If you have lung disease, there are a number of tests that will help you and your doctor determine the severity of your disease. Some of the most common tests that provide these measurements are chest x-rays, CT scans, pulmonary function tests, blood gas (oxygen and carbon dioxide) studies and exercise studies. We will review how some of these tests work, and what information can be gathered from them.
The Chest X-Ray
The chest x-ray can reveal clues about the presence of chronic lung disease (COPD), especially if the amount of disease in the lung is moderate or severe. But it does not provide an accurate measure of disease, and in fact, the x-ray can appear normal even in a significantly diseased lung. The signs of COPD that we look for in an x-ray are:
- Increased lung size.
- Decreased normal lung blood vessels and other markings (both of these findings are more common in emphysema).
- Flattening of the diaphragm (long muscle separating the chest and abdomen).
- Increased air between the front part of the chest and the heart.
In most cases, a diagnosis of COPD should not be made from a chest x-ray. If COPD is suspected on the basis of an x-ray, further studies should be done.
Pulmonary Or Lung Function Studies
Health
Lung function studies are used to better clarify the state of disease in the lungs. The most common of these tests is spirometry.
Spirometry
Spirometry is designed to measure the amount of air you can move in and out of your lungs and how quickly you can move it. You do this test by breathing through a tube connected to a machine that measures volumes and flows. First you take several normal breaths (called tidal breaths) after which you take in as big a breath as you can and blow it out as fast as you can. Recording the volumes and flow rates of these breaths provides a measurement of the maximum volume, or amount, of air you can move in and out of your lungs (vital capacity, or FVC) and how rapidly you can breathe it out of your lungs (forced expiratory volume or FEV).
Normal people can blow most of the air out of their lungs in one second, so the standard measure of how well people move air out of the lungs is FEV1. This measurement is the number most often found to be abnormally low in people who have COPD, but it does not establish the exact nature of the disease - it could be emphysema, chronic bronchitis or some other cause of COPD. These measurements can never be interpreted properly in isolation. In order to make meaningful judgments about these measurements several things must be done.
- The FEV1 must be compared to the FVC and only if the ratio of these two (FEV1/FVC, or flow rate of exhaling to volume of breath exhaled) is less than normal can we say that airway obstruction, which is characteristic of COPD, is present.
- Second, to make a judgment about whether or not these measures are abnormal, they must be compared to measurements taken from people without lung disease who are the same age, sex and height since lung capacities vary depending on these factors. Normal values for different races may also vary by as much as 10-15 percent.
- Third, it is critical that the test be done properly and meets the standards of the American Thoracic Society Guidelines or it will be impossible to interpret accurately.
Spirometry measures volume in the lung, but only the volume you can voluntarily move in and out of the lung. You can never empty your lungs of air completely or they would collapse and it would be very hard to take another breath. The amount of air left in your lungs after you blow out as much air as you can is called the residual volume. This amount of air, like the spirometry values, varies from person to person depending on age, sex, height and, in some cases, race. The residual volume is harder to measure and is only measured under certain circumstances. While there are several ways to indirectly measure the residual volume, usually the most accurate way in people with COPD is with a body plethysmograph or body box. This is a small enclosed "box" in which a person must sit for a few minutes with the doors closed while measurements are made. The volume of air remaining in the lung is measured by pressure changes recorded when you breathe against a pressure-sensitive valve. Some people feel a little claustrophobic during the body-box measurement; however, it can be done relatively quickly and most modern "boxes" are almost entirely glass (usually plexiglass), so you can always see out of it.
If you have COPD, your doctor may want to measure your residual volume, which can be used to calculate the total volume of air that your lungs will hold. This is called total lung capacity and is often greater than normal in people with emphysema and some other forms of COPD. It is found by adding together the residual volume (measured in the body box) and the vital capacity (measured by spirometry) described above.
What Is The Diffusing Capacity?
The surface area of the lung where oxygen can 'get in' to the body is very limited in people with COPD. For example, in patients with emphysema, both the small air sacs (alveoli) and the small blood vessels (capillaries) that run past them are destroyed, leaving a smaller area for oxygen to come in contact with the oxygen-carrying proteins in the blood (hemoglobin).
"Diffusing capacity" refers to the capacity of the lung to release carbon dioxide and take in fresh oxygen. This lung function test measures the amount of area of the lung where oxygen can move into the blood vessels. It is performed much like the spirometry test, except that during this test, you breathe in a small amount of carbon monoxide gas. Carbon monoxide is used because it binds very quickly and well with hemoglobin and the amount is easily measured.
The test is usually performed during a single breath. To measure the diffusing capacity, you have to have certain minimal lung volumes and be able to hold your breath for a brief period of time. Also since diffusing capacity varies with the concentration of hemoglobin in the blood, the values obtained need to be revised if your hemoglobin level is not normal.
Are There Any Other Lung Function Tests I Might Have To Take?
We have only covered the most commonly used lung function tests that are performed to help in the diagnosis and management of COPD. A number of other tests exist and are done in special circumstances. There are tests, for example, that measure the elasticity of the lung tissue, the air-flow through just the smallest of the airways, and the strength of the muscles participating in breathing. If you require a specialized test like this, you should ask your doctor for a full explanation of the test goals and procedure.
Arterial Blood Gases
If your spirometry shows moderate or more severe disease, your doctor may want to measure the levels of oxygen and carbon dioxide in your blood. Normal oxygen levels are pressures of oxygen of more than 80 mmHg (sea level) and normal carbon dioxide pressures are in the low 40's mmHg. The oxygen pressure varies with altitude so that in areas of high altitude where the barometric pressure is lower, the normal values are also lower. In people with COPD, oxygen levels may fall below normal and carbon dioxide levels can rise above normal. Usually, oxygen levels fall before carbon dioxide levels rise. If the carbon dioxide levels rise and stay elevated, the patient is said to be in chronic respiratory failure; this is a sign of advanced lung disease.
The most accurate way to test the levels of these gases in the blood is to take blood from an artery, usually the radial artery at the wrist, and directly measure the pressures of oxygen and carbon dioxide in the drawn blood. It is important to take the blood from an artery, as arterial blood represents blood that has passed through the lungs where it has picked up oxygen and is being pumped to the body by the heart. Therefore, it more accurately represents the amount of oxygen that is being delivered to the tissues. It is also possible to get an estimate of blood oxygen levels without taking blood, by measuring hemoglobin saturations with a finger probe, a small device placed on the finger.
Computerized Tomography, or CT Scans
A CT scan is an imaging tool that provides radiographic images of the body. This scan is not necessary to diagnose COPD, but the images help doctors to distinguish between types of COPD. CT scans can also sometimes pick up unsuspected things like small lung cancers or areas of bronchiectasis (abnormally widened airways).
CT scans can be perfomed in several ways. Different from a chest x-ray, which provides a two-dimensional view of the chest, CT scans offer a more three-dimensional picture. CT scans really look at individual slices of the chest taken at varying intervals from your neck to your waist. New high resolution CT's look at slices of the body as thin as 2 millimeters or less, or slightly larger than a sixteenth of an inch, and this allows the structure of the lung tissue to be seen in fine detail. For example, high resolution CT scans can usually distinguish emphysema very clearly from other diseases, and can give doctors a good idea how the emphysema is distributed in the lung. These scans also offer very sensitive and clear images of bronchiectasis. New spiral CT technology makes it possible to complete a scan in less than a minute and with no more radiation than a chest x-ray. Another benefit of CT scans is their ability to pick up small lung nodules which are not visible on a chest x-ray and which sometimes represent early cancers.
Exercise Studies
There are two major categories of exercise studies: graded exercise studies and non-graded studies.
Graded exercise studies: In graded exercise studies, you usually walk on a treadmill or an exercise bicycle at a determined work-load. Your work-load is progressively increased and various breathing and heart measurements are recorded as you exercise. These tests are not done routinely in patients with COPD.
The test may be done if:
- the diagnosis is uncertain.
- you are participating in a clinical trial where specific improvements in exercise capacity are important outcomes.
This type of exercise test is similar to the ones done by internists and cardiologists to determine if you have coronary artery disease, but in those situations the doctors are primarily interested in the EKG changes during exercise.
Non-graded exercise: Non-graded exercise studies can be very useful in determining how much exercise capacity you have. In a non-graded study, you walk on level ground for a determined period of time (often six minutes). You should walk as fast as you can manage, with stops if necessary. During this test you will likely wear an oxygen saturation probe on your finger or ear to detect whether your blood oxygen value falls below acceptable values, an indication that your lungs do not supply adequate oxygen to your blood when you exercise. Successive six-minute walking tests may be done at intervals to assess the functional benefits of pulmonary rehabilitation programs, or sometimes to assess the functional impact of the progression of your disease. A second type of test is one in which you walk a set distance, usually at your own pace, again with a saturation probe in place to determine if you need supplemental oxygen. This would be done if the six minute walk test revealed falls in blood oxygen values (desaturation) to unacceptable levels.
Conclusion
COPD is a common disease which can be measured with a variety of tests. Chest x-rays may suggest the presence of COPD, but lung measurements are necessary to confirm airway obstruction. CT scans are helpful in distinguishing one type of COPD from another. Exercise tests and other studies may also be used to help make a difficult diagnosis or to get specific about a person's exercise capability. The simpler tests, like spirometry, are often repeated at intervals to get an idea of the progression of disease.
