Pulmonary Function Tests
PKGhatak, MD
Pulmonary Function Tests (PFTs) are aptly named, the tests delineate the functional status of the lung and allow one to follow the progression of a disease or improvement. The tests are also sensitive in the early stages of a pulmonary disease when other tests, like chest x-ray, C scan and blood tests, are likely to be normal. The PFTs are an integral part of good management of asthma where patients themselves can adjust medications on a day to day basis. Tests are simple enough and can be repeated frequently.
A complete PFT is done in a pulmonary lab by a technician and the tests are done in stages but in one sitting. It can be a bit exhausting, particularly for those with a lot of disabilities.
From the patient's side, the patient has to breathe in and out, as best he/she can, into a mouthpiece attached to a machine. And there is no need for a blood sample or any injections.
The electronic machines are amazingly efficient and fast and can spit out numbers instantly. If the report is delayed that is likely due to the time taken to interpret those numbers by an expert.
Why PFTs are needed:
1. It is an essential test to monitor a lung diastase from day to day.
2. Patients use this device to detect early warning signs of trouble and seek proper help ahead of the problem begins
3. In certain pulmonary diseases, the functional state is more important than just a diagnosis. An example is asthma.
4. An essential tool for detecting very early cases of asthma. And then categorize asthma according to severity, in order to prescribe appropriate medications and dose adjustments.
5.PFTs are sensitive tests utilized in detecting the cause when the breathlessness of the patient is the only symptom.
6. These tests differentiate obstructive lung diseases from those diseases that prevent the lung to expand fully to its capacity (restrictive lung disease).
Normal breathing.
We breathe 12 times a minute, one cycle takes about 5 sec, which includes two phases, inhalation and expiration. If one listens with a stethoscope the movement of air during inhalation makes a gentle sound, whereas, expiration is very short and only in the early part of expiration is audible. That is due to the fact that the elastic recoil of the lungs and the chest wall do all the work during expiration.
Let's take a look at this test result together. Note there is a small oval graph within the larger one, and a horizontal line is running across the graphs. The horizontal line is the baseline. The lower half of the graph represents the movement of air into the lungs, and the upper part is air moving out of the lung. For the lower half, the pen inscribes from Right to Left. The pen inscribes on the upper part from Left to Right.
The smaller graph depicts the air moving in and out of the lungs during normal breathing.
The larger graph is inscribed when the patient took in as much air in his lungs as possible and then forced out air as fast as he could and continued to push out air from his lungs with the constant cheering of the technician. This graph represents Forced vital capacity, or simply the Vital Capacity.
The horizontal line within the graphs represents the Volume of air in Liters, the vertical line depicts the Flow Rate of air in Liters per Second.
The difference between the two graphs is the Reserve Volume of the lungs, both in the inspiratory and expiratory phases.
FEV1.
The amount of air that expired under maximum efforts in the First second is known as the forced expired volume in the first one second (FEV1). A normal person can expel 80 % or more at the first second. In obstruction of the airway (COPD) the FEV1 is less than 75%. In asthma, the FEV1 should return to a normal level after a brief inhalation of a bronchodilator. In suspected cases of asthma, the initial EFV1 may be normal but the FEV1 will fall below 80% after a Methacholine challenge test, also known as Bronchial Provocation Test
The results of the PFT are reported with the actual patient's performance numbers and also as a percentage of his performance when compared with a normal person with his age/ sex/ethnic background/ height/ and ideal weight for the age and height.
Methacholine Challenge test.
A forced vital capacity is recorded first. Only if PFT is normal, then the methacholine challenge test is performed; because this test is a test to document hyperactive airways, if that is already demonstrable, then there is no point in doing the test.
The subject is asked to breathe in and out normally a nebulizing solution starting with a dose of 1 to 3 micrograms of methacholine and observing and documenting changes in flow rates of the forceful expiration. The test is continued with an increased dose of methacholine till the maximum dose is given or a reduction of flow rate is observed.
A decrease of 20% of FEV1 is necessary to call a test positive.
Total Lung Capacity.
After a maximum expiratory effort, a significant amount of air is still left in the lungs, because the lungs cannot be squeezed further as the chest wall will not yield because of rigidity. The amount of air that will not be expelled is called Residual Volume.
On adding the residual volume to the forced vital capacity volume, the Total Lung Capacity (TLC) number is obtained.
Direct measurement of TLC.
To determine the TLC, a known amount of inert gas, Helium (harmless, non-absorbable, not soluble in water or blood) is used. A mixture of oxygen and helium is inhaled from a closed system and the subject is allowed to breathe in and out within the closed system to ensure the proper distribution of helium in the lungs. Then the expired air is analyzed for the final concentration of the helium gas. Knowing the initial gas concentration and the final gas concentration of helium, the volume of the Total Air present in the lungs is easily calculated.
In emphysema, the reserve volume increases at the expense of inspiratory reserve volume and the TLC also increased due to the loss of the elastic tissue and the chest expands outwards, commonly referred to as a Barrel Chest.
In chest wall deformities, e.g. Kyphoscoliosis, and in pulmonary fibrosis, the TLC and the FVC are reduced.
The diffusion capacity of the lung is also known as the transfer factor, DLCO (diffusion of carbon monoxide), TLCO. The diffusion capacity is a measure of the health of the delicate tissue at the junction of air sacs (alveoli) and pulmonary capillaries where the oxygen molecules from the alveoli move across the membrane and bind with the Hemoglobin in the RBCs.
In a biological system, multiple factors are at play, so also in DLCO. One has to accept that under the condition of the test, just one factor is variable. After the result is obtained a correction is made, if necessary. As an example, significant anemia will appear as decreased DLCO due to less hemoglobin available to bind oxygen and not due to a damaged membrane.
To test DLCO, the subject exhales forcefully first, then inhales a known amount of a mixture of oxygen, nitrogen, helium and carbon monoxide (CO) from a closed system, and holds breath for 10 seconds. The expired air is sampled at the mid portion of expiration and analyzed by a rapid CO analyzer. The amount of CO transferred is calculated and expressed in CO mmol/min/k Pa or SI units. And a ratio of the expected normal value.
DLCO is reduced in pulmonary emphysema, pulmonary fibrosis, loss of lung tissue from lung surgery, or other diseases. Also damage to the pulmonary capillaries from platelet microthrombi, Pulmonary embolism, poison gases, radiation, etc.
Very Useful gadgets developed from PFT.
Peak Flow Meter.
A peak flow meter is easy to use, and the inexpensive gadget is an integral part of asthma management.
A maker on a tube records the highest flow generated with forceful expiration. A sudden fall in the Peak Flow indicates fatigue is setting in a sustained asthma attack - also called Status Asthmatics. Patients are well coached to seek medical attention right away.
Vitalograph.
A simple vital capacity measuring device uses a rubber bag to collect expired air and a pen records the volume and flow rates on graph paper. Patients with neurological conditions like Gillian Barre syndrome, ascending paralysis, or ALS and their caregivers are taught to use the gadget and call their doctors when detect deterioration. The use of these devices prevents time intervention and avoids ER trips unnecessarily.
Volumetric Incentive Spirometer.
It is still a simpler gadget than a vitalograph, but basically performs the same function of alerting a patient's caregivers of any diverse changes.
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