Pulmonary Function Tests - Diagnosing Asthma: Asthma

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Pulmonary function tests

These tests help you find out how well your lungs are working. Two types of breathing tests, spirometry and peak flow, are often used to assess asthma.

Spirometry

A spirometer (derived from the Greek words for "breath" and "measure") is a device that measures how much air you can exhale and how fast it exits your lungs. This test, known as spirometry, reveals how wide open or narrow your bronchial tubes are (see "Using a spirometer"). If your bronchial tubes are wide open, air from your lungs can flow out rapidly when you exhale. If your tubes are narrowed because of asthmatic constriction, air exits your lungs more slowly.

Using a spirometer Spirometry is one of the tests used to confirm a diagnosis of asthma. You exhale forcefully through a hollow tube attached to the spirometer, which then measures your lung capacity and helps your doctor determine whether your airways are obstructed.

Spirometry is painless and takes five minutes or less to perform. You take as deep a breath as you can, and then you exhale quickly and forcefully through a tube attached to the spirometer until there is no more air to expel. During the test, a soft plastic clip like a clothespin pinches your nostrils together so that all of the air coming out of your lungs will pass through your mouth and into the spirometer's recording equipment. It takes at least six seconds of blowing hard to empty every last ounce of air from your lungs. When you are done and have rested a moment or two, you repeat the test a second time, then usually a third time as well. The best of the three efforts will be recorded as your spirometry results.

The spirometer prints out a graph that shows two key values:

• Forced vital capacity is the maximum volume of air you can exhale with force into the spirometer, measured in liters. This reflects how large your lungs are, how elastic the lung tissue is, and possibly how much air remains trapped in the lungs at the end of an exhalation because of narrowed airways.

• Forced expiratory volume in one second (FEV1) is the maximum volume of air exhaled in the first second, described in liters. This is the best way to characterize the speed at which you can force air out of your lungs.

Your test results will be compared with the normal range for a healthy person of your sex, age, and height. If you have a normal forced vital capacity but your FEV1 is below normal, the combination shows an obstructive pattern, which suggests the airway narrowing typical of asthma. Unfortunately spirometry is not able to distinguish asthma from other diseases that cause an obstructive pattern, such as emphysema and chronic bronchitis, bronchiectasis (a type of ongoing infection of the bronchial tubes), and bronchiolitis (a narrowing of the smallest airways, the bronchioles).

If your breathing tests reveal an obstructive pattern, your doctor will probably ask you to take a quick-acting asthma medication while you are still in the office or testing lab, and then repeat the spirometry test. The medication is an inhaled bronchodilator that acts quickly to relax the muscles surrounding the bronchial tubes. If your condition is due to asthma, the obstructive pattern should largely or completely disappear within 10 to 15 minutes after taking this medication. When you repeat the test, air will empty much faster from your lungs than on the first test, and the FEV1 value should increase by at least 15%. This demonstrates the reversible airway narrowing that is characteristic of asthma.

Although spirometry in and of itself cannot distinguish asthma from other lung diseases, it is useful in confirming a diagnosis suggested by other evidence. Asthma is the most common cause of an obstructive pattern on spirometry in people who are young and have never smoked. What's more, the test also provides some idea of how severe your asthma might be.

Peak flow

As valuable as spirometry is in helping evaluate asthma and other breathing conditions, it is not available in most primary care physicians' offices. The cost of a spirometer, the need for technical support, and the level of expertise needed to interpret the results have prevented its routine use in most general medical practices. You and your doctor may decide that it is too much trouble or too expensive for you to visit a hospital to have your pulmonary function tested. In this case, a potential substitute for assessing your breathing is a peak flow meter.

A peak flow meter is a simple device that measures the maximum speed at which air can empty from your lungs. The device has an indicator that moves when you blow air into it, and a scale of numbers printed along the side to show how far you can move the indicator. The scale records how fast you can blow air from your lungs in liters per minute. You need to exhale forcefully for only about one second (less than in spirometry). The results depend on the strength of your effort, so you need to exhale as hard and fast as you can.

As with FEV1, normal peak flow values vary with age, height, and sex. Your results will be compared with normal reference values based on measurements made in healthy, nonsmoking people. Each time you are tested, you should make at least three efforts to achieve your best value. The highest of the three is considered to be your actual peak flow.

Like a spirometer, a peak flow meter can measure how fast air exits your lungs — an indication of how much your airways are narrowed. Unlike spirometry, however, the peak flow test does not record your vital capacity (the amount of air that can be emptied from your lungs). As a result, the test cannot distinguish diseases such as asthma (which reduce the peak flow because of narrowing of the bronchial tubes) from diseases and conditions such as pneumonia or lung scarring or collapse (which reduce peak flow because the lungs contain less air to start with). The test therefore cannot indicate whether your coughing, wheezing, or chest tightness is being caused by one of the obstructive abnormalities, meaning it is neither as detailed nor as sensitive as spirometry.

Still, a peak flow test is useful in several ways. First, a normal peak flow value is reassuring: There is no sign of serious asthma at that time. Second, if your peak flow is reduced, your doctor will likely ask you to take a bronchodilator (as in spirometry) to see if your peak flow increases afterward. If the peak flow increases within minutes by approximately 20%, the results support a diagnosis of asthma. Third, if you already have an established diagnosis of asthma, a peak flow value is easy and useful to record every time you visit your doctor. Tracking the changes in your peak flow values is an important part of your asthma care (see "Measure peak flow").

Peak flow meters are also helpful for diagnosing asthma when you have had breathing difficulties but the symptoms are not present when you are actually at the doctor's office. Using a peak flow meter at home, you can make measurements of your breathing both when you are well and when you are having respiratory symptoms. This will help you assess whether those symptoms correspond with reductions in your peak flow. Your doctor can write a prescription for a peak flow meter, which you can obtain at a local pharmacy or purchase by mail (see "Resources"). Prices vary, but you can usually find a peak flow meter for $15 to $30.

Exhaled nitric oxide

Doctors continue to look for better, more accurate ways to diagnose asthma. A newer test measures a chemical marker of asthma — nitric oxide — in exhaled air. Evidence has shown that people with asthma have higher nitric oxide levels in their breath. Doctors can also use the test to evaluate your response to anti-inflammatory treatment. A decline in exhaled nitric oxide concentration suggests that the treatment may be decreasing the lung inflammation associated with asthma.

A person taking the test slowly and steadily exhales into a tube. The gases in the breath pass through a computerized analyzer, and the nitric oxide concentration appears on a computer screen. Currently this experimental test is available only in certain specialized asthma centers. A normal level of exhaled nitric oxide is less than 20 parts per billion.

Last updated:	September 27, 2007