Bioimpedance, dry weight and blood pressure control: new methods and consequences

Chronic overhydration contributes to the development of left ventricular hypertrophy and a high cardiovascular mortality in end-stage renal disease. Assessment of dry weight is highly dependent on clinical assessment. Bioimpedance technology offers the potential to quantify body fluid compartments and to facilitate dry weight prescription. This review covers recent innovative approaches to dry weight assessment using bioimpedance technology.

Three different bioimpedance approaches to determine dry weight have been published. The normovolemic/hypervolemic slope method applies whole body multifrequency bioimpedance to assess predialysis total body extracellular fluid volume and compares the extracellular fluid volume/body weight relation at hypervolemia with the standard value in normovolemic individuals. The resistance-reactance graph method uses whole body single frequency bioimpedance for assessment of hydration state and nutritional status from height-adjusted resistance and reactance. The resulting resistance-reactance vector is set in relation to a distribution range in a normovolemic population. An alternative method uses segmental bioimpedance in the form of continuous intradialytic calf bioimpedance to record changes in calf extracellular volume during dialysis. Dry weight by this method is defined as the weight at which calf extracellular volume is not further reduced despite ongoing ultrafiltration.

Although promising, none of these methods has gained much popularity, probably due to the difficulties in understanding bioimpedance and the lack of gold standard methods for dry weight determination. Bioimpedance will improve dry weight assessment, but further refinement of the methods as well as large-scale clinical studies to demonstrate the accuracy and the clinical value of objective dry weight determination are needed.