Basics of Spirometry and Body Plethysmography
Spirometry is the measurement of breath. It is the most frequently used method for pulmonary function testing, more specifically it is the measurement of the speed and the amount of air that can be exhaled and inhaled. In recent years ultrasound has established itself as the most accurate method for measuring lung function parameters. Here, two diagonally opposing piezo-elements send and receive alternatively ultrasonic waves. Without any flow the transit time of the ultrasound waves is the same in both directions. Flow moving the air inside the tube will accelerate the waves in one direction and slow them down in the other. The higher the difference between go and return, the higher the flow.
During the spirometry measurement the patient is instructed to inhale as deeply as possible. Afterwards the patient should exhale as fast as possible and as much volume as possible. This manoeuvre is captured and displayed on screen as a flow-volume curve. Doctors can determine the conditions of the patients lungs from evaluating the form of the curve. The curve is defined by a series of parameters. Important parameters are peak expiratory flow (PEF, maximum air flow reached within the expiration), forced expiratory volume in 1 second (FEV1, the amount of volume the patient can exhale within the first second of expiration) and forced vital capacity (FVC, the total amount of air, which the patient can exhale during the manoeuvre). Additionally, the ratio of FEV1/FVC (FEV1%) is also taken into account.
During a body plethysmography test, the patient is required to sit in an airtight chamber that resembles a small telephone booth. Inside the chamber is an affixed spirometer, which is used to determine the flow properties of the patient. During expiration the chest of the subject expands, thus increasing the pressure within the chamber. During tidal breathing it is now possible to determine the needed pressure, which is needed to cause a certain air flow. Air flow and cabin pressure can now be displayed in one diagram. This so called resistance loop can be a first indicator of several lung diseases.
Furthermore, it is possible to calculate the residual volume, the volume which is still in the lungs after a maximum expiration. The volume, which is still in the lungs after the expiration during tidal breathing, is called thoracic gas volume (TGV). The TGV can be calculated using Boyle-Mariotte’s law, which describes that the product of volume and pressure remains constant. As described before, the movement of the chest during breathing causes expansion and compression of the air inside the lungs. At the end of expiration during tidal breathing, the pressure inside the lungs equals the ambient pressure. During inspiration the chest expands and therefore the pressure in the lungs decreases. In order to measure the pressure in the lungs, a shutter in the mouthpiece closes and the mouth pressure can be measured. In this case where no air flow is present, the mouth pressure is equal to the pressure in the alveoli. The change of the lung volume is calculated while regarding the change of the pressure in the cabin. The only unknown parameter left to calculate would be the TGV. In practice, the mouth pressure, which can be measured after the shutter has closed, is displayed together with the cabin pressure. It is easy to determine the TGV from observing the angle of the curve. The smaller the change of the mouth pressure compared to the cabin pressure, the larger the lung volume at the point when shutter has closed.
The flow-volume curve and the resistance loop are especially significant for making a diagnosis. Experienced pneumologists can identify the early stages of diseases within seconds. Depending on the patient’s medical history it is decided if more medical investigations are required. These could be diffusion testing, ergospirometry or a provocation test.
Below a normal flow-volume curve is displayed. Based on the forced expiration the peak expiratory flow is reached shortly after beginning the manoeuvre. The start of the manoeuvre is highly dependent on the cooperation of the patient. Due to this the patient is asked to be motivated to put as much effort in the manoeuvre as possible. The end of the expiration manoeuvre does not rely so heavily on the patients cooperation, which can be explained by the physiology of the lungs.
The resistance loop, which is pictured below in blue, is in the case of a healthy subject relatively steep. In a pathological case, the curve is more flat and can show a kind of a hysteresis. The TGV curve is displayed in orange. Usually it has an angle of about 45°. If the angle is flatter, the residual volume is bigger.
Read further: Diagnosis of Lung Diseases