The role of bioimpedance and biomarkers in helping to aid clinical decision-making of volume assessments in dialysis patients  Simon J. Davies, Andrew Davenport  Kidney International  Volume 86, Issue 3, Pages 489-496 (September 2014) DOI: 10.1038/ki.2014.207 Copyright © 2014 International Society of Nephrology Terms and Conditions

Figure 1 Impedance to an alternating current can be described as having two components: resistance (R) and reactance (Xc). Body water is inversely proportional to resistance, and thus H2/R (X-axis) reflects higher body water content. Cell membranes act as capacitors, and thus increasing H2/Xc (Y-axis) means less tissue and more extracellular fluid. Patient A is a clinically normovolemic woman; Patient B is a hypertensive hypervolemic man, as shown by his longer and wider vector. In dialysis patients, both the length (Z) and angle (θ) of the vector are independent predictors of mortality. Kidney International 2014 86, 489-496DOI: (10.1038/ki.2014.207) Copyright © 2014 International Society of Nephrology Terms and Conditions

Figure 2 Extracellular water (ECW)/total body water (TBW) ratio as determined by two different bioimpedance analysis (BIA) devices (RJL Quantum and Xitron Hydra 4200) in 85 PD patients. There is a strong correlation, but on an individual level there is significant disagreement; this is not due to the relationship of reactance to resistance as measured by the two devices, but the subsequent equation applied to derive fluid volumes. Kidney International 2014 86, 489-496DOI: (10.1038/ki.2014.207) Copyright © 2014 International Society of Nephrology Terms and Conditions

Figure 3 The different approaches for determining fluid volumes in general show broad agreement when identifying patients with overhydration and underhydration, as shown here for 60 peritoneal dialysis (PD) patients. (a) Compares the use of a single-frequency vector plot (as shown in Figure 1) with the body composition monitor (BCM)-derived overhydration index (overhydration (OH)/extracellular water (ECW)): normal hydration , underhydration , and overhydration . (b) Compares the BCM OH index (same symbols as for a) with the ECW/total body water (TBW) ratio (observed—expected as determined by the Lindley–Lopot equation (see Appendix), in which two standard deviations of normal subjects should fall between ±5%. Kidney International 2014 86, 489-496DOI: (10.1038/ki.2014.207) Copyright © 2014 International Society of Nephrology Terms and Conditions

Figure 4 Serial changes in postdialysis N-terminal probrain natriuretic peptide (NTproBNP; pmol/l) in an individual hemodialysis patient associated with deliberate ultrafiltration over time designed to reduce postdialysis target weight and bioimpedance-derived extracellular water (ECW)/total body water (TBW). Reduction in target weight by 6.0kg and ECW/TBW ratio from 0.46 to 0.393. Kidney International 2014 86, 489-496DOI: (10.1038/ki.2014.207) Copyright © 2014 International Society of Nephrology Terms and Conditions