• Forced Expired Volume in the first second (FEV1):
    This is a measure of how much of the FVC the subject exhaled in the first second. A normal person will exhale around 80% of their FVC in the first second.
  • FEV1/FVC:
    This is the percent of the FVC that person exhaled in the first second (see above). Both measures are important to look at together because if the FVC is low, the FEV1 will also be low (as in restrictive diseases such as idiopathic pulmonary fibrosis; however in these diseases, the person’s ability to exhale is not usually compromised).

Other Flow Measures

  • While the FEV1 and the FEV1/FVC are the most useful flow measures, others are available and often obtained.
  • Forced Expiratory Flow between 25% and 75% (FEF25-75): This is the average flowrate calculated as the subject is exhaling between 25% and 75% of his/her FVC. Think about it this way: As the subject forcibly exhales the FVC, the rate at which the flow is leaving the lungs is not constant, but rather changes over the length of exhalation.
  • Peak Expiratory Flow (PEF or sometimes PEFR): The highest flowrate the subject attains during exhalation. It usually occurs at the beginning of exhalation. Often used as an indication of effort.

Authored by William A. French. Medically reviewed by Dr. Barbara Erickson, PhD, RN, CCRN.

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