Spirometry

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Spirometry (meaning the measuring of breath) is the most common of the Pulmonary Function Tests (PFTs), measuring lung function, specifically the measurement of the amount (volume) and/or speed (flow) of air that can be inhaled and exhaled. Spirometry is an important tool used for assessing conditions such as asthma, cystic fibrosis, and COPD.

Spirometric maneuvers include slow vital capacity (SVC), forced vital capacity (FVC), tidal volume (TV) and maximum voluntary ventilation (MVV). The measurement of forced vital capacity is the most commonly performed maneuver, sometimes in conjunction with slow vital capacity and/or tidal volume. A plethysmograph can be used to measure functional residual capacity (FRC).

Results are usually given in both raw data (liters, liters per second) and percent predicted - the test result as a percent of the "predicted values" for the patients of similar characteristics (height, weight, age, sex, and sometimes race). The interpretation of the results can vary depending on the physician and the source of the predicted values, but generally speaking, results nearest to 100% predicted are the most normal, and generally results over 80% are often considered normal. However, review by a doctor is necessary for accurate diagnosis of any individual situation.

The spirometry test is performed using a device called a spirometer, which comes in many different varieties. Many produce a graph called a Flow-Volume Loop, which graphically depicts the flow of air compared to the total volume inspired or expired. Flow is listed along the Y-Axis and volume along the X-Axis. Most spirometers also display a Volume-Time curve, showing volume (liters) along the Y-Axis and time (seconds) along the X-Axis (not shown).

Image:Flow-volume-loop.png

The basic FVC test varies slightly depending on the equipment used. Generally, the patient is asked to take the deepest breath they can, and then exhale into the sensor as hard as possible, for as long as possible. It is sometimes directly followed by a rapid inhalation (inspiration), in particular when assessing possible inspiratory obstruction or restriction. Sometimes, the test will be preceded by a period of quiet breathing in and out from the sensor (tidal volume), or the rapid breath in (forced inspiratory part) will come before the forced exhalation. During the test, soft nose clips may be used to prevent air escaping through the nose. Filter mouthpieces may be used to prevent the spread of germs, particularly for inspiratory maneuvers.

The maneuver is highly dependent on patient cooperation and effort, and is normally repeated at least three times to ensure reproducibility. Since results are dependent on patient cooperation, FEV₁ and FVC can only be underestimated, never overestimated.

Sometimes, to assess the reversibility of a particular condition, a bronchodilator is administered before performing another round of tests for comparison. This is commonly referred to as a reversibility test, or a post bronchodilation test (Post BD), and is an important part in diagnosing asthma versus COPD.

The most commonly used guidelines for spirometric testing and interpretation are set by the American Thoracic Society (ATS) and the European Respiratory Society (ERS).

Spirometry can also be part of a bronchial challenge test, used to determine bronchial hyperresponsiveness to either rigorous exercise, inhalation of cold/dry air, or with a pharmaceutical agent such as metacholine or histamine.

Due to the patient cooperation required, spirometry can only be used on children old enough to comprehend and follow the instructions given (typically about 4-5 years old), and only on patients that are able to understand and follow instructions - thus, this test is not suitable for patients that are unconscious, heavily sedated, or have limitations that would interfere with vigorous respiratory efforts. Other types of PFTs are available for infants and unconscious persons.

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Explanation of Common Test Values in FVC Tests

FVC: Forced Vital Capacity - This is the total amount of air that you can forcibly blow out after full inspiration, measured in liters.
FEV₁: Forced Expiratory Volume in 1 Second - This is the amount of air that you can forcibly blow out in one second, measured in liters. Along with FVC it is considered one of the primary indicators of lung function.
FEV₁ / FVC - This is the ratio of FEV 1 and FVC, which showing the amount of the FVC that can be expelled in one second. In healthy adults this should be approximately 80%.
PEF: Peak Expiratory Flow - This is the speed of the air moving out of your lungs at the beginning of the expiration, measured in liters per second.
FEF 25-75% or 25-50%: Forced Expiratory Flow 25-75% or 25-50% - This is the average flow (or speed) of air coming out of the lung during the middle portion of the expiration (also sometimes referred to as the MMEF, for maximal mid-expiratory flow).
FIF 25-75% or 25-50%: Forced Inspiratory Flow 25%-75% or 25%-50% - This is similar to FEF 25%-75% or 25%-50% except the measurement is taken during inspiration.
FET: Forced Expiratory Time - This measures the length of the expiration in seconds.