COMPARISON OF HELIUM DILUTION AND PLETHYSMOGRAPHIC LUNG-VOLUMES IN PREGNANT-WOMEN

The multibreath helium equilibration method is the technique recommend ed for routine measurement of static lung volumes in normal subjects, However, pregnancy could be an exception to this general rule, due to airway closure during the second half of gestation, The aim of this st udy was to compare the measurements of lung volumes by plethysmography and helium dilution during pregnancy. Twenty three healthy women were studied at 12, 24 and 36 weeks of pregnancy, and 4 months postpartum, Total lung capacity (TLC), functional residual capacity (FRC) and res idual volume (RV) were measured by multibreath helium equilibration (T LCHe, FRCHe and RVHe) and by plethysmography (TLCbox, FRCbox and RVbox ). Only at 36 weeks were there differences between the two methods, RV box was significantly larger than RVHe (1.01+/-0.18 vs 0.77+/-0.21 L; p<0.001), FRCbox was larger than FRCHe (1.95+/-032 vs 1.60+/-032 L; p< 0.001) and TLCbox was larger than TLCHe (4.83+/-0.52 vs 4.45+/-0.51 L; p<0.05), The 95% limits of agreement for differences between lung vol umes measured by the two techniques (helium dilution - plethysmography ) at 36 weeks were: -0.42 to -0.06 L for RV; -0.54 to -0.17 L for FRC; and -0.66 to -0.11 L for TLC. We conclude that using the multibreath helium equilibration method to measure lung volumes in at-term pregnan t women results in underestimation of functional residual capacity and total lung capacity.