Background: Current concepts of dry weight (DW) prescription are largely based on clinical symptoms because of the difficulty in assessing extracellular fluid volume (ECV) during dialysis. Intradialytic changes in ECV can be recorded as changes in extracellular resistance [Re] by continuous regional calf multifrequency bioimpedance spectroscopy (BIS). We hypothesized that relative changes in calf Re (Re at time '0' over Re at time 't' [R(e-0)/R(e-t)]) will become very small when ECV is reduced towards normal and individual dry weight is reached.
Method: Intradialytic continuous calf BIS was recorded repeatedly in 15 hemodialysis (HD) patients. The first measurement was performed at the prevailing clinical dry weight (CDW). Next measurements were made after post-HD body weight was gradually decreased by 0.2-0.3 kg per treatment. This procedure was iterated over several subsequent treatments until a treatment was observed where changes in R(e-0)/R(e-t) were < 1%. The weight at the end of this treatment was defined as "achieved dry weight (ADW)". Each R(e-0)/R(e-t) curve was fitted using a Matlab program (curve fitting toolbox) to obtain the exact weight at 20 min after beginning of the flattening of the R(e-0)/R(e-t) slope ('dry' weight estimated from BIS, DW-BIS).
Results: Both mean ADW (80.5 +/- 34.1 kg) and mean DW-BIS (80.6 +/- 34.1) were significantly lower than CDW (81.4 +/- 32.0 kg, p < 0.001), but there was no difference between ADW and DW-BIS. However, the average weight reduction from CDW to ADW (0.80 +/- 0.15 kg) was significantly higher than from CDW to DW-BIS (0.66 +/- 0.14 kg, p < 0.001, paired t-test). When ADW was achieved, pre-dialysis systolic blood pressure (SBP) was lower than at CDW (139.3 +/- 32.5 mmHg, vs. 129.4 +/- 33 mmHg, p < 0.05), post-HD SBP did not differ. The incidence of clinical symptoms of underhydration was similar at CDW (15%) and DW-BIS (15%), but higher at ADW (46%).
Conclusion: Intradialytic continuous calf BIS allows the assessment of changes in extracellular calf resistance as an indicator of changes in extracellular fluid volume. Recording of a continuous R(e-0)/R(e-t) slope during dialysis appears to be a promising new tool for the prediction of dry weight in hemodialysis patients.