Diagnosis of Lung Diseases
Below, we would like to present typical examples of lung diseases. These examples should act as a starting point to this topic and do not guarantee perfect correctness. E.g. combined diseases are not always visible in these curves as their single aliments. Furthermore, a normal curve does not necessarily rule out the possibility of a lung disease. Further tests, like diffusion or provocation testing, provide a physician with more information. An ergospirometry test can show diseases, which are not visible at rest.
A normal flow-volume curve shows a fast rise in the expiratory part of the curve, followed by a relatively linear decrease until the point of a normal forced vital capacity. The resistance loop (blue) is steep and closed and does not show a hysteresis. The TGV curve (yellow) has an angle of about 45°, which implies an unobtrusive TGV.
Mistakes during the procedure
A spirometry test is highly dependent on the cooperation of the patient. Therefore, an accurate explanation on how the measurement is performed is crucial. However, many typical mistakes are easily identifiable in the flow-volume curve.
In order to give a correct diagnosis a fast and forced expiration is crucial. It is common for the patient to hesitate. Then the slope of the curve is not straight and the peak is shifted to the right. Available software indicates important reference values in the predictive curve as well as the position where the peak of the curve is assumed. If the patient is coughing, the expiration is not linear but shows a zig-zag-pattern. If the curve is not leading to the x-axis in a linear way, but aborts before, it is likely that the patient did not exhale as long and as motivated enough as requested.
Even if the physics of a body plethysmography test does not appear really complex, the realization of this technique requires long-term experience. A normal flow-volume curve combined with a steeper TGV curve than usual is not typical for any disease. In this case it is likely that the body box leaks, which means that the cabin has a malfunction and a service technician is needed or the cabin needs to be replaced.
The most common occurrences in lung diseases are obstructive changes of the airways. They can be congenital and paroxysmal like asthma. Or like COPD (chronic obstructive pulmonary disease) they can be chronic, caused by environmental pollutants or smoking.
A typical sign of an obstruction is a reduced PEF and a more rapid decline than normal of the flow-volume curve. As a result of this the FEV1 is considerably decreased. FVC is usually within the tolerance of 80% of the reference and therefore FEV1% is considerably decreased as well.
Depending on the characteristics of the obstruction, different curve progressions are possible. Nevertheless, there are specific alterations, which are present all the time. The resistance loop is flatter than usual. This occurs due to an increased flow resistance based on a decreased diameter of the bronchi. For the same flow, the body needs to create higher volumes through the movement of the chest. Sometimes, there is a hysteresis visible, which can signal the early stages of emphysema.
Furthermore the TGV curve shows a smaller angle, which means that TGV is significantly increased. A TGV decrease is a mechanical compensation of the body against the obstruction. A shift of the resting expiratory position results in an enlargement of the cross section of the airways. This lowers the airway resistance and makes breathing easier. However, in this case the flow-volume curve of the ventilation is not ideal anymore and requires an increased effort of the respiratory muscles.
It should be mentioned that during an asthma attack, the results of the measurements do not necessarily differ from those of a COPD. However, a differentiation is crucial for an optimal treatment of the diseases. We will discuss this topic in a further article.
In people with emphysema, the tissues, necessary to support the physical shape and function of the lungs, are destroyed. Together with chronic bronchitis, it is included in the group of chronic obstructive pulmonary diseases. Emphysema is called an obstructive lung disease because the destruction of lung tissue around the alveoli makes these air sacs unable to hold their functional shape upon exhalation. The reduction of lung compliance causes malformation of bronchial cartilage, making them soft and collapsible
There is no cure for emphysema, but an early discovery and treatment can abate the symptoms and can prevent the development of further emphysema.
The flow-volume curve shows here a stronger influence of the expiration compared to a mild obstruction. Shortly after reaching PEF, the airflow rapidly decreases. This buckle is caused by the collapse of the bronchial cartilage during the expiration. Due to the same reason the resistance loop shows a golf club shape similar deformation in the expiratory part. In order to determine the severeness of the emphysema the total lung capacity (TLC), TGV, RV and RV/TLC ratio are important values. In severe cases, TLC can be more than 150% of the reference. Because of the alteration of resting expiratory position the vital capacity is significantly decreased, but it has to be mentioned that this not a restriction in its classical meaning.
In the case of a restriction the compliance of the lungs and the thorax is limited. This is visible through a decrease of the lung volumes. The typical flow-volume curve is clearly smaller than normal. FVC and FEV1 are decreased. As a result the FEV1% is usually unaltered. A restriction is defined through a decline of the total lung capacity, which means that a body plethysmography test is needed to provide a final diagnosis. Reasons for a restriction can be thorax deformation, a reduced ventilation of the alveoli (e.g. because of pneumonia) or reduced gas exchange (e.g. because of pulmonary oedema).
In case of a stenosis the airways are partly constricted. Usually just at one point. The stenosis can be intrathoracic (within the thorax) or extrathoracic (outside of the thorax). A typical sign of a stenosis is a flattening of the flow-volume curve. Below, a fixed stenosis is pictured. “Fixed” means that the stenosis affects inspiration as well as expiration. The question if this stenosis is intrathoracic or extrathoracic cannot be answered with a simple spirometry test. Reasons for a fixed stenosis can be a tumor, cicatrization or a struma.
A variable stenosis acts either inspiratory or expiratory. This means that the flattening of the flow-volume curve is just visible on one side. An intrathoracic stenosis is recognisable by a limitation of the expiratory flow and an extrathoracic stenosis is recognisable by a limitation of the inspiratory flow. This is based on the following: During inspiration, there develops low pressure in the lungs and the trachea. The pressure decrease down the airways is relatively linear. Vice versa, during expiration there develops high pressure compared to ambience. If the position of the stenosis is intrathoracic, i.e. it is within the thorax, then the low pressure (pressure in the lungs is lower than in the trachea) during inspiration will hold the airways open. On the other side, during expiration the high pressure (pressure in the lungs is higher than in the trachea) which works on the stenosis will make them collapse.
If the stenosis is extrathoracic these circumstances are just reversed. Outside of the stenosis are now not the lungs but the ambient conditions. During inspiration, the low pressure in the trachea now causes the airways to collapse. During expiration, the high pressure in the trachea compared to ambient conditions holds the airways open.
A body plethysmography test can help to determine the position of the stenosis. For a secure diagnosis an endoscopy or a computed tomography is essential. But this would mean a more complex and invasive intervention. If there is an intrathoracic stenosis, the resistance loop shows a golf club like deformation, but not as strong as with an emphysema. If the stenosis is extrathoracic the resistance loop shows the shape of an “S”, with a bigger deformation in the inspiratory part of the figure. Furthermore, there is a hysteresis visible most of the time.
As already mentioned before, these hints should act as an introduction into this topic and are probably not 100% correct. Often, further investigations, like a diffusion test, a bronchial challenge test or an ergospirometry measurement are useful. It can be useful to send the patient to a cardiologist. An increase of the diffusion capacity, to name just one example, can be a sign of a cardiac disease (e.g. left-to-right cardiac shunt) and does not necessarily mean that the cause is a respiratory disease.