Breathing Clarity: A Guide to Interpretation of Spirometry

22/05/2023

Interpretation of Spirometry

Gaseous exchange is the basis for existence of life. Lungs are the organs that perform this function. They help in intake of oxygen for metabolic needs, as well as remove metabolic wastes like carbon dioxide from the body. When these organs fail to function as usual, then diagnosing the disease at an accurate time plays a crucial role in further prognosis. The most common and feasible way to diagnose and investigate a lung disease is spirometry. Spirometry measures the amount of air inhaled and exhaled and the time taken for the process to take place. After you have taken the test, your values are compared with the normal values available, which helps in diagnosis of the disease. Interpreting spirometry can be a tedious process, as it requires the patient's cooperation and the interpreter to have the knowledge about the reference values. Now, let’s look at the possible results of spirometry and what that means. 

BASIC SPIROMETRY RESULTS:

Basic spirometry interpretation | PFT Interpretation

You might have recently undergone a spirometry test and in a dilemma about your test results. Don’t worry, here is what your test readings tell about your lung status. Spirometry test values give you an idea preoperative and postoperative whether you would likely develop any pulmonary complication.

Spirometry is usually interpreted using three values: forced vital capacity(FVC), forced expiratory volume in 1st second (FEV1), and the ratio of FVC/FEV1.

FORCED VITAL CAPACITY- It is the volume of air that can be exhaled after a maximum forced inspiration. FVC normally ranges from 80%-120%. Generally, it takes 6 seconds to empty the lungs.

FORCED EXPIRATORY VOLUME IN 1ST SECOND-  it is the amount of air exhaled in the 1st second of expiration. FEV1 normally ranges from 80%-120%.

FEV1/FVC- it is the volume of forced vital capacity exhaled in the 1st second of the expiration. 

  • NORMAL PATTERN: In this, all the three parameters are in normal range. However, that does not exclude the possibility of disease. Many asthmatics have values within normal range during spirometry. In such cases, it becomes all the more important to use secondary diagnostic measures.
  • RESTRICTIVE PATTERN: when there is greater decrease in FVC when compared to FEV1 indicates restrictive lung disorders like pleural effusion, pneumonia, pulmonary fibrosis.
  • OBSTRUCTIVE PATTERN: when there is greater decrease in FEV1 when compared FVC and FEV1/FVC indicates obstructive lung disease like emphysema and bronchiectasis.

The diagram below represents the graphical representation of results obtained through spirometry.
In obstructive lung disorders, the graph shifts towards the left.

In restrictive lung disorders, the graph shifts towards the right.

VARIABILITY OF SPIROMETRY:

Spirometric results usually vary depending on various physiological and pathological factors. Some of the factors that may produce major contrast with respective to results include:

  • Age-Lung function increases up to the age of 25 years, then declines with increasing age.
  • Sex- Pre-puberty, the lung volumes are same in both the genders. Post-puberty the growth of thorax is greater in males, producing significant differences.
  • Height-Taller the person, greater is the volume of the lungs.
  • Weight
  • Whether you are a smoker or non-smoker- Smoking predisposes the individual to various lung disorders.
  • Alcoholic or non-alcoholic.
  • Race/ ethnic background
NORMAL VALUES DURING SPIROMETRY:

RESULTS CLASSIFICATION

FEV1/FVC

%FEV1<>

%FVC<>

Normal

>70%

>80%

>80%

Obstructive Lung Disease

<70%

<80%

>80%

Restrictive Lung Disease

>70%

<80%

<80%

VALIDITY OF SPIROMETRY:

A spirometry is only valid when its following requirements are meet:

  • At least, three spirograms must be recorded before the results can be derived.
  • The exhalation of the patient should last at least for six seconds.
  • The difference between the two largest FVC and FEV1, must not be greater than 0.2L.

OTHER SPIROMETRIC METHODS:

  • Portable spirometers are used in offices to diagnose lung diseases with ease and in an inexpensive way.
  • It is performed in individuals who are over 45 years of age, who are current or former smokers.
  • A primary health care worker is advised to perform spirometry in above mentioned individuals to assess the risk of lung disorders in them.

OCCUPATIONAL SPIROMETRY:

Some occupations expose the individual to particulate matter in the air, such as sugarcane, cotton and coal industries. Overtime, exposure to these particles in the air causes progressive lung damage. To prevent these diseases, the government has introduced many national health policies that recommend regular spirometry tests in these individuals. The employers are also asked to perform these tests before employing the individual as a part of pre-placement examination. This ensures the person is safe, sound and healthy.

FOOTNOTES:

Spirometry is one of the safest and best techniques to diagnose lung diseases. Interpretation of spirometry depends on you as well as the interpreter. Make sure you exhale fully while the test is being performed. With different values mentioned in various journals, interpreting spirometry becomes a difficult task. The interpreter must choose comparative values with careful insights, this will minimise the error in diagnosis of disease.

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