Spirometry Interpretation

Spirometry Tests

There are different types of tests that can be performed with a spirometer. The most important ones are the Forced Vital Capacity and the reversibility test.

Forced Vital Capacity - Flow-Volume Loop

flow-volume loop
a flow-volume loop

The forced vital capacity is by far the most widely used spirometry test.

To perform this test, the patient first needs to inspire fully in the spirometer (not necessarily as forcefully as possible) and then forcefully blow out all the air in his lungs as quickly as possible. After the complete expiration he needs to inspire a second time, fully and with as much force as possible.

For an optimal result, the patient will need to perform this maneuvre at least 3 times to have reproducible results. Most spirometers will calculate reproducibility automatically.

The results are expressed as a flow-volume loop, a volume-time graph and more than 20 parameters.

Reversibility Test

A reversibility test is performed when the results of an FVC test show signs of obstructive lung disease (Tiffeneau<70%).

The patient inhales a fast acting bronchodilator like ventolin and performs another FVC test 15 minutes later. The result of FEV1 is compared to the result of the pre-medication test. If the FEV1 post-bronchofilator is more than 12% higher compared to the pre-bronchodilator test, the patient is likely to suffer from asthma.

Other spirometry tests

The other spirometry tests are rarely performed outside a hospital setting.

the (Slow) Vital Capacity is performed the same way as the FVC but slowly.

The Maximum Voluntary Ventilation is very rarely performed nowadays.

Provocation testing with a bronchoconstrictor, like histamine or metacholine, is performed to induce an asthma attack and should only be done under the strictest supervision and with emergency care available.