Problems with High Small Solute Clearances Large K

Blood Flow and Efficiency of Dialysis Short dialysis with fixed Kt/Vurea leads to maximization of dialysis efficiency by using higher efficiency dia-lyzers and high blood and dialysate flows; however, the influence of blood flow on the efficiency of dialysis is markedly lower than dialysis time. Removal of MMs (including phosphorus) is only slightly dependent on blood and dialysate flows [13]. so compensating shortened dialysis time by increasing blood flow is not effective. This is not only related to the slow diffusion of these molecules through the membrane, but also to multicompartmental behavior, i.e., slow diffusion from the extravascular space to the plasma [78]. This process may be compared to the poor 'plasma-refilling rate' of water and sodium in high ultrafiltration rate hemodialysis. It is worth realizing that even for removal of small molecules, an increased time of dialysis is more effective than increased blood and dialy-sate flows, because spKt/Vurea (single pool) is directly proportional to dialysis time, but K is exponentially, not linearly, proportional to blood and dialysate flows.

High Blood Flow Rates and Retrofiltration The introduction of ultra-short dialysis treatments with high blood flow and high flux dialyzers brought other unexpected, undesirable effects, namely back filtration or retrofiltration of dialysate to the blood compartment [79, 80]. Ronco [81] explained that back filtration (retrofiltration) is particularly pronounced with long dialyzer and the high flows of blood and dialysate. The consequence of bacterial product delivery from the dialy-sate to the blood stream is an acute phase reaction with consequent chronic inflammation, protein-energy malnutrition, and accelerated arteriosclerosis constituting the well-described malnutrition-inflammation-arteriosclerosis syndrome [82, 83].

High Blood Flow Rates and Blood Access Problems

According to the DOPPS, in the US the mean dialysis duration of 213 min is the lowest of the seven nations participating in the study and the prescribed blood flow rate of 401 ml/min is the highest [ 84]. The requirement of high blood flow increases demand on blood access. There are major differences in blood access use between Europe and the US in both genders, in all age groups, and in patients with and without diabetes. In addition, survival of arteriovenous fistulas is better in Europe than in the US [85]. It is my strong suspicion that the differences are related, at least in part, to the differences in required blood flow. For instance, primary arteriovenous wrist fistulae providing blood flows of 300 ml/min may be considered adequate in Europe where the mean prescribed flow is 300 ml/min, but are considered inadequate in the US where the prescribed blood flow is over 400 ml/min [86]. Such fistulae are abandoned and other blood accesses are created instead in the US. Even fistulae providing blood flows of 350 ml/min are in jeopardy because of repeated attempts to achieve higher blood flows using tourniquets and other maneuvers. With these attempts, the intima of the fistula is damaged by suction of the inflow needle, and the survival of the fistula is shortened. Finally, hypo-tensive episodes suddenly reduce fistula blood flow and predispose to clotting. If intravenous catheters are used as blood accesses, large catheter lumens are required to achieve high blood flows. The large diameter catheter fits the vein too tightly and predisposes to damage of the vein wall, vein thrombosis and stenosis [ 87]. Thus, the requirement of very high blood flow may be a contributing factor to poor blood access results in the US.

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