Hemodiafiltration (HDF)

Overview

• HDF is a renal replacement therapy combining diffusion and convection.
• It more effectively removes middle molecular weight molecules compared to hemodialysis.

Mechanism

• Diffusion: Solutes pass through a high-flux dialyzer from blood to dialysate.
• Convection: Uses pressure to drag larger molecules (e.g., beta-2 microglobulin) across the dialyzer.
• Ultrafiltration: Removes 20-30% of plasma water.

Ultrafiltration Volume

• Comprised of substitution volume + net ultrafiltration volume.
• Substitution Volume: Replacement fluid infused to maintain fluid balance.
  • Must be sterile and non-pyrogenic.

High-Volume HDF

• Online HDF: Potable water treated with carbon filters, reverse osmosis, and sterilizing ultra-filters.
  • Delivers sterile, non-pyrogenic fluid.
  • Uses online substitution fluid instead of saline.

HDF Modalities

• Pre-dilution HDF:
  • Substitution fluid before the dialyzer.
  • Uses >35L of substitution fluid, may reduce solute clearances.
• Post-dilution HDF:
  • Substitution fluid after the dialyzer.
  • Uses ≥21L of fluid, increases solute clearance but may cause hemoconcentration.

Treatment Protocol

• Frequency: Prescribed thrice weekly for 4 hours.
• Considerations:
  • Dialysate and blood flow rates.
  • Type of dialyzer, needle size.
  • Filtration fraction, substitution fluid volume.
  • Anticoagulation.

Flow Rates

• Blood Flow Rate (Qb): 300-450 ml/min.
• Dialysate Flow Rate (Qd): 400-500 ml/min.
• Filtration Fraction: 20-30% of blood flow rate.

Substitution Fluid Rates

• Post-dilution HDF: 100 ml/min (24L for 4-hour session).
• Pre-dilution HDF: 200 ml/min (48L for 4-hour session).

Equipment and Technique

• Needle size: 15-16 gauge for matching Qb.
• High-flux dialyzers (1.6-2.5 m² surface area).
• Anticoagulation with heparin.

Transitioning Patients

• Gradual transition from hemodialysis to HDF.
• Customize based on individual patient needs to prevent complications.