[Extrarenal elimination procedures in acute kidney failure]

In spite of continuous progress in intensive care during the past two decades, there is no definite proof that the prognosis of critically ill patients with acute renal failure has actually improved. Nevertheless, the development of hemofiltration and hemodiafiltration and of the continuous methods has extended the therapeutic spectrum and allows therapy to be adapted to the specific cardiovascular and metabolic situation of the individual patient. Detoxification is carried out intracorporeally in peritoneal dialysis or extracorporeally by means of filters and membranes with different filtration properties. Peritoneal dialysis is seldom indicated in operative patients, because of its low efficiency and the hypercatabolism that is often observed, except in situations where continuous abdominal lavage is desired, as in peritonitis or necrotizing pancreatitis. The standard detoxification method is hemodialysis in which the filter is perfused in the opposite direction to blood flow. Solute transport is the result of diffusion, osmosis and, to a minor extent, convection, and the elimination of small molecules is favored. As huge amounts of dialysate have to be used, a central water regeneration unit is the technical prerequisite for hemodialysis. In critically ill patients bicarbonate dialysis should be preferred, because it is hemodynamically better tolerated. Hemofiltration relies on convection rather than diffusion through a membrane with a cut-off between 20,000 and 50,000 daltons. The ultrafiltrate largely resembles plasma water. Fluid loss is substituted by appropriate electrolyte solutions allowing space for negative balances and hypercaloric parenteral nutrition. Hemofiltration is better tolerated than hemodialysis and was introduced to improve the elimination of toxic middle molecules. In intensive care units without access to hemodialysis or intermittent hemofiltration continuous arteriovenous hemofiltration (CAVH), first proposed by P. Kramer, has become an alternative, mainly because of its simplicity and the absence of hemodynamic side effects. The drawback of this method is its limited efficiency. In hypercatabolic patients daily ultrafiltration rates of 10-15 l will not reduce blood urea sufficiently. The same holds true for severe hyperkalemia. One attempt to increase efficiency is predilution, which involves adding part of the substitution fluid before the filter, to enhance blood flow and to increase the effective filtration pressure by lowering the oncotic pressure of the plasma. Predilution will increase the net urea clearance by some 20%. Another way to enhance filtration is to apply negative pressure to the membrane with the aid of an infusion pump at the filtrate side. The development of better filters with smaller resistances may further increase filtration rates.(ABSTRACT TRUNCATED AT 400 WORDS)