**4. Biofeedback system**

While monitoring may help to understand how BV is regulated during UF in an individual patient, the HD prescription often remains empirical. The hemodynamic stability during the previous HD sessions dictates the delivered parameters of the next HD treatments. Dialytic parameters such as UF rate and dialysate sodium concentration are usually set at the start of the HD session, and remain fixed throughout the treatment, with the assumption that they continue to be adequate for the whole treatment duration. However, this is not taking into account that patients may undergo physiologic variations during HD, and that fixed parameters, creating labile gradients, may not always be appropriate and may promote IDH. In fact, the standard HD prescription lacks a rapid retroactive response in case of variation of the monitored parameter, as the action to bring it back towards the desired value are taken manually (by the operator, a nurse or physician) or semi-automatically (authorized by the operator) (Locatelli et al., 2005), thus implying a certain lag of time that may be deleterious if IDH is to be prevented.

The method used to traditionally prescribe HD parameters is far from how the kidneys really behave to maintain internal homeostasis, keeping biologic variables in very tight ranges through instantaneous adjustments in response to precise negative or positive feedback loops. To try to get closer to what would be called « physiological dialysis », technological advances of the last decades have conducted to the development of sophisticated softwares that allow automated biofeedback. The concept of biofeedback is based on repetitive on-line measurements of the patient's biochemical parameters with biosensors, which are then constantly analysed by an automatic controller as being within the target values pre-set by the operator, or not. If the measured parameter is within the desired values considered safe, the treatment continues unchanged. If not, an action to bring it back towards the aimed values is immediately and automatically undertaken through the effectors, in a closed-loop that insures the stability of the monitored parameter.

The theoretical advantage of such devices is that they not only rapidly detect physiological abnormalities which may predict hemodynamic instability (blood volume reduction for example), but they automatically adjust one or more dialytic parameters to correct the situation. This obviates the need to perform manual changes by the operator and, at the same time, avoids the time lag before the action is undertaken. By modulating on-line some of the delivered dialysis parameter, these devices also address the physiological variations occurring during HD (and the variability of the patient's parameters from session to session), and thus provide more physiological dialysis, which may be more suited to prevent IDH.

At the present time, biofeedback systems are available for different parameters: relative blood volume, thermal energy balance, plasmatic conductivity, to which arterial pressure feedback using fuzzy logic control can be added. These devices are described here, with emphasis on blood volume biofeedback.
