The functional residual capacity (FRC) was measured in 24 healthy subjects, 1-by the helium dilution method using a closed system and an open system during washout, and 2-using a constant pressure plethysmograph. Results obtained with the three methods were not significantly different. Furthermore, FRC was determined by helium dilution in 177 healthy men and in 100 healthy women, aged between 18 and 77 years. In both sexes, FRC was dependent on age, height (H) and weight, being greater in the taller, older subjects. For a given height and age, FRC decreased with increasing weight. Correlation coefficients between logarithm of (FRC/H3) and age and/or weight were highly significant. From these allometric relationships, it was possible to calculate FRC reference values, and then, reference values for other functional tests such as steady state TLCO and lung compliance.
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