ATS Pulmonary Function Laboratory Manual

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

Effective Date: 2014 Version #1

Chapter 9

Procedure Name: Airway Resistance and Related Indices Measured by Body Plethysmography

Purpose or Principle There are three main measurements of resistance of the respiratory system: ( 1 ) lung resistance, ( 2 ) airway resistance (Raw), and ( 3 ) total respiratory resistance (Rtotal). Lung resistance refers to the collective resistance of the lung tis- sue and airways and is usually determined with an esophageal balloon in place. Raw refers to the flow resistance in the airways between the mouth and alveoli. Rtotal, as determined with forced oscillation or interrupter techniques, is the sum of the chest wall, lung tissue, and Raw. There are several ways to measure the various resistances including: using an esophageal balloon, forced oscil- lation, interrupter device, and the body plethysmograph. This procedure describes the measurement of Raw and related indices (specific conductance and specific resistance) using the body plethysmograph and panting technique (1). Raw can be described mathematically as follows: R =Δ P/ ˙ V , where R is Raw, Δ P is the pressure difference between the alveoli and the mouth, and ˙ V is the flow during panting. Raw is most frequently measured while the patient is enclosed in a whole-body plethysmograph, designed to measure pressure changes and flow. Two maneuvers are required: ( 1 ) panting with the shutter open, and ( 2 ) panting with the shutter closed. The following explanation assumes a constant-volume type plethysmograph, though other types can be used. During the open-shutter panting maneuver, the body plethysmograph (body box) pressure (Pbox) and flow are measured and displayed so that the slope of the relationship (i.e., Δ ˙ V / Δ Pbox) can be determined. The resulting display is shown in Figure 9.1. Note that during expiration, Pbox decreases because gas in alveoli is compressed by the pressure needed to generate ˙ V . Immediately after the open-shutter panting maneuver, the shutter in the system is closed and the patient pants against the closed shutter. The change in mouth pressure ( Δ Pao) and the change in body plethysmograph pressure ( Δ Pbox) are then measured and displayed so that the slope of the relationship (i.e., Δ Pao/ Δ Pbox) can be deter- mined. The resulting display is shown in Figure 9.2.

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