- Introduction to lung function tests
- Why are lung function tests performed?
- What happens during a lung function test?
- What do the results show?
- Risks and benefits of lung function tests
Pulmonary or lung function tests are simple and safe investigations routinely used in clinical practice for the diagnosis and evaluation of a variety of respiratory diseases. The most important of these tests include spirometry, measurement of lung volumes and measurement of how well the air spaces in the lungs transfer oxygen to the blood (called diffusion capacity). Results of these tests can help doctors determine whether you suffer from obstructive or restrictive lung disease so they can decide on appropriate management. Pulmonary function tests are generally well tolerated by patients and cause minimal side-effects or complications.
As forementioned, lung function tests are performed routinely in general practice and other medical settings to diagnose and assess respiratory disease. Specific reasons for performing these investigations include:
- Assessing the physiological type (e.g. obstructive versus restrictive) and extent of lung dysfunction.
- Diagnosing the causes of shortness of breath and cough.
- Detecting early evidence of lung dysfunction.
- Following up the natural history of disease and response to therapy.
- Assisting in the determination of respiratory impairment for medico-legal purposes.
- Monitoring patients with occupational exposures to substances such as asbestos.
- Evaluating your respiratory function prior to undergoing surgery. Lung function tests will give an idea of how well your body will tolerate the surgery.
- Evaluating any degree of disability.
Pulmonary/lung function tests are relatively simple and important tests to evaluate lung function and evidence of respiratory disease. Most tests can be performed at the bedside and are important investigations in primary practice. The results of the tests must be compared against reference values according to age, sex and height, for healthy individuals. There are large variations between results so lung function tests are probably best used for ongoing monitoring in an individual patient.
The major types of lung function tests include spirometry, measurement of lung volumes, and quantitation of diffusing capacity (discussed below) but your doctor may order other tests if they deem them necessary. Prior to the tests you should not eat a large meal as your stomach may prevent your lungs expanding. You should also not smoke or exercise heavily for at least 6 hours before as this can interfere with results
Peak expiratory Flow (PEF)
This is an extremely simple and cheap test used frequently by GPs to determine if there is obstruction or narrowing of your airways.
For the procedure, you will need to sit upright in a chair, take a full breath in, and then expel the air as fast and hard as you can into a small tube device (called a peak flow meter). You need to hold the device in a horizontal position and make sure your lips are tightly sealed around the mouthpiece. Your doctor will normally demonstrate how to do this first! The device is calibrated to measure how fast the air flows from your mouth. You will have to repeat the procedure three times and the best result is recorded. It is really important you put in your best effort otherwise the results may be inaccurate.
This investigation is best for monitoring diseases such as asthma that cause narrowing of the airways.
This test is similar and looks at how effectively and quickly the lungs can be emptied and filled. It is probably the most common investigation used in the diagnosis of respiratory disease.
For the procedure you will need to sit upright in a chair, with your feet on the floor and your nose clipped shut. Once again you will need to breath into a mouth piece (held securely between your lips and teeth) which is attached to a recorder (spirometer). You need to breathe in to your full capacity, put the device into your mouth, then breathe out forcefully until your lungs are completely empty. This is normally for at least 6 seconds. Your doctor will help and encourage you to achieve this. Sometimes after complete expiration you may be advised to breathe in fully again through the device, depending on the type of particular device the doctor is using. The information can then be printed out onto a special chart called a spirogram which the doctor will carefully analyse. This plots the volume expired against time. The information can also be converted to a flow-volume loop.
The main parameters measured include:
- Forced vital capacity (FVC)- This measures the amount of air you can exhale after you have inhaled completely.
- Forced expiratory volume in 1 second (FEV1)- This measures the amount of air you exhale in the first second of a full forced expiartion. This is less effort-dependent than PEF. The FEV1 is often expressed as a percentage of FVC as this is an excellent measure of airway obstruction or limitation. In normal individuals the value is approximately 70%.
- Forced expiratory flow (FEF 25% to 75%)- This measures the airflow halfway throgh exhalation and is often reduced in small airways disease.
- Peak expiratory flow (PEF)- This can be devised from the flow-volume loop or can be measured by the absolute method discussed above. It measures the speed of exhalation.
If airflow obstruction is present, you may need to repeat these measurements 10-20 minutes after taking an inhaled bronchodilator (puffer). This determines whether the obstruction is reversible (as in diseases such as asthma) and whether you will benefit from bronchodilator medications (e.g. Ventolin).
The above methods mainly look at the flow of air during exhalation. However, every time you exhale there is some air left in the lungs called the residual volume (RV). This can be measured using gas dilution tests.
For this you will need to breathe a known amount of gas (usually helium) in and out of a container. The change in concentration of gas gives an idea of the volume of the lungs as the gases used are not absorbed into the body. This method also determines the total lung capacity (TLC) which is how much air is in your lungs following a full inhalation. This includes the amount of air you just breathed in and the air that is always stuck in your lungs. Another more accurate method for measuring lung volumes is body plethysomography. For this you need to sit in an airtight container/booth and breathe in and out of a mouthpiece. Pressure, volume and flow measurements are taken.
Chest x-rays can also give a vague idea of lung volume.
Diffusion capacity / gas transfer factor
This is a more complicated investigation that measures the amount of gas transferred across the alveolar-capillary membrane within the lungs. This is the membrane that exists between the air spaces (alveoli) in the lungs and blood vessels. Normally oxygen travels across this membrane into the blood and carbon dioxide is expelled in the opposite direction. Certain lung diseases such as pulmonary fibrosis (thickening and scarring of the airway walls), emphysema and congestive heart failure can inhibit this process.
To measure the capacity of your lungs to uptake and exchange gases, you will need to inhale a small concentration of carbon monoxide from a container. Some of this will enter your lungs and be transferred into your blood stream. The doctor will take a sample of blood from an artery at your wrist to determine the concentration of carbon monoxide in the blood. This can be quite painful but is not particularly dangerous. The concentration reflects how well your lungs take up gases (including) oxygen into the body.
The results can be influenced by haemoglobin concentrations (e.g. in anaemia) so your doctor may take this into account when interpreting results. In addition, recent cigarette smoking can influence the results so you should not some for the 24 hours leading up to procedure.
All pulmonary function results are compared to the normal ranges for a healthy person of the same age, sex, height and race. The most important factor to determine is whether the pattern of respiratory disease is either obstructive (over-inflation) or restrictive (difficulty expanding the lungs). The doctor can then get an idea of the specific underlying disease.
- Obstructive defects are characterised by airway narrowing which increases the time it takes to empty your lungs. FEV1 and FVC are both reduced but FEV1 is reduced to a greater degree causing a reduction in the FEV1/FVC ratio. PEF also tends to be reduced whilst lung volumes (such as TLC and RV) are increased. Examples of obstructive airways disease include asthma, chronic obstructive pulmonary disease (COPD), bronchitis and emphysema. The latter also causes a reduction in gas diffusion capacity.
- Restrictive defects are characterised by a loss of lung tissue or reduced ability for the lung to expand. Like obstructive disease, the FVC is still reduced but the ratio of FEV1 to FVC is normal or increased (often due to the increased elastic recoil or of the lungs). PEFR may be normal or increased. The TLC tends to be reduced (due to as the lungs are more ‘tight’ so can’t expand). Examples of restrictive lung diseases include pulmonary fibrosis, pneumonias, pleural effusions, obesity and other disorders damaging the nerves and muscles that help expand the lungs.
The lung function tests are all pretty simple to perform although measurement of lung volumes and gas transfer requires specialised and sophisticated equipment. They provide valuable information about the type of pulmonary deficits and can be used to monitor a variety of different respiratory tract disorders. Furthermore, lung function tests are generally pretty safe and are accepted by most patients as they don’t require any invasive steps.
Occasionally the heavy breathing required for the tests may cause you to become tired or light-headed but this only lasts for a short time and your doctor will give you time to rest if needed. The body plethysmography booths may cause you to feel quite uncomfortable also but your doctor will be standing in view for support.
If you have recently had severe chest pain or a heart attack or are allergic to any medications be sure to tell your doctor so they can minimise the risks from the investigation. These tests may also not be suitable if you have acute severe asthma, angina aggravated by testing, pneumothorax or active tuberculosis.
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