Lung function tests

Practical medical physiology Pulmonary function test
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6- Lung Volumes and Pulmonary Function Tests
Pulmonary function tests (PFTs) are noninvasive diagnostic tests that
provide measurable feedback about the function of the lungs. By assessing
lung volumes, capacities, rates of flow, and gas exchange, PFTs provide
information that, when evaluated by doctor, can help in diagnosis of certain
lung disorders.
Some medical conditions may interfere with ventilation. These conditions
may lead to chronic lung disease. Conditions that interfere with normal
ventilation are categorized as obstructive or restrictive. An obstructive
condition occurs when air has difficulty flowing out of the lungs due to
resistance, causing a decreased flow of air. A restrictive condition occurs
when the lungs or chest muscles are unable to expand adequately, creating
a disruption in air flow.
Why the Test is Performed
Pulmonary function tests are done to:
? Diagnose certain types of lung disease (such as asthma, bronchitis, and
emphysema)
? Find the cause of shortness of breath
? Measure whether exposure to chemicals at work affects lung function
? Check lung function before someone has surgery
? Assess the effect of medication measure progress in disease treatment.
Lung volumes:-
There are four different volumes, they are:
1- The tidal volume (TV):- It is the volume of air inspired or expired with
each normal breath. It is about 500 ml. in the normal young adult subject.Practical medical physiology Pulmonary function test
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2-The inspiratory reserve volume (IRV):- It is extra lung volume that can be
inspired over the normal tidal volume. It is about 3000 ml.
3- The expiratory reserve volume (ERV):- It is the amount of air that can
still be expired by forceful expiration after the end of a normal tidal
expiration. It is about 1100 ml.
4- The residual volume (RV):- It is the volume of air still remaining in the
lung after the most forceful expiration. It is about 1200 ml. this volume
cannot be measured by spirometer.
Lung capacities:-
There are four different capacities. The lung capacity includes two or more
lung volumes. They are:
1- The inspiratory capacity (IC):- Maximal volume of air inhaled after a
normal expiration. It equals the TV plus the IRV (about 3500 ml).
2- The functional residual capacity (FRC):- The volume of gas that remains
in the lung at the end of a passive expiration. It equals the ERV plus RV
(2300 ml).
3- The vital capacity (VC):- The total volume of air that can be forcibly
expired after maximal inspiration. It equals the IRV plus the TV plus the ERV
(4600 ml).
4- The total lung capacity (TLC):- It is equal to the vital capacity plus the
residual volume (5800 ml). It is the maximum volume to which the lung can
be expanded with greatest possible inspiratory effort.Practical medical physiology Pulmonary function test
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Figure 6-1: Lung volumes and capacities
Pulmonary function tests encompasses a wide variety of objective tests to
assess lung function which provide objective and standardized
measurements for assessing the presence and severity of respiratory
dysfunction, they include:
1. Forced vital capacity (FVC). This is the amount of air exhaled forcefully
and quickly after maximum inspiration.
2. Forced expiratory volume in one second (FEV1). This is the volume of air
expired during the first second of the FVC test.
3. FEV1/FVC: the ratio of FEV1 to FVC X 100 (expressed as a percent); an
important value because a reduction of this ratio from expected values is
specific for obstructive rather than restrictive diseases.
4. Peak expiratory flow rate (PEFR). This is the maximum volume during
forced expiration.
Some PFTs involve the use of a spirometer which is an instrument that
measures the amount of air breathed in and/or out and how quickly the air
is inhaled and expelled from the lungs while breathing through a
mouthpiece. The measurements are recorded on a device called a
spirograph. Spirometer can be used to diagnose lung diseases & toPractical medical physiology Pulmonary function test
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differentiate between obstructive & restrictive lung diseases.
Spirometer cannot measure residual volume (RV) & dead space.
It is measured by whole body plethysmography & Helium dilution method
& many other methods.
The normal values for PFTs vary from person to person. The amount of air
inhaled and exhaled in your test results are compared to the expected
average in someone of the same age, height, sex, and race. In addition,
results are compared to your previous test results, if previous testing has
been done. If you have abnormal PFT measurements or if your results are
different from previous tests, you may be referred for other diagnostic
tests to establish a medical diagnosis.
Interpretation of Spirometry:
Step 1: obstruction or not?
Low FEV1/FVC (<70%) = obstruction
Step 2: Interpret severity (based upon FEV1)
Restriction: FEV1 and FVC reduced in proportion (i.e. normal FEV1/FVC
ratio)
Flow/Volume Loops
Obstruction – concave, scooped appearing
Restriction – decreased VC, normal shape
Figure 6-2: shape of spirographPractical medical physiology Pulmonary function test
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Figure 6-3: Spirograph
Figure 6-4: Spirobank 2 Figure 6-5:SpirolabPractical medical physiology Pulmonary function test
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Objective:
Measurement of respiratory volumes and function tests.
Materials and instruments:
1- Subject.
2- Spirometer.
Procedure:
1- The subject is sitting.
2- Use clean disposable mouthpiece.
3- Ask subject to breath in or out of the spirometer.