What Spirometry Measures
Spirometry is a physiological test that measures how an individual inhales or exhales volumes of air as a function of time, with the primary signals measured being either volume or flow. 1
Key Measurements in Spirometry
Primary Parameters
- Forced Vital Capacity (FVC): The volume of air delivered during an expiration made as forcefully and completely as possible starting from full inspiration, expressed in liters 1
- Forced Expiratory Volume in 1 second (FEV1): The maximal volume of air exhaled in the first second of a forced expiration from a position of full inspiration, expressed in liters 1
- FEV1/FVC Ratio: The proportion of the FVC exhaled in the first second, a key indicator for detecting airflow obstruction 2
Additional Parameters
- Peak Expiratory Flow (PEF): Maximum expiratory flow achieved from a maximum forced expiration 1, 2
- FEF25-75%: Mean forced expiratory flow between 25% and 75% of the FVC; provides information about small airway function 1, 2
- FEV6: Volume exhaled in the first 6 seconds; can serve as a reliable surrogate for FVC in some cases 2
- FEV1/FEV6: An alternative to FEV1/FVC ratio 2
Clinical Significance
Spirometry serves as a screening test of general respiratory health, similar to how blood pressure provides information about cardiovascular health 1. The measurements obtained through spirometry allow clinicians to:
- Detect and classify ventilatory defects (obstructive, restrictive, or mixed)
- Assess disease severity
- Monitor disease progression
- Evaluate response to treatment
- Screen individuals at risk of pulmonary disease
Technical Considerations
For accurate measurements, spirometry requires:
- Proper equipment calibration (daily volume checks with a 3-L syringe) 1
- Standardized testing procedures
- Patient cooperation and proper technique
- Quality control measures to ensure reproducibility (the two largest values of FEV1 and FVC must be within 0.150 L of each other) 2
Interpretation Framework
When interpreting spirometry results:
- Compare measured values to appropriate reference values based on the patient's ethnicity, age, sex, and height 2
- Normal FEV1 is generally considered to be ≥80% of predicted values, though using the lower limit of normal (LLN) is more accurate 2
- The FEV1/FVC ratio is particularly sensitive for detecting early airway obstruction 2
- Examine multiple parameters together rather than in isolation for comprehensive assessment
Common Pitfalls to Avoid
- Inadequate patient effort leading to falsely low values
- Improper technique affecting the start-of-test and end-of-test criteria
- Using fixed percentage cutoffs rather than statistically derived lower limits of normal
- Failing to verify test quality before interpretation
- Interpreting FEV1 in isolation without considering the FEV1/FVC ratio
Spirometry is an essential tool in respiratory medicine that provides objective measurements of lung function, enabling accurate diagnosis and management of respiratory conditions that impact morbidity, mortality, and quality of life.