Chapter 52: Renal Replacement Therapies

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 ...