Solute transport

Hemofiltration uses convective solute transport; a pressure gradient drives an ultrafiltrate, of composition similar to plasma water, through a high-flux membrane. Blood purification is achieved by partial or complete substitution of the ultrafiltrate with a replacement solution that is administered into the venous (postdilution) or arterial (predilution) limb of the circuit. The clearance of all solutes with molecular weight below the cut-off of the membrane is identical and only limited by the filtration rate. (CD Figure, 1

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CD Figure 1. Schematic representation of hemofiltration. A high-volume ultrafiltrate is driven through a highly permeable membrane by a pressure gradient; this process is called convection. Convective solute clearance is independent of molecular weight for all solutes that are smaller than the membrane cut-off point. Blood epuration is achieved by partial or complete substitution of the ultrafiltrate with a replacement solution that can be administered in the prefilter line (predilution) or (as shown in the figure) in the postfilter line (postdilution).

During hemodialysis the dialysate is directed in countercurrent through the outer side of a hemodialyzer, while solute transport is based on diffusion driven by the concentration difference between blood and dialysate. In contrast with convection, diffusion depends on molecular weight and provides superior elimination of low-molecular-weight solutes but limited elimination of medium-molecular-weight compounds. (CD Fig.uie.,,2.)

CD Figure 2. Schematic representation of hemodialysis. Blood and dialysate flow in countercurrent directions on the opposite sides of a semipermeable membrane. Solute transfer is based on diffusion driven by a concentration gradient between the two compartments. Small solutes diffuse more easily. In order to achieve the desired fluid balance, a small transmembrane hydrostatic pressure gradient produces an ultrafiltrate that mixes with the dialysate.

Diffusion and convection can be combined in hemodiafiltration. ( CD Fjgure^S)

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CD Figure 3. Hemodiafiltration combines diffusive and convective solute transport.

Vascular access

Arteriovenous methods, using the patient's blood pressure to drive the blood through the extracorporeal circuit, require access to a large artery and vein.

Percutaneous cannulation of the femoral artery and vein is the usual method ( Fig 1). Catheters must be short (8-11 cm) and large bore (> 2 mm) in order to reduce resistance. Blood flows of 50 to 120 ml/min are generally observed.

Most complications of arteriovenous hemofiltration are associated with the arterial access. Vessel perforation may lead to retroperitoneal hemorrhage. Other access-related complications include local bleeding, thromboembolic phenomena, aneurysm formation, and infection. The need for arterial access precludes application of arteriovenous techniques in patients with severe arteriosclerosis or a vascular prosthesis.

In venovenous techniques the driving pressure is provided by a blood pump (rate 100-200 ml/min) and a single venous access with a double-lumen catheter suffices

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