**2.2.1 Problems and solutions**

### **2.2.1.1 Changes of body position**

It has been shown that resistance/reactance curves are modified reversibly by changes in body position (reclining, sitting, upright position). Physical studies have demonstrated a modification in the lines of current passing through the body with changes in body position. In addition, length and resistance measures vary [55]. In the healthy subject, comparison of whole body and segmental measures shows an increase in resistance in all sectors (peripheral and central compartments) when moving from the upright to the reclining position [56, 57]. This is logical for the peripheral sector, related to the decrease in water in this sector which is redistributed to the central sector by cancelation of the gravity effect. However, surprisingly, resistance increases in the trunk, despite the increase in water. One explanation is that impedancemetry poorly evaluates the central sector. In the dialysis patient, and using the segmental mode, resistances vary with electrode position. The volumes calculated using the segmental mode are much higher than with the whole body mode and than with the anthropometric formulas [58]. In conclusion, segmental measures are insufficient to eliminate artifacts related to changes in body position.

#### **2.2.1.2 Underestimation of extracellular volume variations in dialysis patients**

These calculations are made from variations in plasma sodium content, accepting the hypothesis of zero sodium exchange between intra- and extra-cellular compartments or with the dialysis solute (obviously a false assumption). These results show that the contribution of the extracellular volume is superior to that measured by impedancemetry, with certain aberrant results [59, 60]. Conductivity variability is well known in dialysis and is most likely responsible for these problems of fluctuating resistivity for different compartments during dialysis.
