Overview
Spirometry is a type of pulmonary function test that measures lung function. It works by measuring the volume and rate of flow of air during inhalation and exhalation. It is particularly useful in the diagnosis of respiratory conditions such as asthma, chronic obstructive pulmonary disease (COPD), and pulmonary fibrosis.
Spirometry heavily depends on patient cooperation and effort and requires repeats to ensure results are reproducible. For this reason, it may be difficult to carry out in children or those who have severe lung disease.
FEV1
The forced expiratory volume in one second (FEV1) is the volume of air exhaled in the first second of a forced expiration:
- This is usually reduced in both obstructive and restrictive disease
FVC
The forced vital capacity (FVC) is the maximum amount of air forcibly exhaled after taking in the deepest breath possible:
- This may be reduced in obstructive and restrictive disease
FEV1/FVC ratio
This is a calculated ratio between the FEV1 and FVC ratio and is often expressed as a decimal or percentage. In general, a normal FEV1/FVC ratio is 0.7 (70%).
It is important to interpret this ratio in light of clinical findings, as the elderly may have a value of less than 0.7 and have no signs or symptoms of airway obstruction.
- In obstructive disease, the FEV1/FVC ratio is generally <0.7
- In restrictive disease, the FEV1/FVC ratio may be normal or raised (i.e. ≥0.7)
Obstructive lung diseases
In general:
- FEV1 – reduced
- FVC – reduced, but may be normal
- FEV1/FVC – reduced (<0.7)
Example conditions may be:
- Asthma
- COPD
- Bronchiectasis
- Cystic fibrosis
- Lung tumours
Restrictive lung diseases
In general:
- FEV1 – reduced
- FVC – reduced
- FEV1/FVC – normal or increased
Example conditions may be:
- Pulmonary fibrosis
- Asbestosis
- Kyphosis/scoliosis
- Neuromuscular disorders
- Severe obesity
