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Measuring Air Flow


Updated September 29, 2008


Breathing into a spirometer

Photo © A.D.A.M

Spirometry, which measures air movement, is the most commonly prescribed pulmonary function test (PFT). To do the test, a soft clip is placed on the patient’s nose to direct all of the air flow to the mouth. The patient takes a big, deep breath, inhaling as deeply as possible, then blows the air out as quickly and completely as possible into a mouthpiece that is connected to a machine called a spirometer. The inhale/exhale sequence is repeated at least two more times, or more if necessary for the technician to get a good reading.

The spirometer measures air flow in liters, but the results are reported in percentages. The percentage represents how the patient’s air flow compares to the air flow expected in an average person with the same physical characteristics. There are many measurements taken during spirometry. Let’s take a look at some of them.

FEV1 - Forced Expiratory Volume

FEV1 is the amount of air that can be forcefully exhaled in the first second, which indicates how well the large and medium-sized airways are functioning. FEV1 can be used as a guideline to describe the severity of lung disease:

FEV1 80 - 100%: Normal
FEV1 60 – 79%: Mild
FEV1 40 – 59%: Moderate
FEV1 35 - 40%: Severe
FEV1 <35%: Very Severe

FEF 25-75 – Forced Expiratory Flow

Sometimes called the "maximal mid expiratory flow rate", FEF 25-75 measures the air flow in the middle of a forced exhalation, which indicates how well the smaller airways are working.

FVC – Forced Vital Capacity

FVC is the total amount of air that can be forcefully blown out after taking as deep a breath as possible, which indicates how much air the lungs can hold.


FEV1/FVC is the ratio of the amount of air forcefully exhaled in the first second divided by the total amount of air forcefully exhaled. The FEV1/FVC is used to help determine the severity of lung disease.

Chandrasekhar, A. 2006 “Pulmonary Function Test Basics”. Loyola University Chicago School of Medicine. 2008 September 25
Allen, J. 1997. “Interpretation of Pulmonary Function Tests“ 2008 September 25

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