Effective control of fluid overload is a vital component of an adequate dialysis treatment but, unlike solute removal, it cannot easily be measured and linked to outcome. The main difficulty in quantifying fluid status is uncertainty about the patient’s dry weight. This presentation will describe how two technologies can help nursing staff to assess dry weight.

Blood volume monitoring, using continuous measurements of haematocrit or albumin concentration during haemodialysis, was first described in the early 90’s. The rate of change in blood volume during haemodialysis shows how easily the vascular space is being refilled from the tissues. Once staff have learned to interpret blood volume changes, the information provided by blood volume monitoring can help staff identify over or underhydration. This technology is now widely used and user-friendly monitoring devices are incorporated in many dialysis machines.

Bioelectrical impedance has been used to evaluate fluid status in dialysis patients since the 1970’s. Tissue hydration can be assessed using vector analysis of measurements at a standard frequency (50kHz). Bioimpedance spectroscopy, using data over a range of frequencies, can provide estimates of the intra- and extracellular fluid volume. In theory, correctly interpreted bioimpedance measurements should give a more accurate indication of a patient’s fluid status than blood volume changes. Bioimpedance also has the advantage that its use is not limited to haemodialysis, but currently this technology is rarely used routinely. This may change as bioimpedance systems that are easy to use in the ward become available.