Pulmonary function tests (Lufu for short, spirometry is often used as a synonym) are a series of medical tests that check the function of the lungs. These tests determine how much air you can breathe in and out of the lungs, how fast you can breathe in and out of the lungs, and how much oxygen is transferred from the air into the bloodstream. When a lung function test is to be performed, there can be many different reasons for this.
Pulmonary function tests are often done to determine the cause of a long lasting cough or shortness of breath. In addition, lung function tests can be used to characterize a known lung disease more precisely and to monitor its course. These lung diseases include asthma, chronic bronchitis or chronic obstructive pulmonary disease (COPD).
In addition to testing for these diseases, lung function tests can also be used to check how well a respiratory spray works or whether the lungs are functioning well enough to survive surgery. In order for gas exchange to take place, the inhaled air must first pass through the main bronchi and bronchioles into the pulmonary alveoli. Only there does the gas exchange between blood and air take place.
Procedure of a pulmonary function test
Since there are different tests for measuring lung function, there are also different procedures. Pulmonary function tests generally serve to determine various pneumological parameters. Basically, the procedure for the patient is quite similar in many procedures.
In so-called “open” measurements, such as spirometry, ergospirometry, peak flow meter or DLCO (carbon monoxide diffusion capacity), the test person must inhale test air through a mouthpiece or mask. Measurements of various lung parameters are then taken. There are also closed procedures such as whole-body plethysmography.
1 Spirometry: In spirometry, the test person inhales and exhales through a mouthpiece. Nasal breathing is interrupted by a nose clip. In addition to normal breathing, breathing maneuvers such as maximum inhalation and exhalation are performed.
The ergometer is either a treadmill or a bicycle ergometer on which the patient must perform. The load can be increased here as required. Both cardiovascular (e.g. blood pressure and heart rate) and pulmonary parameters are recorded.
The latter are determined with the help of the connected spirometer. 3. the peak flow meter: This device measures the maximum exhalation and is mainly used to monitor the course of bronchial asthma. The peak flow meter is a tube with a built-in resistor.
Against this resistance the patient exhales as forcefully as possible in one breath. The patient holds the device horizontally in front of him and inhales once as deeply as possible. Then he puts the mouthpiece firmly into his mouth and exhales with a maximum breath pulse.
4. The DLCO: In this procedure, the test person inhales test air containing carbon monoxide, which he then exhales again through the device after briefly holding the air. This test measures the ability of the lungs to absorb oxygen and release carbon dioxide.
5 Blood gas analysis: Blood gas analysis does not require the active cooperation of the patient. Either capillary blood from the fingertip or arterial whole blood from the radial artery or femoral artery is collected and mechanically analyzed within minutes. The oxygen and carbon dioxide saturation, the pH value and the acid-base balance are checked.
6. whole-body plethysmography: this is a closed procedure in which the patient sits in an airtight cabin. The patient breathes normally in the cabin. This changes the pressure conditions in the cabin, from which the respiratory resistance, the total gas volume in the thorax and the total lung capacity can be determined.
7 The helium inhalation method: The patient inhales a certain amount of helium gas, which has the property of being distributed only in those parts of the lung that are involved in exhalation. The test can therefore show whether there are larger areas of the lung, e.g. emphysema, which are no longer involved in exhalation. Spirometry is the most commonly used lung function test.
This test can usually be carried out by your family doctor.In spirometry, the patient must first inhale as deeply as possible and then exhale as quickly and firmly as possible into a tube. This tube is connected to a spirometer via a tube. The spirometer measures exactly how much air can be inhaled into the lungs and how much air is then exhaled again (vital capacity, FVC).
In addition, it can measure how much air can be exhaled within one second with maximum force (one-second capacity, FEV1). During the test, the patient can receive certain medications via a spray and then breathe back into the spirometer. This makes it possible to see whether these drugs have a benefit for the patient, for example whether the asthma spray really does lead to improved ventilation of the lungs.
For chronically ill patients who need to check their lung function regularly, for example to find out how much of a medication they need to take, there are also small digital lung function tests for use at home or on the road. One disadvantage of spirometry is that the values measured are highly dependent on the patient’s cooperation. This means that the test result is easy for the patient to manipulate.
In addition, small children or particularly ill people cannot perform this test. This lung function test examines the lung’s ability to release the inhaled gases, especially oxygen, into the blood and then filter them out of the blood and release them into the ambient air. In this test, the patient inhales a certain gas and then exhales it back into a tube.
This can determine how much of the inhaled gas is exhaled again and thus the ability of the lungs to transfer oxygen or other gases into the blood and filter them out of the blood again. The causes of a disturbance in the gas transfer in the lungs can be an obstruction of a vessel in the lung (pulmonary embolism) or over-inflation of the lungs (pulmonary emphysema). During this lung function test, the exact amount of air that can fit into the lungs (total capacity, TLC) and the amount of air remaining in the lungs after exhalation is measured.
This remaining air cannot be exhaled and serves to prevent the lung from collapsing after each exhalation. This volume remaining in the lungs is called residual volume. In some diseases of the lung, there is less air in the lungs, but in other diseases there is more air than a healthy subject.
In whole-body plethysmography, the patient sits in a glass box that looks like a telephone booth. Since the amount of air in the glass box and the pressure of the air are known, a pressure difference in the glass box can be used to measure exactly how much air the patient has in his lungs when breathing in and out and how much the chest is stretched or compressed when breathing. In this pulmonary function test, the test person must also inhale and exhale through a tube connected to a measuring system.
Often, whole-body plethysmography is combined with spirometry to obtain more parameters for evaluation. In arterial blood gas determination, the blood is examined directly. For this, blood must first be taken from an artery and then analyzed in the laboratory.
The amount of oxygen in the blood can also give an indication of lung function, but may also be influenced by other factors. The results of the various lung function tests are evaluated according to the patient’s sex, age and physical constitution and are thus assessed within an objective framework. Of particular importance are the vital capacity, which represents the amount of air that can subsequently be exhaled by the patient after maximum inhalation, and the one-second capacity, which describes the amount of air that the patient can force to exhale in one second after maximum inhalation.
The vital capacity is an indication of the stretching ability of the lungs and chest. As a guideline, a younger man of normal height and weight can be assumed to have about 5 liters. The vital capacity decreases as you get older, as the lung is not as flexible and therefore less air can enter the lungs.
In addition, the so-called dead space volume can be determined. Dead space volume is the amount of air that is inhaled but does not participate in the gas exchange with the blood vessels, i.e. the air that does not reach the alveoli but remains in the bronchi.The dead space volume increases when parts of the lung no longer participate in the gas exchange, for example as a result of a vascular occlusion of an artery within the lung. The function of the lung is usually determined by means of a spirometer.
In this lung function test, certain values are analyzed. One of these values is the respiratory tract volume, i.e. the volume that is inhaled and exhaled during each normal breath without strain or exertion. During normal breathing, this volume is approximately 0.5l per breath.
If the patient now breathes in to the maximum, this is the value of the inspiratory reserve volume. This volume is still mobilizable during physical exertion and should contain about 2.5l of air per breath. The breath volume and the inspiratory reserve volume are combined to form the inspiratory capacity.
Next, the patient must exhale to the maximum. This maximum exhalation corresponds to the expiratory reserve volume, which should be about 1.5 l per breath. The inspiratory reserve volume, the breath volume and the expiratory reserve volume are combined to form the vital capacity.
This value is determined during pulmonary function tests and provides information on how much volume a patient can inhale or exhale with maximum effort. The total vital capacity should be around 5l. As this is a mobilizable volume, this value is determined using the spirometer.
The so-called residual volume (approx. 1.5l) cannot be mobilized, but is always in our lungs and can therefore only be determined with a whole-body pletysmograph. Vital capacity and residual volume together are called total lung capacity.
With the help of the lung function test further values can be determined. These include the one-second capacity. The patient inhales as deeply as possible and then exhales everything as quickly as possible.
The volume that is exhaled within one second is called the one-second capacity. This procedure is also known as tiffeneau test. The relative one-second capacity is given in percent and indicates what percentage of the vital capacity can be exhaled within 1 second.
This value should be 70-80%. If a patient can exhale less in one second and the percentage is therefore lower, this indicates increased resistance in the bronchial tubes (for example due to asthma). This resistance is another value that is determined using a pulmonary function test.
This resistance is called the airway resistance. The resistance depends on many factors, including the width of the bronchi. The wider the bronchi, the lower the resistance for the air.
In asthma, on the other hand, the bronchial tubes become narrower, which increases the resistance and makes it harder for the air to reach the end of the lungs, the alveoli. Another value that is determined in the lung function test is the maximum expiratory flow (MEV). This determines how strong the patient’s expiratory flow still is when he has already exhaled 75% of his vital capacity, or when he has exhaled 50% of the vital capacity, or when he has exhaled 25% of the vital capacity.
Another value of the pulmonary function test is the respiratory threshold value. This value indicates how many liters of air a patient can maximally exhale and inhale within one minute. For this purpose, the patient breathes in and out as much as possible for about 10-15 seconds (hyperventilation).
The volume that was breathed within this time is then extrapolated to one minute. The normal range here is 120-170 l/min. Values below 120 l/min indicate increased resistance in the bronchi (increased resistance), for example in bronchial asthma.
Finally, the so-called peak flow is measured, which is particularly important for self-control in asthma. Here, a pneumatograph is used to measure the maximum number of liters a subject can exhale. The value for a healthy patient should be around 10 liters per second.
In general, a distinction is made between two types of respiratory disorders (ventilation disorders). In the case of obstructive lung dysfunction, there is usually a foreign body in the airways, for example a swallowed Lego brick, a tumor that presses on the airways or the lungs, or diseases such as asthma and chronic bronchitis. These events increase the resistance of the airways.
Due to the disturbance of the ventilation, the patient cannot exhale as quickly as healthy subjects, so that the one-second capacity is increased. With the restrictive ventilation disorder, the vital capacity of the lungs is reduced. This is usually caused by the fact that the lung’s ability to stretch (compliance) is no longer large enough as a result of illness.As a result, the patient can no longer inhale as well as healthy test persons and a larger amount of air always remains in the lungs.
These complaints often occur in the case of adhesions in the lung area, as this limits the elasticity and extensibility, or in diseases that restrict the mobility of the lungs, such as scoliosis. The lung function test can be used to detect possible diseases such as bronchial asthma. To do this, a patient is allowed to breathe through a spirometer (device for measuring air volume, etc.).
In the case of asthma, expiration is particularly difficult because the resistance in the bronchial tubes (the resistance) is increased and thus also the volume that the patient cannot exhale (residual volume). The patient finds it difficult to exhale as much volume as possible within one second, so the relative one-second capacity is reduced (below 80%). The respiratory burst and breathing limit are also lowered.
This is called obstructive lung disease. In order for the doctor to determine whether a patient has asthma, the lung function test involves a provocation test, which means that the patient inhales a light dose of histamine. Since the asthmatic already has a lot of histamine in his lungs, he reacts more strongly than a healthy patient.
A stress test is also possible, since an asthmatic attack often occurs under stress. In a patient with an asthmatic attack, the airway resistance (the resistance) in the bronchi is increased because the bronchi are narrowed due to increased muscle activity (contraction). The messenger substance (neurotransmitter) histamine is responsible for this.
This is released by the mucous membrane in the bronchi and then causes an asthmatic attack. Since the bronchi are constricted by the histamine, not enough air with new oxygen reaches the alveoli. The alveoli are the final stage of respiration and ensure that oxygen is absorbed and carbon dioxide (CO2) is released.
Due to the narrowing, not enough air gets into the alveoli and the patient tries to compensate for this by breathing more and faster (hyperventilation), but makes the situation even worse. At the same time, not enough CO2 comes out of the lungs because the bronchi become too narrow. It is therefore important to avoid an asthmatic attack.
A lung function test, the so-called peak flow meter, can be helpful in this regard. This allows the patient to exhale with maximum force after inhalation (inspiration). Here the patient can measure at home how well he can still exhale.
If his values deteriorate, the patient knows from the pulmonary function test that asthma may recur. This is because the bronchial tubes become narrower due to inflammatory substances such as histamine or leukotrienes or prostaglandins, which have the same effect as histamine. As a result, the patient can exhale less easily, which may not be obvious to him or her at first, but can be easily determined by the peak flow meter. Thus, the lung function test can be used to prevent an asthma attack. For example, the patient can now take atropine, which dilates the bronchi and thus counteracts an attack.