The Forced Expiratory Volume in 1 second (FEV1)

The FEV1 represents the volume of air expired in the first second of a maximal velocity VC manoeuvre. The volume of air that can be forced out in one second from total lung capacity (full breath of air) is an important measure of pulmonary function. The FEV1 should be expressed as a ratio of the FVC or VC whichever is greater, (FEV1/FVC or FEV1/VC) and reported as a percentage (FEV1/FVC × 100), this measurement is independent of lung size.

Around three-quarters of the FVC can be exhaled in the first second (FEV1) in normal healthy subjects; this is illustrated graphically below.

FEV1 on Volume-time graph

This gives a normal FEV1/FVC ratio or FEV1/FVC% in a range of around 0.7 to 0.9 or 70% to 90%, respectively. To avoid confusing this with the percentage of predicted however (this will be examined later), use the ratio rather then the percentage.

A common mistake, particularly when learning about spirometry is confusing the FEV1/FVC percentage with the percent predicted for FEV1.

Multiplying this value by 100 tells you what percentage of the FVC is blown out in the first second (FEV1); an important marker of airway health.

The normal range for the FEV1/FVC ratio typically depends on the subject’s age. Consider that in some patients, particularly over the age of 60, an FEV1/FVC ratio of below 0.7 may be absolutely normal for them. A holistic approach must be applied before a diagnosis of COPD is made.

This ratio also declines in patients with airways disease. The FEV1 has therefore become critical in the assessment of airway calibre and function.

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