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Pulmonary Function Tests


About Pulmonary Function Tests

Spirometry provides an objective assessment of airflow obstruction and is important in staging asthma severity. It should be done on initial diagnosis of asthma, after treatment is started and symptoms have stabilized, and every 1 to 2 years afterward. Spirometry is used to measure the rate of airflow during maximal expiratory effort after maximal inhalation. It can be useful in differentiating between obstructive and restrictive lung disorders. In asthma (an obstructive lung disorder) the forced expiratory volume in 1 second (FEV1) is usually decreased, the forced vital capacity (FVC) is usually normal and the ratio FEV1/FVC is decreased. In restrictive disorders the FEV1 and FVC are both decreased, leaving a normal FEV1/FVC.
Spirometry measurements are usually done before and after administration of a beta2 agonist. Reversibility with the use of a bronchodilator is defined as an increase in FEV1 of 12% or 200 ml. Patients with severe asthma may need a short course of oral steroid therapy before they demonstrate reversibility.

Common Terms in Spirometry

Below is an example of a volume-time curve. It shows the amount of air expired from the lungs as a function of time.

Below is a short explanation of the terms used in spirometry.

FVC (Forced Vital Capacity) -- This is the total volume of air expired after a full inspiration. Patients with obstructive lung disease usually have a normal or only slightly decreased vital capacity. Patients with restrictive lung disease have a decreased vital capacity.

FEV1 (Forced Expiratory Volume in 1 Second) -- This is the volume of air expired in the first second during maximal expiratory effort. The FEV1 is reduced in both obstructive and restrictive lung disease. The FEV1 is reduced in obstructive lung disease because of increased airway resistance. It is reduced in restrictive lung disease because of the low vital capacity.

FEV1/FVC -- This is the percentage of the vital capacity which is expired in the first second of maximal expiration. In healthy patients the FEV1/FVC is usually around 70%. In patients with obstructive lung disease FEV1/FVC decreases and can be as low as 20-30% in severe obstructive airway disease. Restrictive disorders have a near normal FEV1/FVC.

FEF25-75% (Forced Midexpiratory Flow Rate) -- This is the average rate of airflow during the midportion of the forced vital capacity. This is reduced in both obstructive and restrictive disorders.

DLCO (Diffusing Capacity of the Lung for Carbon Monoxide) -- Carbon monoxide can be used to measure the diffusing capacity of the lung. The diffusing capacity of the lung is decreased in parenchymal lung disease and COPD (especially emphysema) but is normal in asthma.

Links to Other Spirometry Sites

University of Iowa Virtual Hospital Spirometry Tutorial
A good tutorial on spirometry from the extensive virtual hospital site at the University of Iowa

United Kingdom National Asthma Campaign
A brief tutorial on spirometry.

Australian National Asthma Campaign
A very detailed site about spirometry commissioned by the thoracic society of Australia and New Zealand.