igh-flux dialysis is a form of dialysis therapy where both diffusive and convective solute removal take place. A dialyzer described as high flux has a permeable membrane that allows small and large molecular weight solute to be diffused across the membrane.

Requirement of high-flux dialysis:

1.Dialyzer:

 High flux membrane (Synthetic membranes) include: Polysulfone , Polyacrylonitrile (PAN), Polymethy lmethacrylate (PMMA), Polycarbonate,  polymide, Ethylene-vinylalcohol copolymer and Cellulose triacetate]

                                                           High Flux Dialyzer

Nature of high flux membrane:

a. They are a thin , smooth luminal surface, supported by a sponge-like wall structure.

b. They have solute removal clearance of substance in  the molecular weight range of 5000(insulin) to 12000(B2-microglobulins).

c. The surface area ranges from 0.6-2 msq 

d All have ultrafiltration coefficients (KUF) of 20 to 80 ml/hr/mmHg or more.

Preparation of the dialyzer: according to the instruction of manufacturer  

2.Dialysis machine with :

 a-Ultrafiltration control device is required is essential  to control fluid loss

 b-Bicarbonate dialysate delivery system for better cardiovascular stability.

 c-Dialysate flow rate: 1000 ml/minute .

3.Blood flow rate : High blood flow rates (300-500 ml/minute) are needed (a) to maximize the permeability and achieve higher clearance.(b)to increase the pressure at dialyzer outlet. Blood flow rate influenced by:

a. Blood pump design in terms of rotor speed and torque. It should be capable of higher speed with adequate occlusion.

b. Tubing segment design in terms of diameter and thickness which determine the ability to deliver a constant stroke volume. It should be designed to prevent pump segment from softening and losing elastic recoil.. It was found that the actual blood flow reduced by 10% due to softening of the tubing segment as treatment progresses.

c. Fistula needle design with 15-14 gauge and ultrathin wall is required. The resistance of the fistula needle is inversely related to the fourth power of the inside diameter. The difference between a 16 and a 14 gauge needle is significant.

d. Vascular access

4. Water: Good quality of water is required

5. Disinfection: The equipment and dialysate bicarbonate containers must be disinfected on regular basis to avoid bacterial growth.

Advantages of high flux dialysis:

1.It allows reduction of time on dialysis ,up to 6-9 hours per week.

2.Prescribed for patient with amyloidosis in dialysis patients to remove B2 microglobulin. B2 microglobulin is removed by adsorption to the membrane 100 mg /treatment.

3.Membrane used in high flux dialysis have better biocompatibility with blood than cellulose membrane.

NB. Amyloid deposits are polymerized forms of the protein B2 microglobulin.

Backfiltration (Reverse filtration)

·      The ultrafiltration controller generates a dialysate pressure profile that creates reverse filtration from the dialysate to blood in distal portion of the dialyzer. It depends on the pressure distribution along the length of the dialyzer in both the blood and dialysate compartment. At the blood inlet there is a region of filtration, as the blood pressure exceeds the dialysate pressure. Backfiltration occurs when the dialysate pressure is greater than that of the blood. 

                  High UF rate              Critical UF rate          Low UF rate-BF

                 No Backfiltration         No backfiltration        Backfiltration

·      Contamination of blood by pyrogenic material and endotoxin fragments is created. It depends on the phenomenon of backfiltration occurring with high flux membranes [Highly permeable membrane]. As much as 180-480 ng of endotoxin could transferred to the patient during a single high flux dialysis compared to the minimum pyrogenic dose for man about1-2 ng/kg body weight.  Addition of a molecular filter or ultrafilter to dialysate path immediately ahead of the dialyzer is necessary to reject the intact or fragmented endotoxins.

Negative features related to high flux membrane:

1.Very expensive.

2.Automated ultrafiltration control is required because of very high water permeability 3.Significant loss of protein by adsorption.

4.Backfiltration from the dialysate and risk of bacterial or endotoxin contamination, due to high hydraulic permeability

Complications related to high-flux dialysis:

1.Pyrogen reactions caused by passage of endotoxin into the blood

2.Osmotic disequilibrium syndrome.

3.Transient hypokalaemia

4.Cardiovascular instability because of rapid fluid removal and the accompanying hypotension.

5.Access problem ;rebound and cardiopulmonary circulation

6.Other complications ; as in conventional dialysis

Limitation of high flux dialysis:

1.Fluid removal:

Shortening of the dialysis time by high flux dialysis may be ultimately limited by the ability to correct the intradialytic weight gain while maintaining cardiovascular stability. As a general rule, it has been noted that when patients intradialytic weight gain more than 5 kg and the dialysis time shortened to less than 3 hours, a significant increase in hypotension may occur.

2.Cardiovascular instability:

Cardiovascular instability , unrelated to excessive weight gain could be a limiting factor in application of high flux dialysis. It account 2-3% of all patients failing this type of dialysis. Patients who have poor cardiovascular reserve and high incidence of intradialytic hypotension on standard dialysis are not a good candidate for high flux dialysis.

3.Extracoprporeal blood flow:

Inability to achieve higher blood flow rate has been noted to be a limitation to deliver higher clearance. Access with recirculation rate more than 20% or in patients whose arterial line collapses when blood flow rates exceed 300 ml/min , the ability to deliver higher clearance needed for shorter treatments may be compromised.