Spirometry and lung function tests

Main points

1 Spirometry measures airflow and lung volumes, and is the preferred lung function test in COPD.
2 The forced vital capacity (FVC) is the total volume of air that can be exhaled with maximum force, starting from maximum inhalation and continuing to maximum
exhalation.
3 The forced expiratory volume in one second (FEV1) is the amount of air that can be exhaled in the first second of a forced blow from maximum inhalation.
4 Both the FVC and the FEV1 are expressed as volumes (in litres) and as a percentage of the predicted values. Predicted values have been determined from large population studies, and are dependent on age, height,gender and ethnicity.
5 The ratio of FEV1 to FVC (FEV1/FVC) is expressed as a percentage.Values of less than 70% indicate airflow obstruction.
6 In diseases that cause airflow obstruction the FEV1 will be below 80% of the predicted value and the FEV1/FVC ratio will be less than 70%. (In severe COPD the FVC may also be less than 80% of predicted.)
7 In restrictive lung diseases both the FEV1 and the FVC will be below 80% of the predicted value but the FEV1/FVC ratio will be normal or high.
8 The volume/time trace must be smooth, upward and free of irregularities.The graph must reach a plateau, demonstrating that the patient has blown to FVC.

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