Hemofiltration during cardiopulmonary bypass

Several factors combine to facilitate the evolution towards heart and multi-organ failure following cardiac surgery. Some of these factors are related to pure cardiac aspects, for example, the existence of a preoperative heart disease, the use of aortic cross clamping or performance of cardiotomy. Cardiopulmonary bypass (CPB) also plays an important role in the occurrence of postoperative organ dysfunctions by two principal means. It induces a profound hemodilution, which impairs oxygen transport through tissues. This phenomenon becomes obvious in the postoperative period by the existence of increased transpulmonary O-2 gradients, extravascular lung water volume and subsequent impairments of O-2 transport. (2) Cardiopulmonary bypass is deleterious by triggering an important inflammatory reaction. This reaction is largely related to the ratio of the circuit area to the patient's body surface area and is therefore maximal in children. It has been widely demonstrated that the very early paths of this reaction imply several humoral factors including kinins, coagulation factor XII and complement fragments. The activation of these factors is self-amplified and triggers both expression and release of numerous mediators by endothelial cells and leukocytes. Finally, these mediators are responsible for the well described "post-bypass syndrome" which is, from a clinical viewpoint, very close to hyperkinetic septic shock. Several methods have been proposed to reduce the deleterious effects of both cardiac surgery and CPB. The older one is hypothermia that considerably reduces the triggering of the inflammatory mediator network. Heparin-coated circuits may also reduce this reaction to some extent. Hemofiltration has been introduced in the 1990s in CPB management. Because of its very high tolerance in patients with compromised circulatory status this technique was already used in the postoperative period to treat patients with acute renal failure. Initially hemofiltration was intended to correct the accumulation of extravascular water during or immediately following the surgical procedure. Nevertheless, several of its side-effects appeared to be useful, such as the reduction of postoperative blood loss and immediate improvement in hemodynamics. Several studies attempted to point out the mechanism of action of hemofiltration and although removal of inflammatory mediator occurs, there is currently no proof that this removal is the actual mechanism by which this technique acts.