Effect of Thoracic Gas Compression on the Flow-Volume Curve of the Forced Vital Capacity1

Boyle's law states that the volume of any gas varies inversely as the pressure to which it is subjected, i.e., gas is compressible. A common application of Boyle's law is the plethysmographic determination of thoracic gas volume (1). However, in pulmonary mechanics a change in the degree of lung inflation or thoracic gas volume is often equated with the change in volume displaced at the mouth as measured with a spirometer, i.e., the gas in the chest is treated as being incompressible. This assumption can result in confusion or error in the interpretation and comparison of volumetric data obtained with a plethysmograph and spirometer. The significance of alveolar gas compressibility and its effect on the static pressure-volume diagram has been described (2). Jaeger and Otis (3) in a theoretical and experimental analysis studied the effect of gas compressibility by simultaneously measuring changes in thoracic gas volume and volume displacement at the mouth. They found that the volume displacement at the mouth was less than the volume change of the thorax and this difference increased with increased airway resistance, respiratory rate, and lung volume. They described differences in the measurement of the mechanical work of respiration when the change in lung volume rather than the change in volume at the mouth was used to construct the pressure-volume diagram. The analysis of Jaeger and Otis was c~nfined to the study of slow and rapid cyclic respiration. It is the purpose of the present paper to report the effect of gas compressibility on the forced vital capacity maneuver and the flow-volume relationships (4, 5) during this maneuver.