Skip to Main Content

THE PHYSIOLOGY OF RENAL REPLACEMENT THERAPY

Renal replacement therapy (RRT), or “dialysis,” uses the basic concept of molecular movement across a semipermeable membrane to provide particle and water removal from the blood.1

This requires two types of movements:

  • Diffusion: Solute exchange across a membrane between two solutions based on concentration gradient (from high to low concentration), permeability of the membrane, and surface area of the membrane

    • Used in HEMODIALYSIS or whenever a dialysate is used

    • Favors small particle movement

    • Faster movement with a large concentration gradient

  • Convection: Solute movement or “drag” with filtration across a membrane driven either by hydrostatic or osmotic pressures independent of concentration gradient

    • ULTRAFILTRATION: Water removal across a membrane using a pressure gradient

    • Particles AND water move together; if removing large amounts, will have to provide replacement fluid with electrolytes to compensate for filtration losses

    • Removes small and medium-sized particles; amount depends on amount of filtered water and the sieving coefficient of the membrane

    • Large particles will not be removed if they are larger than the pores of the membrane

RENAL REPLACEMENT THERAPY MODALITIES

PERITONEAL DIALYSIS (PD)

  • Description: Uses the peritoneum as the membrane for both convection- and diffusion-based solute clearance. Need a healthy, intact peritoneum (no diaphragmatic hernias, adhesions, or active peritonitis).

  • Advantages: Can be run emergently and continuously without vascular access; PD catheters can be placed at the bedside percutaneously by the intensivist or interventional radiologist if pediatric surgeons are unavailable.

  • Indications: Efficacious in fluid overload and is less invasive and has little hemodynamic impact, making it safe in neonates and infants.

    • Does not work in hyperammonemia or for drug clearance in toxic exposures

  • Complications: Hernias, peritonitis, hyperglycemia, respiratory compromise if giving dwells of more than 60 mL/kg, dialysate leakage, pleural effusions.

INTERMITTENT HEMODIALYSIS (IHD)

  • Description: Removes venous blood from the patient into an extracorporeal circuit past a membrane to provide mainly diffusion in a rapid manner via a 3- or 4-hour session

  • Advantages: Effective for small molecule clearance

  • Indications: Good for hyperkalemia, toxic exposures, tumor lysis syndrome

    • Contraindicated in hemodynamic instability, severe uremia (see “Disequilibrium Syndrome” later)

  • Considerations: Solute clearance depends on molecular weight, dialysate flow, membrane properties, and blood flow

  • Complications: Discussed in detail later

CONTINUOUS RENAL REPLACEMENT THERAPY (CRRT)

  • Description: Removes venous blood from the patient into an extracorporeal circuit past a membrane to provide diffusion and/or convention, intended to run 24 hours a day

  • Advantages: Provides slow, gentle adjustable removal of fluid and waste over time, more precise in reaching solute clearance and ultrafiltration goals than PD

  • Indications: Hemodynamically unstable patients, effective in all indications for RRT

  • Complications: Discussed in detail later

  • Types of CRRT

    • SCUF (slow continuous ultra-filtration): Free water and some small molecule clearance, no replacement fluid or dialysate is typically used

    • CVVH (continuous veno-venous hemofiltration): Convective-based solute clearance that ...

Pop-up div Successfully Displayed

This div only appears when the trigger link is hovered over. Otherwise it is hidden from view.