L'hémofiltration continue : méthode d'épuration extrarénale en réanimation

Abstract

Continuous haemofiltration (CHF) mimics physiological glomerular filtration. Blood flows through a haemofilter, which is permeable to water and to all those substances not bound to plasma proteins, of up to about 6.000 d molecular weight. Ten to twenty liters of ultrafiltrate (UF) can be filtered daily. Solute concentration in this UF is very similar to that in plasma water. Because of the large volumes involved, the UF must be replaced continuously with an electrolyte solution. Electrolyte and acid-base disturbances can thus be easily and rapidly corrected. There are different techniques of CHF. Continuous arteriovenous haemofiltration (CAVH) avoids the use of an external blood pump, as the patient's own arterial pressure is used to drive the blood through the filter via a large-bore arterial catheter. On the other hand, continuous venovenous haemofiltration (CVVH) requires the use of a blood pump with a pressure alarm and an air bubble detector. Supplementary diffusive transport [CAVH(D), CVVH(D)] can improve the clearance of low molecular weight toxins, such as urea. In these techniques, there is a continuous flow of dialysate in the UF compartment of the haemofilter. One of the major problems with CHF is the anticoagulation of patients who are at risk of developing haemorrhagic complications. Unfractionated heparin is used most often, but other drugs have been used : low molecular weight heparin, prostacycline, nafamostat, or sodium citrate. The neutralization of heparin has also been suggested. Because the fluid balance can be easily managed by CHF, patients in acute renal failure can be given standard intravenous feeding. Many small endogenous molecules, such as gastrin, are probably removed by CHF. However, most drugs have a molecular weight <6.000 d, and are not totally protein-bound. They are therefore likely to be ultrafiltered, and so, become inefficient. As a result, the drugs used should be adapted to the haemofilter, and vice versa. More than any extracorporeal circulation, CHF increases the incidence of bacterial blood contamination, because of its continuous use. Routine blood cultures should be carried out. Moreover, blood is cooled during its passage in the extracorporeal circuit, leading to hypothermia. There are some devices which prevent this. Renal function can be completely replaced with the production of 12 to 15 1 UF a day. CHF must be started early on in the course of the renal failure. When the concentration of blood urea is greater than 40 mmol · l −1 diffuse transport must also be used. Water clearance depends on ultrafiltration rate, which itself depends on blood flow, in the range 80 to 230 ml · min −1, CHF will not lower potassium levels rapidly, but sodium bicarbonate can be given in the replacement fluid to control hyperkaliemia. Hypernatraemia or excessive alkalaemia can be avoided by withholding sodium or bicarbonate ion from the replacement fluid. The best indication for CHF is the haemodynamically unstable patient with acute renal failure. Many authors have shown that CHF is better tolerated than haemodialysis. Osmotic shifts are a major cause of hypotension during haemodialysis, whereas plasma osmolarity does not vary during CHF. Even if CHF is an easy technique to use, it requires specially trained staff in sufficient numbers for it to be carried out in the proper conditions of safety.