Body plethysmography – Its principles and clinical use

Abstract

Body plethysmography allows to assess functional residual capacity (FRC(pleth)) and specific airway resistance (sRaw) as primary measures. In combination with deep expirations and inspirations, total lung capacity (TLC) and residual volume (RV) can be determined. Airway resistance (Raw) is calculated as the ratio of sRaw to FRC(pleth). Raw is a measure of airway obstruction and indicates the alveolar pressure needed to establish a flow rate of 1 L s(-1). In contrast, sRaw can be interpreted as the work to be performed by volume displacement to establish this flow rate. These measures represent different functional aspects and should both be considered. The measurement relies on the fact that generation of airflow needs generation of pressure. Pressure generation means that a mass of air is compressed or decompressed relative to its equilibrium volume. This difference is called "shift volume". As the body box is sealed and has rigid walls, its free volume experiences the same, mirror image-like shift volume as the lung. This shift volume can be measured via the variation of box pressure. The relationship between shift volume and alveolar pressure is assessed in a shutter maneuver, by identifying mouth and alveolar pressure under zero-flow conditions. These variables are combined to obtain FRC(pleth), sRaw and Raw. This presentation aims at providing the reader with a thorough and precise but non-technical understanding of the working principle of body plethysmography. It also aims at showing that this method yields significant additional information compared to spirometry and even bears a potential for further development.

What Does Airway Resistance Tell Us About Lung Function?

“…2,20 This latter expression reveals that sR aw reflects not only R aw but also lung volume, and is not a resistance per se, but rather a form of work (pressure ϫ volume). 2,20 Thus, sR aw reflects the work related to changes in lung volume while overcoming a fixed resistance, while R aw reflects the pressure related to air flow. Because sR aw encompasses lung volume, and R aw and lung volume vary inversely, sR aw is relatively stable with respect to changes in lung volume (ie, as lung volume decreases, R aw increases, so the product sR aw remains the same, and vice versa).…”

“…Instead, flow is related to the small shifts in lung volume that occur during tidal breathing to derive sR aw . 1,2,20,23,24 Mathematically, the relationship between pressure and flow during panting and its relationship to TGV (ie, R aw ) is equivalent to dividing sR aw by TGV, so R aw ϭ sR aw /TGV, or sR aw ϭ R aw ϫ TGV. 2,20 This latter expression reveals that sR aw reflects not only R aw but also lung volume, and is not a resistance per se, but rather a form of work (pressure ϫ volume).…”

“…1,2,20,23,24 Mathematically, the relationship between pressure and flow during panting and its relationship to TGV (ie, R aw ) is equivalent to dividing sR aw by TGV, so R aw ϭ sR aw /TGV, or sR aw ϭ R aw ϫ TGV. 2,20 This latter expression reveals that sR aw reflects not only R aw but also lung volume, and is not a resistance per se, but rather a form of work (pressure ϫ volume). 2,20 Thus, sR aw reflects the work related to changes in lung volume while overcoming a fixed resistance, while R aw reflects the pressure related to air flow.…”

“…This may be illustrated by considering an obstructed patient with hyperinflation versus an obstructed patient without hyperinflation. 2 In the former, R aw is elevated and sR aw is increased (increased work to move air through a higher lung volume), but the increased TGV associated with hyperinflation corrects conductance back to normal (normal sG aw ). In the latter, R aw is increased and sR aw is increased (increased work to move air through an increased resistance), but sG aw is reduced (elevated R aw is not reduced by altered lung volume).…”

“…Traditionally, airway resistance has been measured by relating air flow and driving pressure through the use of body plethysmography, thus deriving airway resistance (R aw ), specific airway resistance (sR aw ), and specific airway conductance (sG aw ). [1][2][3][4] Other methods to measure airway resistance, which require less cumbersome equipment and less patient technique and cooperation, are the forced oscillation technique (FOT), [5][6][7] which allows calculation of respiratory system resistance (R RS ) and reactance (X RS ), and the interrupter technique, which allows calculation of interrupter resistance (R int ). [8][9][10][11] All of these methods may be used clinically, but a major question is whether they add anything to the traditional measures derived from spirometry (FEV 1 , FVC, and FEV 1 /FVC).…”