Ultrafiltration

ultrafiltration during hemodialysis and peritoneal dialysis

1. Ultrafiltration Yousaf Khan Renal Dialysis Lecturer IPMS-KMU

2. Ultrafiltration Process of water removal from the blood stream As means of solute clearance (by convection) To achieve adequate solute clearance by filtration requires large volume of solute replacement Fluid removed is the ultra filtrate

3. Ultrafiltration

4. Method of ultrafiltration during hemodialysis Pressure control: Blood compartment is usually +50 – 100 mmHg depending on blood flow rate In dialysate compartment can be lower by reducing the dialysate inflow rate with a clamp when pump is on the outflow line. Negative pressures of up to -500 mmHg can be achieved in this way. Pressure greater than this can lead to rupture of dialyzer Pressure are usually measured in the blood and dialysate outflow line. TMP needed to remove a given volume of excess water.

5. Method of ultrafiltration during hemodialysis Volumetric control: Much more accurate method UF control Increasingly important with the use of higher flux membrane Small error in measurement of TMP may cause huge volume change. Kuf of dialyzer can change during dialyzer can change during a dialysis session as a result of protein deposition, partial blood clotting and change in haematocrit . Measure the UF rate directly by quantifying the volume of dialysate passing through the dialyzer

6. Schematic diagram of osmotic ultrafiltration No hydrostatic pressure is applied Osmotic agent introduced to right compartment this cause water shift to the right but this later reversed. If osmotic agent is also small enough to diffuse along the concentration gradient from right to left.

7. Schematic diagram of osmotic ultrafiltration

8. Schematic diagram of hydrostatic ultrafiltration The application of external pressure force movement of water from left to right. Low molecular weight constituents will be swept through the membrane with this water. In dialysis setting the pressure gradient is generated by manipulation if dialysis fluid parameters such as pressure volume or flow.

9. High efficiency dialysis High clearance rate of urea (>210 ml/min) High urea KoA (mass transfer coefficient x surface area) of the dialyzer (>600 ml/min) The KUF of the dialyzer may be high or low and membrane synthetic and cellulosic At low blood flow rates (<200 ml/min) urea clearance of both high and low efficiency dialyzers is the same High efficiency dialysis requires large surface area membrane high KoA , high blood flow and dialysate blood flow and bicarbonate dialysate Excellent access to provide high blood flow Higher dialysate flow rate >500 ml/mint

10. High flux dialysis Refer to the rate of water transfer across the dialysis membrane and high permeability membrane ( B2-microglobulin clearance >20 ml/min) Requires volumetric UF control Middle and large molecule clearance is better because of larger membrane pore size. Dialyzer can by synthetic and cellulosic

11. Hemofiltration Provide solute clearance solely by convection As solute are dragged down a pressure gradient with water Large volume of filtrate are removed and need to be replaced (> 40 liters each session)

12. Haemofiltration

13. Hemodiafiltration Combine dialysis with large volume UF i.e convective and diffusive removal of solute.

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