Dialysis

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    Introduction

    • Two basic methods:
      • Hemodialysis
      • Hemofiltration
    • Indications for Renal Replacement Therapy:

        1.  Uremic Encephalopathy

        2.  Volume Overload

        3.  Life Threatening Hyperkalemia

        4.  Metabolic Acidosis

        5.  Toxins

    Principles

    • Hemodialysis removes solutes by diffusion across a semi-permeable membrane.
    • Water (fluid) removal is passive (follows solutes as they are removed).
    • Removal of solutes is driven by concentration gradient across membrane. 
    • Concentration gradient decreases as solute concentrations equilibrate, hence both blood and dialysate are moved in opposite directions known as countercurrent exchange.
      • Blood moves 200-300 mL/min (2-3x faster than normal GFR)
      • Dialysate moves even faster 500-800 mL/min

     

    Vascular Access

    • Temporary access can be established by using a large-bore double-lumen catheter in the Internal Jugular or Femoral Veins (subclavian is not advised due to high risk of stenosis)

    Methods

    Hemofiltration

    • Uses hydrostatic pressure gradient to drie fluid through a semi-permeable membrane.
    • Small solutes come with the fluid (called solvent drag)
    • Can remove large volumes of fluid (3L/h), but solute clearance is poor... therefore must be performed continuously.
    • As ultrafiltrate is removed with solutes, another fluid must be given back to the patient to avoid hypovolemia.
    • Vascular Access
      • In the past: brachial artery and a large vein (IJ or femoral), arterial-to-venous pressure used to drive hemofiltration.
      • Now: venovenous hemofiltration, a pump creates filtration pressure.
    • Advantages:
      • More gradual fluid removal (less hemodynamic compromise)
      • Removes larger molecules (including large cytokines in pts with sepsis etc..)
    • Disadvantages:
      • Must be provided continuously to provide effective dialysis. (time consuming).
        • hemodiafiltration (dialysis and hemofiltration -> better for renal failure)
      • Requires anticoagulation to keep circut patent.

     

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