QS Study

Forced Expiratory Volume in one second (FEV1)

The fraction of the vital capacity expired in the first second of a forced expiration is known as FEV1. Normal value: FEV1 is 83% to 84% in 1st sec. It is the volume of air that can be forced out in one second after taking a deep breath, an important measure of pulmonary function. It calculates the amount of air that a person can force out of their lungs in 1 second.


  1. Measurement of FEV1 is a much more sensitive index of the severity of obstructive diseases such as asthma in which FEV1 is decreased to less than 20%.
  2. By measuring FEV1 obstructive disease and restrictive disease can be differentiated.

FEV1 measurements allow doctors to recognize how harsh the disease is, and path its evolution over time. In restrictive disease, vital capacity is reduced to the proportion of the vital capacity, FEV1 is normal. In obstructive disease (asthma) FEV1 is reduced but vital capacity is usually normal.

Fig: FEV1 measurements

  • An FEV1 of 80> of the expected value indicates mild Chronic Obstructive Pulmonary Disease (COPD).
  • An FEV1 between 50-80 percent indicates moderate COPD.
  • An FEV1 between 30-50 percent indicates severe COPD.
  • An FEV1 of <30 percent indicates very severe COPD.