Normal spirometry

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.

Restrictive pattern

Due to conditions in which the lung volume is reduced eg fibrosing alveolitis, scoliosis. The FEV1 and FVC are reduced proportionately.

Obstructive pattern

Due to conditions in which the airways are obstructed eg asthma or COPD. The FEV1 and FVC are reduced disproportionately.

Severity of COPD

National guidelines vary:

FEV1 as a % of predicted

  • <80% = mild
  • <60% = moderate
  • <40% = severe
  • <80% = mild
  • <50% = moderate
  • <30% = severe
    (NICE guidelines)

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 inhaled 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)
FEV1/FVC =90%		 FVC = 3.50 litres (98% predicted)
FEV1 = 1.80 litres (58% predicted)
FEV1/FVC =51%
  FVC reduced <80%
FEV1 reduced
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.

More on spirometry