What is a PFT Test?

5.0 Airways Resistance (by Plethysmography)

Airway resistance (Raw) is a measure of the resistance (measured in cm h1O) to airflow (measured in L/second) afforded by all anatomical structures between the atmosphere and the lung alveoli, including the mouth, nasopharynx, and the central and peripheral airways. It is used for evaluation of airway responsiveness, provocation testing, characterization of various types of obstructive lung disease, localization of the primary site of flow limitation, and evaluation of localized obstruction.
Volume standardization of the airway resistance may be accomplished by dividing the conductance, SGaw (the reciprocal of the resistance) by the TGV at which the resistance measurement was made.

While a single small airway provides more resistance than a single large airway, resistance to air flow depends on the number of parallel pathways present. For this reason, the large and particularly the medium-sized airways actually provide greater resistance to flow than do the more numerous small airways.

Airway resistance decreases as lung volume increases because the airways distend as the lungs inflate, and wider airways have lower resistance.

• Parameters:
  • Raw  Airways Resistance
  • SGaw Specific Conductance
  • Gaw Conductance
  • VTG Volume of Thoracic Gas

The example below shows a typical airway resistance test performed in the following order:

1. Quiet, regular breathing to establish the end-tidal FRC baseline (FRC 0).
2. The subject is then asked to begin shallow breathing at a slightly faster rate (approximately 1 breath per second) and Flow -v- Box Pressure is recorded.
3. The mouthpiece shutter is closed and Mouth Pressure -v- Box Pressure is recorded. The shutter opens and the test is complete.

The VTG is an important factor when standardizing the airway resistance to the volume at which it was measured. This allows for the reporting of specific conductance (SGaw).