Pulmonary function tests are the primary diagnostic tools for COPD, after the medical history and physical examination. These tests demonstrate characteristic abnormalities in lung function that, in the proper clinical context (i.e., medical history, physical examination, chest x-ray) confirm or support the diagnosis of COPD and give some idea of the degree of impairment and prognosis. Lung biopsy is rarely used to diagnose emphysema.
There are four components to pulmonary function testing: spirometry, postbronchodilator spirometry, lung volumes, and diffusion capacity. In the initial evaluation, all four components are often performed. Periodically, an individual component, most commonly spirometry, is performed to assess progression of disease and to determine the effectiveness of medication.
The most reliable way to determine reversible airway obstruction is with spirometry, a procedure that measures the amount of air entering and leaving the lungs. This simple test can be performed in most physicians' offices, with the patient sitting comfortably in front of the spirometry machine. The machine measures airflow that passes through the inhalation port attached to the machine. The inhalation device is usually a disposable cardboard tube or a reusable tube that can be sterilized after use.
The patient inhales as deeply as possible and forms a seal around the tube with their mouth. Then the patient exhales, as forcefully and rapidly as they can, until they can exhale no more. To be an adequate test, the patient must exhale all the air they possibly can continue exhaling for at least another 6 seconds. Usually, three separate attempts are made and the best result is used for evaluation.
Multiple measurements are obtained from this maneuver. Those most commonly used for interpretation are (1) forced expiratory volume after 1 second [FEV1], (2) forced vital capacity [FVC], and (3) forced expiratory flow at 25%-75% of maximal lung volume [FEF25-75]. They are expressed as percentages of what is predicted for normal lung function, depending on the variables of height, age, race, and sex.
COPD produces characteristic results in this test. The amount of air exhaled (forced vital capacity, or FVC) is reduced, compared to a person with normal lung function. Futhermore, the amount of air exhaled during the initial 1 second (FEV1) is reduced and is reduced to a greater degree than the entire FVC. Therefore, the ratio of air exhaled after 1 second is low compared to the total amount of air exhaled. In healthy lungs, 70%-75% of all the air exhaled after maximum inhalation (FVC) is exhaled within the first second (FEV1), known as the FEV1/FVC ratio. In lungs with COPD, the FEV1/FVC ratio falls below 70–75%.
The absolute value of the FEV1 is also reduced. The FEV1 can be reduced in another disease process, termed restrictive ventilatory defects. However, in restrictive ventilatory defects the FVC is reduced proportionally, preserving a normal FEV1/FVC ratio. The FEV1 is used to quantify the severity of obstruction with a FEV1 < 70% of what is predicted for age, height, weight and race considered mild; < 50% to 69%, moderate; < 35–49%, severe; and < 35%, very severe. Sometimes the only abnormality is a reduction in the FEF25-75. Isolated reduction in the FEF25–75 is considered an early detector of very mild obstruction. It can also be a normal variant.
Spirometry is often repeated after giving the patient a bronchodilator, such as an inhaled beta-agonist. If the FEV1 (forced expiratory volume after 1 second) improves more than 12%, the obstruction may be reversible or partially reversible. This procedure provides some information on the potential responsiveness of the airways to medication. It is also useful for determining whether steroid treatment has been beneficial, a few weeks after initiating therapy.
Peak expiratory flow rate (PEFR) also can be obtained. PEFR can be compared with readings the patient obtains at home with a peak flow meter. A peak flow meter is a portable device that consists of a small tube with a gauge that measures the maximum force with which one blows air through the tube.
Lung volumes are measured in two ways, gas dilution or body plethysmography. The gas dilution method is performed after the patient inhales a gas, such as nitrogen or helium. The amount of volume in which the gas is distributed is used to calculate the volume of air the lungs can hold. Body plethysmography requires the patient to sit in an airtight chamber (usually transparent to prevent claustrophobia) and inhale and exhale into a tube. The pressure changes in the plethysmograph are used to calculate the volumes of air in the lungs.
The most important measurements obtained are residual volume and total lung capacity (TLC). These measurements vary with age, height, weight, and race and are usually expressed as an absolute number and a percentage of what is predicted for a person with normal lung function. A high TLC demonstrates hyperinflation of the lungs, which is consistent with emphysema. Increased residual volume signifies air trapping. This demonstrates an obstruction to exhalation.
Diffusion capacity is a measurement of gases transfered from the alveoli to the capillary. The patient inhales a very small amount (very safe) of carbon monoxide. How much of it is taken into the blood is measured. A reduced diffusion capacity is consistent with emphysema but is seen in a many other lung diseases as well.