Submitted to: Nutrition
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 5/3/2007
Publication Date: 7/1/2007
Citation: Lukaski, H.C., Hall, C.B., Siders, W.A. 2007. Assessment of change in hydration in women during pregnancy and postpartum with bioelectrical impedance vectors. Nutrition. 23:543-550. Interpretive Summary: Pregnancy is a physiological state in which the volume of water in the body increases. Both excessive increases and failure to increase body water levels are linked to adverse outcomes in pregnancy. The ability to assess over- and under-hydration is limited by the availability of a safe, rapid, cost-effective, valid, and practical method. One approach is bioelectrical impedance analysis that uses a low-level, radiofrequency, electrical current, and measures the conduction of this current in the body. Although this method has been used to estimate the total volume of water in pregnant women, this application fails to characterize hydration status because of a lack of appropriate standards. This paper describes a new approach in which individual impedance factors, resistance and reactance, are standardized for body height, and then used to determine a vector. These vectors were found to be significantly related to the volume of water measure in 15 women before, during and after pregnancy. When the individual vectors were compared to a group vector determined in non-pregnant women, the pregnant women with excessive water levels were identified. This new method, called impedance vector analysis, provides a sensitive tool for assessment of hydration that is non-invasive and practical without need to prediction models. It will be useful in clinical studies and medical applications that seek to monitor hydration status at the bed-side.
Technical Abstract: Increases in total body water (TBW) are typical of late-stage pregnancy. Because excessive TBW expansion or contraction can lead to adverse outcomes, a safe non-invasive method for routine assessment of TBW would be useful clinically. Impedance vectors are derived from resistance (R) and reactance (Xc), determined with 800 'A to 50 kHz and standardized for height (H), and plotted on a bivariate graph. To ascertain the validity of impedance vectors, we measured TBW, assessed with D20 dilution, and R and Xc in 15 women, aged 21 to 37 y, before and during pregnancy, and postpartum (PP). Body weight (61.9±2.3 to 75.5±2.3 kg) and TBW (31.4±1.1 to 38.2±1.1 L) increased (p<0.05 from pre-pregnancy to the 3rd trimester of pregnancy, and decreased (67.0±2.3 kg and 32.7±1.1; p<0.05 L) PP. R/H and Xc/H decreased during pregnancy (p<0.05; 361±10 to 318±10 and 44±1 to 36±1 omega/m, respectively) and increased PP (p<0.05; 355±10 and 41±1 omega/m). Vector length decreased (p<0.05; 363±10 to 320±10 omega) during pregnancy, and increased PP (p<0.05; 357±10 omega). Changes in vector length and TBW during pregnancy and PP were correlated (r = 0.599; p<0.001). These findings indicate that impedance vectors provide quantitative evidence of hydration status during pregnancy, and suggest that the impedance method is useful in monitoring hydration status in pregnancy.