[Functional Residual Capacity Measurement Based on Lung Structure Model Analysis Using the Helium Gas Dilution Technique].

In the helium gas dilution method, functional residual capacity (FRC) is calculated from a helium concentra- tion equilibrium curve. In this study, we analyzed the helium concentration equilibrium curves of healthy patients, clarified the determinants of the equilibrium concentration, and studied the effects of an uneven lung distribution. We collected data from 200 patients (92 males and 108 females) whose FRC values had been measured at our institution over the past 6 years. Their FRC values ranged from 80% to 120%, and theit other pulmo- nary function values were within the normal range. In the compartmental model analysis, we discovered that the helium concentration equilibrium curve was composed of one compartment, and that it did not have a polyphasic structure. Each 0.25-minute (15-second) segment of the helium concentration equilibrium curve obtained from the patients was evaluated using univariate and multivariate regression analyses. The helium concentration equilibrium curve decreased exponentially over the time course of the analysis, and the multiple correlation coefficient for the relationship between the 0.25-minute to 0.75-minute segments and the 1.00-minute to 1.50-minute segments in the final model was 0.949. Finally, we examined the influence of an uneven peripheral lung distribution. A model based on the con- centration change seen between the initial and middle periods during at rest ventilation indicated that the latter parameter was not affected by the ventilation volume of the peripheral lung. [Original].