The Forced vital capacity (FVC) of the lung is the volume of air that can be forcibly expelled from the lung from the maximum inspiration to the maximum expiration.
Forced expiratory volume in 1 second (FEV1) is the volume of air that can be expelled from maximum inspiration in the first second.
Due to conditions in which the lung volume is reduced eg fibrosing alveolitis, scoliosis. The FEV1 and FVC are reduced proportionately.
Severity of COPD
National guidelines vary:
FEV1 as a % of predicted
Forced expiratory ratio (FEV1/FVC ratio of FEV1%)
The FEV1/FVC ration is the FEV1 expressed as a percentage of the FVC (or VC if that is greater) ie the proportion of the vital capacity exhaled in the first second. It distinguishes between reduced FEV1 due to restricted lung volume and that due to obstruction. Obstruction is defined as an FEV1/FVC ratio of <70%
|Restrictive ventilatory pattern||Obstructive ventilatory pattern|
|FVC = 2.00 litres (40% predicted)
FEV1 = 1.80 litres (45% predicted)
|FVC = 3.50 litres (98% predicted)
FEV1 = 1.80 litres (58% predicted)
|FVC reduced <80%
FEV1/FVC ratio normal
|FVC normal or reduced
FEV1 reduced <80%
FEV1/FVC ratio reduced
Flow volume loops
This is the same forced expiration converted electronically to illustrate flow rate as the lung empties. The x axis represents volume - from full inspiration to full expiration. The y axis represents the flow rate. The shape of the flow volume loop depends on the mechanical properties of the lung, and the shape can give important clues about diagnosis. The dotted line is normal.
|Asthma||Typically the curve is a smooth concave shape as the airway obstruction is relatively stable throughout expiration.|
|COPD||Typically the curve is angled as the damaged lungs in COPD collapse with forced expiration.|
|Restrictive||Typically the curve is of normal height, but very steep as the lung volume is decreased.|