Fresenius - Hemodiafiltration

Definitions & Key takeaways

Hemodiafiltration is a medical procedure that is used to clean blood. It is done by removing waste products and toxins from the blood with a special machine. The cleaned blood is then sent back into the body. This procedure can be done as an outpatient procedure or while you are hospitalized. It is usually done on people with kidney failure or other health problems that cause high levels of toxins in their blood.

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Hemodiafiltration, or HDF, is a renal replacement modality that combines diffusion and convection to improve removal of molecules in the middle molecular weight range versus hemodialysis.
Like hemodialysis, blood and dialysate flow in opposite directions in the dialyzer, cleaning the blood of toxins and excess fluid.While diffusion allows solutes to passively filter through a high-flux dialyzer down a concentration gradient from blood to dialysate, convection uses pressure to actively “drag” larger, potentially uremic molecules, such as Beta-2-microglobulin, across the dialyzer into the ultrafiltrate.
During an HDF treatment, approximately 20-30% of the plasma water is removed by ultrafiltration.The convection volume, or total ultrafiltration volume, consists of the substitution volume plus the net ultrafiltration volume for fluid removal.
The substitution volume is the amount of replacement fluid infused into the patient’s blood line to maintain fluid balance.
This fluid must be sterile and nonpyrogenic. In high volume HDF, a large volume of plasma water is ultra filtered and must be replaced.In online HDF, potable water undergoes extensive treatment with carbon filters, reverse osmosis, and sterilizing ultrafilters, with one ultrafilter being installed into the circuit before the site of infusion.
This allows delivery of sterile, non-pyrogenic fluid to the patient and use of online substitution fluid, instead of saline, for priming, infusion, or as a bolus administration.Commonly used HDF treatment modalities include pre-dilution and post-dilution HDF.
During pre-dilution, substitution fluid is administered before the dialyzer, meanwhile, in post-dilution, the fluid is administered after the dialyzer.
Pre-dilution HDF can use greater than 35 L of substitution fluid, leading to hemodilution. Such large volumes could also reduce solute clearances, hematocrit, viscosity and oncotic pressure, and increase membrane fouling.
Post-dilution HDF uses 21 L or more or substitution fluid. It can increase solute clearance with lower substitution volumes but can cause hemoconcentration.
Like in-center hemodialysis, OL-HDF is usually prescribed thrice weekly for four hours. When starting the patients on hemodiafiltration dialysate and blood flow rates, type of dialyzer, needle size, filtration fraction, volume of substitution fluid, and anticoagulation should be considered.
The patient should also have a functioning vascular access such as an arteriovenous fistula or graft. Typical blood flow rate, Qb, and dialysate flow rate, Qd, range from 300-450 mL/min and 400-500 mL/min, respectively.
Filtration fraction is the ratio or percentage of convective flow to blood flow and is usually between 20-30% of the achieved blood flow rate.
A typical high-volume, post-dilution HDF treatment uses a substitution fluid infusion rate of 100 mL/min or 24 liters for a 4-hour session.
Pre-dilution HDF often requires double the substitution infusion flow rate to 200 mL/min or 48 liters in a 4-hour session.
Needles size should match Qb, and 15-16-gauge have been used successfully in HDF treatments. High flux dialyzers with surface areas of 1.6-2.5 m2 can provide efficient removal of middle molecules while minimizing albumin loss.
Anticoagulation usually involves heparin. Importantly, the transition from hemodialysis to HDF must be done gradually and modified according to the individual patients’ characteristics to avoid any potential complications.
transition from hemodialysis to hdf must be done gradually and modified according to the individual patients characteristics to