ATS Pulmonary Function Laboratory Manual

ATS Pulmonary Function Laboratory Management & Procedure Manual | 3rd Edition

The Raw is obtained from the ratio of the two slopes in the following way: Raw = Δ Pao/ Δ Pbox Δ ˙ V/ Δ Pbox or Raw = Δ Pao Δ ˙ V There is an inverse and hyperbolic relationship between Raw and lung volume. That is, as one takes a bigger breath and lung volume increases, Raw decreases. Conversely, as lung volume decreases, Raw increases. To adjust for this volume effect, Raw is commonly expressed as the reciprocal form (1/Raw), airway conductance (Gaw). Unlike Raw, there is a direct (almost linear) relationship between lung volume and Gaw. That is, as lung volume increases, Gaw increases in a linear fashion. Conversely, as lung volume decreases, Gaw decreases. As Raw is most commonly measured in a body plethysmograph, lung volume–adjusted measurements may also be made. The lung volume–adjusted Raw and Gaw measurements are called specific resistance (sRaw = Raw × lung volume) and specific conductance (sGaw = Gaw / lung volume), respectively. Lung volume–adjusted values are important parameters to assess the effects of varying lung volumes and reducing the variability due to differences in lung inflation. For example, in the healthy lung, increasing lung volume will correspond to a decrease in Raw. The sGaw at the increased lung volume would be near the sGaw obtained at functional residual capacity (FRC). However, when individuals with airflow limitation (e.g., asthma, chronic bronchitis, emphysema, and peripheral airways disease), with or without hyperinflation, are instructed to pant, they invariably perform Rawmeasurements at lung volumes above FRC. The Raw value at the higher lung volume will be lower relative to the Raw at FRC. But unlike in the healthy lung, the lung volume adjusted measurement (sGaw) will be reduced, which indicates airflow limitation (2, 3). In the United States, most commercial manufacturers supply constant volume, variable-pressure total-body plethysmographs. Because of their design and with resultant frequency–response characteristics, the “panting” method is employed when determining Raw. An alternative technique that measures Raw during quiet breathing may be available on some systems, but is not as well validated as the standard panting technique. During Raw measurements, the patient is seated with the lips firmly attached to a flow-sensing device, wears a nose clip, and supports the cheeks with fingertips. The patient is instructed to gently pant (moving ≃ 50 to 100 ml of air/breath). The pant frequency is generally in the range of 1.5 to 2 breaths/sec. During this panting phase, Δ ˙ V / Δ Pbox is displayed on the computer monitor. When enough technically acceptable pants are obtained (typi- cally three), the shutter is closed and Δ Pao/ Δ Pbox is measured, allowing for the measurement of the lung volume at which Raw was obtained. Therefore, during Raw measurements performed in a total-body plethysmograph, the patient should be allowed to select the lung volume at which he/she feels most comfortable performing the maneuver (and should not be forced to pant at FRC), since derived values will be lung volume–adjusted (as sRaw and/or sGaw). Having the patient feel comfortable during the testing process will generally result in more consistent (technically acceptable) tests being obtained in a shorter period of time. Raw is reported in cm H 2 O/L/s. Specific conductance is reported in L/s/cm H 2 O/L, which is the recipro- cal of Raw (1/Raw) divided by the lung volume at which the Raw measurement was made. Raw is generally measured and reported to reflect the mean of both inspiratory resistance (Raw,insp) and expiratory resistance (Raw,exp) values. However, Raw,exp and Raw,insp can be measured and reported separately. As with forced expiratory and forced inspiratory spirometry (flow–volume loops), open-shutter Raw loops have definite shapes (morphologies) that are indicative of specific airway disorders. Because of these specific morphologies

88