TESTING THE LUNGS
Methods for testing the lungs remain relatively crude. They consist of measuring the ability of the lungs to:
- Move air in and out,
- Exchange gases.
Measuring the Ability of Your Lungs to Move Air In and Out
The technical term for this is spirometry. This measurement is usually obtained by having you take as deep of a breath in as possible, followed by a command to exhale forcefully through a mouthpiece to the fullest extent for a count of at least 10 seconds, followed by a deep rapid inhalation. Your nose is pinched tightly shut during this manoeuvre to prevent leaks that might lead to spurious results and an incorrect diagnosis. You are usually required to perform at least 3 of these manoeuvres. A respiratory therapist coaches you through each manoeuvre and reviews the results to insure that there are no technical defects. If you have asthma or another form of lung disease in which there is narrowing of the airways, you may be given inhaled medication part-way through the test to determine your response to the medication.
Most labs are computerized. The printout of the results usually contains a tracing of your best manoeuvre superimposed on what would be expected of the "average" North American Caucasian person of your sex, age, height, and weight. A table of parameters derived from the curves is included. This table can be quite confusing, even to physicians. Fortunately, you only need to pay attention to 3 parameters. These are:
|Table of Parameters||Printout of Spirometry|
FEV1 stands for forced expiratory volume in the 1st second of exhalation. This parameter is decreased in patients who have narrowing of the air passages or whose lungs are smaller than normal.
FVC stands for forced vital capacity and is the maximum volume of air that you can force out of your lungs. This parameter is usually normal in patients with narrowing of the air passages. The FVC is decreased in patients whose lungs are smaller than normal.
The ratio, FEV1/FVC, provides a useful index of the severity of the narrowing of the air passages.
An expert can obtain additional information about your lungs by looking at the shape of your tracings.
There are 2 kinds of medications that might be given during spirometry. These are:
- Bronchoconstrictors - Methacholine or Histamine
- Bronchodilators - Ventolin (otherwise known as Salbutamol)
A bronchoconstrictor is given to see if your air passages are more irritable than average. Your air passages are considered more irritable than average if the FEV1 starts to fall at a low concentration of the bronchoconstrictor.
A bronchodilator is given to determine whether the narrowing in your air passages is reversible.
Measuring the Ability of Your Lungs to Exchange Gases
There are basically 2 methods of measuring the ability of your lungs to exchange gases. These are:
- Diffusing capacity
- Arterial blood gases
The Diffusing Capacity is measured by first having you breath in a gas containing known concentrations of helium and carbon monoxide. You are then asked to hold your breath for 10 seconds before exhaling. The change in helium concentration in the exhaled gas is used to calculate the lung volume. The change in concentration in the carbon monoxide concentration in the exhaled gas is used to calculate the Diffusing Capacity (after correcting for the dilution as determined from the helium dilution method).
Arterial blood gases are obtained by inserting a small needle into an artery in your wrist. The blood sample is analyzed to provide:
- Partial pressure of oxygen
- Partial pressure of carbon dioxide
Unless the disease is quite advanced, the arterial blood gases will be normal.
Both methods are limited by the fact that the measurements are obtained when you are at rest. In many patients, defects in the gas exchange mechanism only become apparent during exercise.
There are many other tests that can be performed on the lungs but these are of limited use in the everyday care of patients with lung disease. If the doctor cannot explain to you why the test is being performed, it probably isn't necessary. More than 90 % of the time, spirometry will be all that is required.
Food for Thought
The functions of moving air in and out of the lungs and gas exchange are interlocking. Narrowing of the air passages changes the distribution of the airflow into different parts of the lung which causes a mismatch with the blood flow and decreases the efficiency of gas exchange. Conversely, the destruction of gas exchange units in the lung (which occurs in emphysema) deprives the air passages of support tissues that help to keep them open.
At present, we test the lungs by trying to isolate each function under static conditions. As a result, the tests lack sensitivity and the results often don't correspond well with symptoms or functional capacity. In the future, testing the lungs will consist of seeing how the lungs function under dynamic conditions and as part of the whole organism.
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