Electrolyte disorders and substitution fluid in continuous renal replacement therapy

Electrolyte balances during acute renal failure treated with continuous convective techniques, such as continuous arteriovenous hemofiltration (CAVH) and its pumped variants, are largely dependent on the eloctrolyte plasma concentration available for ultrafiltration, the ultrafiltration rate and the composition of the replacement solution. As blood sodium concentrations measured by potentiometry (Na+P) and the total ultrafiltrate sodium concentration are very similar, Na+P can be taken as the value of ultrafilterable sodium when choosing the correct sodium concentration in the substitution fluid. In CAVH, the ultrafiltrate contains about 3 mEq/liter of calcium and 1 mEq/liter of magnesium that must be replaced by the substitution fluid in order to prevent hypocalcemia and hypomagnesemia. In addition, if plasma potassium levels are normal, 3 to 4 mEq/liter of potassium should be added to the replacement fluid to avoid hypokalemia. Although convection and diffusion are combined in continuous hemodialysis, solute transport is largely mediated by convection; however, the net removal of sodium and calcium is significantly influenced by their concentrations in the dialysate, and the risk of hypomagnesemia and hypokalemia can be attenuated by adjusting magnesium and potassium concentrations in the dialysis solution to levels near to the plasma water values. Since critically ill patients are prone to developing dialysis-induced hypophosphatemia, phosphorous must be monitored and supplemented if necessary. Since CRRT works continuously, serious derangement in fluid and electrolyte homeostasis may occur in the absence of careful prescription and extremely vigilant monitoring.