Fluid and electrolyte balance during extracorporeal therapies

Abstract

The aims of depurative therapies in renal insufficiency are to obtain an efficient removal of metabolic waste products, and to restore and maintain body fluid, electrolyte, and acid-base homeostasis. These results may be achieved by means of the controlled transfer of solutes and water through a porous membrane placed between the patient’s blood and a properly designed solution. The membranes used for extracorporeal depurative therapies are made with a wide variety of polymeric materials, and are conventionally divided into two basic categories: those derived from the natural polymer cellulose, and those that can only be obtained by means of chemical synthesis. Both of these types of membrane are semi-permeable insofar as they allow the passage of small, but not large molecules. The permeability of dialysis membranes depends on a number of factors, including their porosity (the size and distribution of the pores), their structure and thickness, their hydrophilicity and their surface electrical charge. In the dialyzer (the device in which the semi-permeable membranes are contained), the membrane separates flowing blood (i.e. the blood compartment) from the dialysate stream (i.e. the dialysate compartment), and allows the removal of excess body water by means of ultrafiltration and the transfer of solutes from the blood to the dialysate and vice versa. The other basic elements necessary for extracorporeal depurative therapy are a vascular access, which provides the dialyzer with a large blood flow, and the dialysis machine, which prepares the dialysis solution on line, regulates blood and dialysate flows and pressures, ensures safe therapy monitoring against blood leakage and air embolism, and controls the temperature and composition of the dialysate.