**1. Introduction**

26 Progress in Hemodialysis – From Emergent Biotechnology to Clinical Practice

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*systemach fizjologicznych* Wydawnictwo Komunikacji i Łączności, 83-206-0284-X,

Intradialytic hypotension (IDH) is the most common complication of hemodialysis (HD), occurring in up to 20 to 33% of sessions (Daugirdas, 2001). IDH is responsible for various minor symptoms (nausea, vomiting, muscle cramps, dizziness, and fatigue) during dialysis, but is also associated with more severe adverse events such as myocardial infarction (Burton et al., 2009) and cerebral ischemia (Mizumasa et al., 2004). Moreover, as a result of frequent interruption of sessions and repetitive administration of intravenous fluids, underdialysis and inability to reach dry weight, with subsequent chronic overhydration, can follow.

Traditionally, HD prescriptions are based on clinical evaluation and laboratory measurements, and are re-evaluated periodically or when an adverse event, such as hypotension, commands it. The drawback of this prescription is that it relies on previous observations, with the assumption that the same will hold true for the next sessions. Hence, it implies discomfort for the patients, as the actions to remediate to IDH, for example, by stopping ultrafiltration (UF) or adjusting dry weight, are taken on an *a posteriori* basis (Locatelli et al., 2005).

In an attempt to prevent IDH, technological advances have made possible the detection of subclinical predictors of hemodynamic instability, for example relative blood volume variations. With repetitive measurement of such specific parameters during HD (Mercadal & Petitclerc, 2009), actions can be implemented to correct the monitored parameter toward a desired target, with the aim of preventing overt IDH. When this action is automatic and regulated by a closed feedback loop, it is called biofeedback.

In this chapter, we will review some of the physiological basis of IDH and blood volume reduction during HD, and we will examine the technical aspects of the various devices used to adjust blood volume during dialysis, with special emphasis on biofeedback systems. Finally, we will study the literature published on the effects of automated blood volume regulation devices on the occurrence of hypotensive episodes, volume overload control, hypertension management and quality of life in chronic HD patients.
