How To Interpret A Spirometer Graph

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Wondering what a spirometer is and how you interpret a spriometer graph? Find out helpful info and tips on spirometer readings with Safey Health. 

If you have breathing problems, including asthma, it is likely that you will be asked to undertake a spirometer test by a GP, if this is the case, a spirometer graph will be interpreted by the practitioner who will, in turn, deliver a diagnosis on the severity of your asthma problem.

This article will help you understand what spirometry is, what the procedure includes and how you can interpret a spirometer graph yourself.

You’ll also be able to understand what each reading will mean for you and the health implications of each result may mean, however, this guide should only act as a framework and your General Practitioner should be the final authority when delivering diagnosis.

The background and measurements of spirometry tests

The Lung Health Study was carried out from 1988 to 1994 and its findings concluded that there were huge numbers of people who had undiagnosed and untreated respiratory problems, using tests which measured the vital capacity of an individual’s lungs.

A number of other measurements were also taken, which make up the modern spirometry test.

Here is the full list of the readings which are spirometry includes.

  •       Forced vital capacity (FVC) – The total volume of air that can be exhaled during a maximal forced expiration effort
  •       Forced expiratory volume in one second (FEV1) – The volume of air exhaled in the first second under force after a maximal inhalation
  •       FEV1/ FVC ratio—The percentage of the FVC expired in one second
  •       FEV6 —Forced expiratory volume in six seconds
  •       FEF25–75%—Forced expiratory flow over the middle one half of the FVC; the average flow from the point at which 25 percent of the FVC has been exhaled to the point at which 75 percent of the FVC has been exhaled
  •       MVV—Maximal voluntary ventilation.

Lung volumes

  •       Expiratory reserve volume (ERV) – The maximal volume of air exhaled from end-expiration
  •       Inspiratory reserve volume (IRV) –  The maximal volume of air inhaled from end-inspiration
  •       Residual volume (RV) – The volume of air remaining in the lungs after a maximal exhalation
  •       Tidal volume (VT) – The volume of air inhaled or exhaled during each respiratory cycle

Lung capacities

  •       Functional residual capacity (FRC) – The volume of air in the lungs at resting end-expiration.
  •       Inspiratory capacity (; the maximal volume of air that can be inhaled from the resting expiratory level.
  •       Total lung capacity (TLC) –  The volume of air in the lungs at maximal inflation.
  •       Vital capacity (VC) –  The largest volume measured on complete exhalation after full inspiration.

Spirometry graph readings and interpretation

A spirometry test is simple and quick to undertake, as all that is required from a patient is for them to blow into a machine tube, similar to the type of equipment used to measure the amount of alcohol a person has drank.

Once the full volume of air has been blown into the machine, it will be graphed for reading.

Here are a few more examples of spirometer graph readings;

Unfortunately, getting an accurate reading can be troublesome when it comes to spirometer tests as they are only reliable if the patient provides maximum effort when blowing into the machine.

It is only once 2-3 tests have been performed and a GP is happy with the consistency of readings that a determination can be made.

This flow diagram highlights the complexity and difficulty of delivering an astute reading from a spirometer graph and test.

The key aspect to keep in mind when trying to understand how to interpret a spirometer graph, is that a number of calculations need to be made in order to get a final reading and the process is a complex one.

Therefore, it is important to trust the professional opinion of your GP when trying to interpret a spirometer graph, however, generally speaking, a healthy adult should be able to exhale three-quarters of their forced vital capacity (FVC) in the first second of blowing into the spirometer.

If not, there may be a respiratory issue which is what will be covered in the next section.

What a spirometer graph reading could reveal

If you go for a spirometer test and a successful reading is taken, a number of results could be given for a patient, including;

  •       Obstructive lung disease
  •       Enlarged thyroid gland
  •       Tumour
  •       Inhaled foreign body
  •       Epiglottic closure
  •       Vocal cord dysfunction
  •       Strained throat
  •       Restrictive lung disease

The causes of these problems are also numerous, though a spirometer may not be able to diagnose what the root cause would be.

The following are the some of the most common causes of extrapulmonary restrictive lung disease;

  •       Scoliosis
  •       Kyphosis
  •       Ankylosing spondylitis
  •       Diaphragm paralysis
  •       Neuromuscular disease (including polio, Myasthenia gravis)
  •       Obesity  

While the following are common causes of intrapulmonary restrictive lung disease;

  •       Pulmonary fibrosis
  •       Allergic Alveolitis
  •       Sarcoidosis
  •       Asbestosis

Understanding a Spirometry graph – Conclusion

A trained professional spends years studying and practicing procedures in order to deliver medical care which is why it is best left to them when it comes to interpreting a spirometer graph.

There are a huge number of variables, as has been highlighted throughout this guide, which contributes towards a final diagnosis so it would not be advisable to attempt to extract an interpretation of a graph if you are shown one after taking the test.

The most important thing to keep in mind before a test, is to give it your all when blowing into the machine, this will give you the best chance of getting an accurate graph reading.

Furthermore, a number of tests may follow your initial consultation so do not worry if the results are inconclusive immediately.

If you are having problems with your breathing, make an appointment with your GP as soon as possible as problems can be halted if they are caught early enough.