A Brief Overview Renal Replacement Therapy

Hemodialysis performs the functions of the human kidney externally to the body.

Indications include: Fluid overload, metabolic acidosis, acidema (pH less than 7.1, uremia, oliguria, hypocalcemia and Azotemia.

Types of Dialysis:

• PD – peritoneal Dialysis

• IHD – Intermittent HemoDialysis

• SCUF – Slow Continuous UltraFiltration

• CVVH – Continuous VenoVenous HemoFiltration

• CVVHD – Continuous VenoVenous HemoDialysis

• CVVHDF – Continuous VenoVenous HemoDiaFiltration

How dialysis works:

• It is a process by which solutes move passively down their concentration gradients, from one fluid compartment into the other (either blood or dialysate).

• Urea, Creatinine, and Potassium move from the blood to the dialysate fluid.

• Calcium, Bicarbonate, and other solutes move from the dialysate to the blood.

• The dialysate creates a “countercurrent” to the blood flow, maximizing the concentration gradient between the compartments, which in turn maximizes the rate of solute removal.

The goal is to: 

• Waste removal

• Electrolyte balance

• Fluid balance

• Acid-base balance

• Removal of septic mediators and/or drugs

Mechanism of Dialysis

Dialysis therapy uses the mechanisms of ultrafiltration, hemofiltration, and hemodialysis. The

different modes utilize a selection of one or more of these mechanisms.

• Ultrafiltration

  • Plasma water with solutes is drawn from the patient‟s blood across the semipermeable membrane in the filter. The effluent pump controls the ultrafiltration rate automatically according to the set flow rates.

• Hemofiltration

  • Plasma water with solutes is drawn from the patient‟s blood across the semi-permeable membrane by means of ultrafiltration. A replacement solution is simultaneously infused into the blood flow path.

  • The replacement solution replaces some or all of the water removed, as well as the desired solutes.

  • Unwanted solutes are not replaced, thus their concentration decreases in the patient‟s blood. Solute

  • removal is achieved by convection (solvent drag across the membrane).

• Hemodialysis

  • Unwanted solutes pass from the patient‟s blood across the semi-permeable membrane and into the dialysate which is flowing in the opposite direction through the fluid compartment of the filter.

  • The concentration of unwanted solutes is lower in the dialysate than in the blood, causing the solutes to diffuse from an area of greater concentration to an area of lower concentration. Therefore, solute clearance is achieved by diffusion.

• HemoDiaFiltration

  • Both hemodialysis and hemofiltration are used. Solute removal occurs by convection and diffusion. The fluid is removed by ultrafiltration.

  • Dialysate fluid is pumped through the fluid compartment of the filter. At the same time, the effluent pump controls the ultrafiltration rate. A replacement solution is infused into the blood flow path either before or after the filter – (pre or post-dilution). An equivalent amount of fluid is withdrawn via the effluent pump thereby maintaining a neutral balance.

  • Pre-dilution of the filter is useful because it decreases blood viscosity and can, therefore, prolong the life of the filter. It also increases solute removal via convection as more fluid is moving across the membrane

• Peritoneal Dialysis

  • A catheter is inserted in the peritoneal cavity.

  • A glucose-rich fluid is drained in over a set period of time.

  • The peritoneal lining acts as a membrane.

  • The fluid remains inside the cavity for a set period of time allowing osmosis, diffusion, and convection to occur.

  • The fluid is then drained out over a set period of time into a collection bag.

  • The fluid contains waste products and excess fluid.

  • Twenty-minute rule.

  • Water is taken out of the patient by the dextrose solution.

    • Advantages:

      •  No anticoagulation required

      •  No direct blood access needed

      •  No specialized staff required

      •  Inexpensive

      •  Maintains hemodynamic stability

      •  Gentle removal of fluid and solutes

      •  Good fluid removal

    • Disadvantages:

      • Time-consuming

      • Needs intact peritoneal cavity

      • Increased rate of infection

      • Strict fluid balance required

      • Daily weight documentation

      • Can compromise the respiratory status

      • Requires specialized glucose enriched dialysate bags

      • No control over fluid removal volume

• Intermittent HemoDialysis (IHD)

  • Normally done for a patient awaiting a kidney transplant or with severe chronic renal failure where a transplant is not possible.

  • Requires the patient to have a graft site.

  • It does not use an anticoagulant, replacement solution, or dialysate.

    • Advantages

      • Rapid fluid removal

      • No anticoagulation

      • Large diffuse volumes

      • No hospitalization required

    • Disadvantages

      • Graft site needed

      • Continual interruption of lifestyle

      • Daily weight documentation

      • Large swings in electrolyte levels between treatments

      • Strict fluid balance

• Slow Continuous Ultra Filtration (SCUF)

  • This is the process of slow fluid removal across a semipermeable membrane. Because the ultrafiltrate will have a similar composition to that of blood, the clearance of solutes is not as effective as dialysis. To perform SCUF you require a circuit that has a specific filter, it needs to have an access, a return line, and an effluent line.

  • The blood is taken out through the access line and passes through the filter and solutes and fluid are removed and the blood is returned to the patient. Anticoagulation may be required.

    • Advantages

      • Controls fluid removal volumes

      • Maintains cardiovascular stability

    • Disadvantages

      • May require anticoagulation

      • Minimal solute clearance

      • The patient has to hospitalized

• Continuous Veno-Venous Hemofiltration (CVVH)

  • Hemofiltration uses the same process as SCUF

  • To perform CVVH you require a circuit that has a specific filter, it needs to have an access, a return line, effluent line, and a pre/post replacement line.

  • The blood is taken out through the access line and passes through the filter and solutes and fluid are removed and the blood is returned to the patient. Anticoagulation is usually required.

    • Advantages

      • Controlled fluid removal volumes

      • Effective clearance of large solutes

      • Maintains cardiovascular stability

    • Disadvantages

      • Requires anticoagulation

      • Requires venous access

      • Requires specialized staff

      • Requires replacement fluid

• Continuous Veno-Venous Hemofiltration (CVVHD)

  • In CVVHD a dialysate solution is passed countercurrent to the blood to encourage the diffusion of solutes across the membrane.

  • This means urea clearance is 12-15 ml/min.

  • Ultrafiltration rate is kept low at 3-5 ml/min thus negating the need to administer replacement fluid

    • Advantages

      • Convective and diffusive clearance

      • Controlled fluid removal

      • Slow correction of electrolyte and acid/base derangements

      • Good small solute removal

    • Disadvantages

      • Requires anticoagulation

      • Requires venous access

      • Requires specialized staff

      • Requires dialysate fluid

• Continuous VenoVenous HemoDiafiltration (CVVHDF)

  • The replacement solution is set to run post filter.

    • Advantages

      • Rapid controlled fluid removal

      • Large diffuse volumes

      • Rapid correction of electrolyte and acid/base derangement

      • Immediate, effective clearance of small molecules

    • Disadvantages

      • Requires anticoagulation

      • Requires venous access

      • Requires specialized staff

      • Tends to have a negative impact on cardiovascular functioning (lowers BP)

      • Requires dialysate and replacement fluid

      • Slower clearance of large molecules

Learn more about renal replacement therapy at merkmanuals.com