Asthma Medicines

A challenge test may be used by the physician to demonstrate that a patient with a normal result on pulmonary function testing may indeed have asthma. This bronchial challenge or provocation testing would only be performed if the patient’s history and physical findings suggested that the patient is asthmatic but spirometry was normal. It is not a routine part of pulmonary function testing. The substances or agents commonly used for challenge testing include histamine, methacholine, and cold air.

Histamine is stored in allergy cells such as mast cells and is released during allergic or asthmatic attacks. It is thought to be one of the mediators of asthma. For this reason it is very suitable for provoking asthma in challenge testing. Methacholine is a chemical that stimulates one part of the nervous system called the parasympathetic nervous system to fire. If inhaled into the bronchial tubes in an asthmatic subject, methacholine will trigger impulses that produce airway constriction. Cold air irritates the bronchial tubes and may also be used for challenge testing. In an asthmatic subject with hyperreactive airways, inhaling cold air will produce significant tightening of the bronchial tubes.

In the patient thought to have occupational asthma the specific offending substance may be used to confirm the direct link between the substance and the patient’s asthmatic reaction. A similar challenge test has been used in patients to confirm allergy to sulfites and aspirin. With any challenge test there is a risk of a severe asthmatic reaction and for this reason these tests are reserved for difficult diagnostic situations and are only performed under careful observation and control.

Guidelines have been developed for performing and interpreting bronchial challenge testing. It is vital to standardize this type of testing to avoid “false positive” or “false negative” results. Generally, for a provocation test to be positive, there must be at least a 15 percent fall in airflow after inhaling the challenge material.

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Besides simple spirometry, the physician may perform other pulmonary function tests to better assess and define a patient’s condition. Patients with a variety of illnesses may have reduced capacities and flow rates and further testing may be needed. These tests include measurement of lung volume in which the different divisions or “compartments” of the lungs are measured. These divisions represent quantities of air that are distributed throughout the lung. One example would be the quantity of air that remains in the lung at all times to keep it expanded.

Measuring of lung volumes may be performed by two methods. A common technique requires inhaling a special gas mixture containing helium that the patient breathes for several minutes. Analyzing the amount of helium exhaled allows the physician to calculate how the air was distributed in the different air divisions of the lung. Another technique for measuring lung volumes requires an airtight box called a body plethysmograph. In this technique the patient sits in a clear box that resembles a phone booth and breathes against a mouthpiece. By analyzing pressure changes in the box as the patient breathes it is possible to determine the volume of gas in the lungs.

Another important pulmonary function test is known as a diffusion capacity. This is a sensitive test for the loss of gas exchanging units of the lung as in emphysema. In this test the patient again breathes a special gas mixture and an amount of exhaled gas is collected. By determining how fast the inhaled gas has disappeared, it is possible to determine whether the air sacs are exchanging gases normally.

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