Airway resistance is a concept in respiratory physiology that describes the resistance of the respiratory tract to airflow during inspiration and expiration. Airway resistance can be indirectly measured with body plethysmography. A plethysmograph is an instrument for measuring changes in volume within an organ or whole body (usually resulting from fluctuations in the amount of blood or air it contains).
Functional Residual Capacity
Pulmonary plethysmographs are commonly used to measure the functional residual capacity (FRC) of the lungs—the volume in the lungs when the muscles of respiration are relaxed—and total lung capacity. In a traditional plethysmograph, the test subject is placed inside a sealed chamber the size of a small telephone booth with a single mouthpiece. At the end of normal expiration, the mouthpiece is closed. The patient is then asked to make an inspiratory effort. As the patient tries to inhale (a maneuver which looks and feels like panting), the lungs expand, decreasing pressure within the lungs and increasing lung volume. This, in turn, increases the pressure within the box since it is a closed system and the volume of the box compartment has decreased to accommodate the new volume of the subject.
Boyle's Law is used to calculate the unknown volume within the lungs. First, the change in volume of the chest is computed. The initial pressure and volume of the box are set equal to the known pressure after expansion times the unknown new volume. Once the new volume is found, the original volume minus the new volume is the change in volume in the box and also the change in volume in the chest. With this information, Boyle's Law is used again to determine the original volume of gas in the chest: the initial volume (unknown) times the initial pressure is equal to the final volume times the final pressure.
The difference between full and empty lungs can be used to assess diseases and airway passage restrictions. An obstructive disease will show increased FRC because some airways do not empty normally, while a restrictive disease will show decreased FRC. Body plethysmography is particularly appropriate for patients who have air spaces which do not communicate with the bronchial tree; in such patients helium dilution would give an incorrectly low reading.
Another important parameter, which can be calculated with a body plethysmograph is the airway resistance. During inhalation the chest expands, which increases the pressure within the box. While oberserving the so-called resistance loop (cabin pressure and flow), diseases can easily be recognized. If the resistance loop is plain, this shows a bad compliance of the lung. A COPD, for instance, can easily be discovered because of the unique shape of the corresponding resistance loop.
There are several important determinants of airway resistance including: the diameter of the airways and whether airflow is laminar or turbulent.
Airway resistance is not constant. As shown above, airway resistance is markedly affected by changes in the diameter of the airways, therefore diseases affecting the respiratory tract can increase airway resistance. Airway resistance can change over time, for example, in asthma during an attack the airways constrict causing an increase in airway resistance. Airway resistance can also vary between inspiration and expiration, for example, in emphysema there is destruction of the elastic tissue of the lungs which help hold the small airways open, therefore during expiration, particularly forced expiration, these airways may collapse causing increased airway resistance.