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Spirometry (Pulmonary Function Testing, PFT)

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Spirometry, also known as Pulmonary Function Testing (PFT), is the measurement of lung function, specifically by measuring the volume and speed of air that can be inhaled and exhaled. Spirometry is an important tool used for assessing lung diseases such as asthma, cystic fibrosis, and COPD. 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 patient's height, weight, age, sex, and race. Results over 80% predicted are considered within normal limits, but review by a doctor is necessary for accurate diagnosis of any individual situation.

Spirometry also produces 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 X-Axis and volume along the Y-Axis.

Flow-Volume Loop. Positive values represent expiration, negative values represent inspiration. The trace moves clockwise for expiration followed by inspiration. (Note the FEV1, FEVA1/2 and FEV3 values are arbitrary in this graph and just shown for illustrative purposes, they must be recorded as part of the experiment). The basic spirometry test itself is simple, and varies slightly depending on the equipment used. With older or less expensive machines, the patient is asked to take the deepest breath they can, and then exhale into a machine as hard as possible, for as long as possible. With newer machines, the patient first breathes normally through the mouthpiece, then continues the test in the same fashion. For the test, soft clips are used to prevent air escaping through the nose, and filter mouthpieces are used to prevent the spread of germs - both are disposed when the patient finishes testing. The test is repeated, often three or more times to insure that the results are accurate and repeatable. Sometimes, a bronchodilator or other medication is administered, and then another round of tests taken to asses the effect on the patient.

Due to the patient input required, standard PFTs can only be used on children old enough to comprehend and follow the instructions given. Infant PFTs are possible, but it is an entirely separate process which requires sedation.

Explanation of Common Test Values

  • FVC: Forced Vital Capacity - This is the total amount of air that you can forcibly blow out after full inspiration, measured in liters.

  • FEV 1: 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.

  • FEV 1 / 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 similar to PEF, except the measurement (again liters per second) is taken at the listed portion of the breath out (e.g. between 25% of the expiration and 75%).

  • FIF 25-75% or 25-50%: Forced Inspirtory 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.

Flow-Volume Loop.  Positive values represent expiration, negative values represent inspiration.  The trace moves clockwise for expiration followed by inspiration.  (Note the FEV1, FEVA1/2 and FEV3 values are arbitrary in this graph and just shown for illustrative purposes, they must be recorded as part of the experiment).

Flow-Volume Loop. Positive values represent expiration, negative values represent inspiration. The trace moves clockwise for expiration followed by inspiration. (Note the FEV1, FEVA1/2 and FEV3 values are arbitrary in this graph and just shown for illustrative purposes, they must be recorded as part of the experiment).

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