A spirometer is a medical device that measures lung function by recording how much air a person can breathe out and how quickly the air moves. It is commonly used to help diagnose and monitor asthma, chronic obstructive pulmonary disease, and other breathing conditions. The main results are shown as volumes, flow rates, and graphs that compare a patient with expected values for their age, sex, height, and health.
This makes spirometry an important tool for turning breathing ability into measurable data.
During a test, a patient seals their lips around a mouthpiece, inhales fully, then exhales as hard and as long as possible into the spirometer. Sensors in the device detect airflow and use that information to calculate lung volumes over time. A flow-volume loop or volume-time graph helps clinicians see whether airways are narrowed, lungs are restricted, or effort was incomplete.
Because technique strongly affects the results, clear instructions and repeated trials are essential for accurate measurements.
Key Facts
- Spirometry measures both air volume and airflow rate during breathing.
- FVC = forced vital capacity, the total volume of air exhaled after a full breath in.
- FEV1 = forced expiratory volume in 1 second, the volume exhaled during the first second of a forced breath.
- FEV1/FVC ratio = FEV1 ÷ FVC, a key measure for detecting airflow obstruction.
- Flow rate can be calculated as flow = change in volume ÷ change in time.
- Peak expiratory flow is the highest airflow reached during a forced exhalation.
Vocabulary
- Spirometer
- A spirometer is a device that measures the volume and speed of air a person breathes in or out.
- Forced vital capacity
- Forced vital capacity is the total amount of air a person can forcefully exhale after taking the deepest possible breath.
- Forced expiratory volume in 1 second
- Forced expiratory volume in 1 second is the amount of air exhaled during the first second of a forced exhalation.
- Flow-volume loop
- A flow-volume loop is a graph that shows airflow rate compared with lung volume during a breathing maneuver.
- Airway obstruction
- Airway obstruction is a narrowing or blockage of the air passages that makes it harder to move air out of the lungs.
Common Mistakes to Avoid
- Not sealing the lips tightly around the mouthpiece, because air leaks make the measured volume and flow lower than the true values.
- Starting the exhale slowly, because spirometry depends on a fast, forceful blast to measure accurate FEV1 and peak flow.
- Stopping the exhale too early, because FVC requires breathing out as much air as possible until the lungs are nearly empty.
- Interpreting one number by itself, because FEV1, FVC, the FEV1/FVC ratio, and graph shape must be considered together.
Practice Questions
- 1 A patient exhales 3.2 L in the first second and has an FVC of 4.0 L. Calculate the FEV1/FVC ratio.
- 2 During a forced exhalation, a spirometer records a volume change of 2.4 L over 0.8 s. Calculate the average airflow rate in L/s.
- 3 A patient has a normal FVC but a low FEV1/FVC ratio. Explain what this pattern suggests about the airways and why.