires the individual to inhale deeply and then forcefully exhale into a spirometer. By recording the volume and flow of air during this maneuver, healthcare providers generate objective measurements of lung mechanics. These measurements are compared against reference data to determine if lung function is within expected limits. The primary goal of this procedure is to diagnose and monitor various respiratory conditions, such as asthma and Chronic Obstructive Pulmonary Disease (COPD). Analyzing the results helps determine the presence of airflow limitations or reductions in lung volume, which guides treatment decisions.
Key Metrics of Lung Function
Interpreting a spirometry report begins with understanding the two main measurements. The Forced Vital Capacity (FVC) represents the total amount of air an individual can forcefully exhale after taking a maximal breath, indicating the overall size and capacity of the lungs. The Forced Expiratory Volume in 1 Second (FEV1) measures the volume of air expelled during the first second of the FVC maneuver. A healthy person can expel a large majority of their total lung capacity in that initial second, demonstrating unrestricted airflow. These measured values are compared to “predicted values,” which are statistically derived averages for healthy individuals. Predicted values are calculated using a formula that accounts for a patient’s age, height, sex, and ethnicity, as these factors naturally influence lung size and capacity. The results are typically expressed as a percentage of this predicted value. If both the FVC and FEV1 percentages of predicted are above 80%, the lung function is generally considered to be within the normal range.
Identifying Obstructive and Restrictive Patterns
The primary step in interpreting spirometry results is determining the FEV1/FVC ratio. This ratio is expressed as a percentage and indicates the proportion of total air expelled in the first second of forced exhalation. For adults, a ratio lower than approximately 70% defines an obstructive pattern, signaling a problem with airflow leaving the lungs.
Obstructive Pattern
An obstructive pattern occurs when the airways are narrowed or blocked, making it difficult to push air out quickly. The total lung volume (FVC) may be normal or only slightly reduced. Conditions like COPD, asthma, and emphysema are typically characterized by a low FEV1/FVC ratio because the FEV1 decreases disproportionately compared to the FVC.
Restrictive Pattern
A restrictive pattern is suggested when the FEV1/FVC ratio is normal or sometimes higher than normal. This occurs because, while the total lung capacity (FVC) is significantly reduced, the flow rate during the first second remains efficient for the volume of air available. In a restrictive pattern, the lungs have difficulty fully expanding, which leads to a low FVC. This limitation can be caused by conditions that stiffen the lung tissue, such as pulmonary fibrosis, or by issues affecting the chest wall or respiratory muscles. When spirometry shows a low FVC but a normal FEV1/FVC ratio, it suggests a reduced lung volume, requiring further testing to confirm a definitive restrictive diagnosis.
Grading the Level of Impairment
Once an obstructive pattern is identified by a low FEV1/FVC ratio, the next step is to quantify the degree of lung impairment using the FEV1 value. The severity of the obstruction is determined by comparing the patient’s FEV1 to the predicted value, expressed as the FEV1 percentage of predicted (FEV1% predicted). This percentage provides a standardized way to grade the disease severity based on airflow limitation.
The system for grading obstructive disease severity classifies the impairment into stages based on FEV1% predicted thresholds:
- Mild obstruction: FEV1 greater than 80% of the predicted value.
- Moderate obstruction: FEV1 falls into the 50% to 79% range.
- Severe obstruction: FEV1 values between 30% and 49%.
- Very severe impairment: FEV1 value below 30%.
In cases of obstruction, doctors often perform a reversibility test by repeating the spirometry maneuver after administering an inhaled bronchodilator medication. This test assesses whether the narrowing of the airways can be reversed or improved. A significant positive response is defined as an increase in the FEV1 of at least 12% and a minimum of 200 mL compared to the baseline measurement. A strong reversible component frequently suggests a diagnosis of asthma, while a lack of significant reversibility is more common in conditions like COPD.