A spirometer is a medical device used to assess lung function by measuring the movement of air into and out of the lungs. This information, recorded during a forceful breathing maneuver, helps healthcare providers evaluate ventilation and the mechanical properties of the lungs. Spirometry identifies and monitors conditions that affect airflow and lung capacity (such as asthma, COPD, and pulmonary fibrosis), tracks disease progression, and assesses treatment effectiveness.
The Technology Behind the Measurement
Spirometers translate a patient’s breath using one of two primary mechanical principles: volume displacement or flow sensing. Older, traditional models often use the volume displacement method, where exhaled air physically moves a component within the machine. A classic example is the water-seal spirometer, which uses an inverted, floating bell or drum suspended over a water-filled chamber.
As the patient exhales into the device, the air collects beneath the bell, causing it to rise. A recording system, historically a pen on a rotating drum or modern digital sensors, tracks the vertical movement of the bell over time. Another volume-displacement type uses a rolling seal, where a piston or bellows moves within a cylinder, measuring the total volume of air displaced.
Modern, portable, and handheld spirometers rely on flow-sensing technology, which measures the rate of airflow rather than volume. The most common mechanism is the pneumotachograph, which works by introducing a slight resistance into the airway, such as a fine mesh screen or capillary tubes. Air moving through this resistance creates a small pressure drop, which is measured by a differential pressure transducer.
This pressure difference is directly proportional to the rate of airflow. Other electronic flow sensors include turbine spirometers, where the breath spins a vane, and ultrasonic spirometers, which measure how the speed of sound waves is altered by the velocity of the moving air. The flow rate signal is then electronically integrated over the duration of the breath. This mathematical process converts the raw flow data into the required volume measurements.
Key Measurements Recorded
The spirometry test generates several metrics derived from a forceful breathing maneuver where the patient inhales completely and then exhales as hard and fast as possible. The primary measurement of lung capacity is the Forced Vital Capacity (FVC), which represents the total volume of air exhaled during this maximum effort.
The FVC measures the total amount of air the lungs can hold and expel. For instance, a healthy adult male may have an FVC ranging from 4.75 to 5.5 liters. This volume is often reported alongside the Peak Expiratory Flow (PEF), which is the maximum speed achieved during the forceful exhalation.
A second metric is the Forced Expiratory Volume in 1 second (FEV1). This measures the volume of air expelled from the lungs during the first second of the FVC maneuver. The FEV1 reflects the ease and speed with which air can be pushed out of the lungs.
The relationship between these two values is expressed as the FEV1/FVC ratio. This ratio is calculated by dividing the FEV1 by the FVC and is usually presented as a percentage. It represents the proportion of the total capacity that can be expelled in the first second, and healthy adults typically have a ratio between 70% and 85%.
Interpreting Test Results
The data collected by the spirometer is compared against predicted normal values to determine if a patient’s lung function is impaired. These predicted values are calculated using established formulas that factor in the individual’s sex, age, height, and ethnicity. A result is typically considered normal if it falls within the expected range, often defined as 80% or more of the predicted value.
A low FEV1/FVC ratio indicates an obstructive ventilatory impairment. This pattern suggests difficulty in expelling air due to narrowing or blockage of the airways, which is characteristic of conditions like asthma or COPD. In this case, the FVC might be normal, but the FEV1 is disproportionately reduced because the flow of air is slowed.
Conversely, a restrictive pattern is suggested when the FEV1/FVC ratio remains normal or high, but the Forced Vital Capacity itself is reduced. This finding points to a problem with lung volume, where the lungs cannot fully expand. Conditions such as pulmonary fibrosis, which causes scarring and stiffening of lung tissue, typically present with a restrictive pattern.
When both the FEV1 and FVC are low, the FEV1/FVC ratio is used to differentiate between a restrictive pattern, an obstructive pattern, or a mixed defect. The severity of any impairment, whether obstructive or restrictive, is then graded by looking at how far the FEV1 value falls below the predicted percentage. This comparison of actual measurements to predicted norms allows clinicians to categorize the functional impact of a patient’s respiratory health.