Bronchial Responsiveness and Lung Function in a Cohort of College Students of Medical Technology Ⅲ.Plethysmgraphic Measurement of Thoracic Gas Volume

Abstract

The thoracic gas volume (Vtg) measured by the body plethysmographic method is often considered to be numerically equal to the functional residual capacity (FRC) recorded by the closed circuit helium dilution method. In the clinical practice, however, this is not entirely true even in normal subjects. To investigate the differences in absolute lung volumes measured by this two methods in healthy young Chinese adults, we made both measurements of FRC and Vtg on 42 healthy college students. The mean difference between the Vtg and the communicating gas volume measured by helium dilution in these subjects, regardless of gender, was 260 ± 225 ml. The values for Vtg were equal ( differed by less than 100 ml) to those for FRC in only one- third of the study population. In addition, none of these normal subjects showed a Vtg smaller than the communicating gas volume. We conclude that the Vtg, by virtue of including abdominal gas , is consistently equal to or larger than the FRC in normal Chinese young adults. Consequently, when the Vtg is used to measure FRC, a larger total lung capacity may be anticipated.