Spirometry is a fundamental test to evaluate lung function, especially in children. Its correct interpretation is key to an accurate diagnosis and adequate management of respiratory diseases.
This interactive guide will take you step-by-step through the principles of interpretation, making sure you understand each concept before moving forward. Let's get started!
Before interpreting any values, it is crucial to ensure that the spirometric test was performed correctly. A poor quality test can lead to erroneous interpretations.
Important! If the quality is not acceptable, the test must be repeated.
Once the quality is good, we compare the obtained values with the reference values predicted for a child of the same age, sex, height and ethnicity.
The results are expressed as a percentage of the predicted value (e.g. FVC % predicted) and as a Z-score (standard deviations from the mean). A Z-score is a more robust measure for children.
The graph above illustrates the distribution of Z-scores, where values between -1.64 and 1.64 are considered within the normal range (approximately 90% of the healthy population).
These are the two most important parameters to start the interpretation.
It represents the total volume of air that a person can exhale with maximum force after a maximum inhalation. It is a main indicator of lung restriction.
It is the volume of air exhaled during the first second of the FVC maneuver. It is the best indicator of airway obstruction.
Illustrative graph of normal ranges of FVC and FEV1.
The FEV1/FVC ratio is the key parameter to identify the presence of airflow obstruction.
A low FEV1/FVC ratio (Z-score ≤ -1.64) indicates obstruction. This is the most sensitive and specific criterion for the diagnosis of obstruction in children.
Graph that illustrates the different patterns according to the FEV1/FVC and FVC ratio.
Forced Expiratory Flow between 25% and 75% of FVC (FEF25-75%) is an indicator of small airway function.
If an obstructive pattern is detected, a reversibility test is performed by administering a bronchodilator (e.g. salbutamol) and repeating spirometry.
A significant bronchodilator response is considered if there is an increase in FEV1 or FVC (or both) of:
In children, an increase in FEV1 Z-score of ≥ 0.29 as significant.
Now let's apply what we've learned with some real cases! Try to interpret the results before seeing the solution.
Patient: 8 year old boy, height 128 cm, male.
Spirometric Results (Z-scores):
Interpretation:
This pattern is consistent with a obstructive pattern. The FEV1 and the FEV1/FVC ratio are reduced, while the FVC is normal. FEF25-75% is also markedly reduced, supporting the presence of airway obstruction, especially small ones.
Patient: 10-year-old girl, height 140 cm, female.
Spirometric Results (Z-scores):
Interpretation:
This pattern is consistent with a restrictive pattern. The FVC is reduced, but the FEV1/FVC ratio is normal. This suggests a limitation in lung volume, typical of restrictive diseases.
Patient: 7-year-old boy, height 125 cm, male.
Spirometric Results (Z-scores):
Interpretation:
All values are within the normal range (Z-scores > -1.64). This indicates a normal lung function.
Read the following case and select the correct interpretation.
Patient: 9-year-old girl, height 135 cm, female.
Spirometric Results (Z-scores):
You have gone through the essential steps to interpret pediatric spirometry. Remember that practice and experience are key to mastering this skill.
If you have any additional questions or want to review a concept, feel free to explore the guide again.