Bioimpedance resistance ratios for the evaluation of dry weight in hemodialysis

Material: Restoration of body water compartments to normal by ultrafiltrati on is a major goal of hemodialysis. Dry weight is the term used to define n ormal body water in dialysis patients, but it is limited, as it is based so lely on clinical observations. Bioimpedance spectroscopy can accurately mea sure the resistance of body fluid compartments. The ratio of the resistance s of the intracellular to extracellular water should reflect the relative v olume of these compartments. As dialysis patients accumulate excess fluid i n their extracellular compartment, this ratio may prove useful in the evalu ation of dry weight. Methods: We measured the resistances of the intracellu lar and extracellular fluid compartments in normal subjects to define the n ormal ratio of the resistances of these compartments. Women had a slightly higher ratio than men (women: 2.41 +/- 0.23 vs. men: 2.08 +/- 0.23 vs. p < 0.0001). The ratios determined in the normal population were taken as the n ormal physiologic ratio and were used to define physiologic dry weight. We then compared dialysis patients both pre- and post-dialysis to this normal population. Results: We found that most patients (67%, n = 18) had an eleva ted ratio pre-dialysis suggesting excess extracellular fluid. Of the 38 tre atments in which patients achieved their clinical dry weight, 19 (50%) had persistently elevated R-i/R-e ratios, suggesting they had not reached physi ologic dry weight. Conclusion: These data suggest that many dialysis patien ts carry excess extracellular fluid post dialysis despite achieving their c linical dry weight. Furthermore, the resistance ratio derived from bioimped ance spectroscopy may be a useful clinical tool in determining dry weight.