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Pediatric CRRT: Terminology and Physiology

Pediatric CRRT: Terminology and Physiology. Jordan M. Symons, MD University of Washington School of Medicine Seattle Children’s Hospital. CRRT: What is it?. Strict definition : any form of kidney dialysis therapy that operates continuously, rather than intermittently

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Pediatric CRRT: Terminology and Physiology

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  1. Pediatric CRRT: Terminology and Physiology Jordan M. Symons, MD University of Washington School of Medicine Seattle Children’s Hospital

  2. CRRT: What is it? • Strict definition: any form of kidney dialysis therapy that operates continuously, rather than intermittently • More common definition: continuous hemofiltration technique, often used for hemodynamically unstable patients Continuous Renal Replacement Therapy

  3. Current Nomenclature for CRRT SCUF: Slow Continuous Ultrafiltration CVVH: Continuous Veno-Venous Hemofiltration CVVHD: Continuous Veno-Venous Hemodialysis CVVHDF: Continuous Veno-Venous Hemodiafiltration

  4. Rate/Interval for Therapy Blood Access Method for Solute Removal Basis for CRRT Nomenclature C VV H

  5. Solute Removal Mechanisms in RRT • Diffusion • transmembrane solute movement in response to a concentration gradient • importance inversely proportional to solute size • Convection • transmembrane solute movement in association with ultrafiltered plasma water (“solvent drag”) • mass transfer determined by UF rate (pressure gradient) and membrane sieving properties • importance directly proportional to solute size

  6. Diffusion

  7. Convection

  8. Clearance: Convection vs. Diffusion

  9. R D UF CRRT Schematic • SCUF • CVVH • CVVHD • CVVHDF

  10. BP Rate Limitations of Volume Removal Extra-Vascular Compartment Vascular Compartment

  11. Extra-Vascular Compartment Vascular Compartment Improved Volume Removal with Slower Ultrafiltration Rates BP Stable

  12. CRRT for Metabolic Control

  13. Hollow Fiber Hemofilter

  14. Cross Section Blood Side Capillary Hemofiltration Membranes

  15. Hemofilter Characteristics • Pore size • “High Flux” vs. “High cut-off” • Surface area; porosity • Effects on maximum ultrafiltration capacity • Membrane material • polysulfone, PAN, etc.; modifications • Adsorption • Prime volume

  16. Effect of Pore Size on Membrane Selectivity Creatinine 113 D Vancomycin ~1,500 D Urea 60 D Glucose 180 D IL-6 ~25,000 D Albumin ~66,000 D

  17. Effect of Pore Size on Membrane Selectivity Creatinine 113 D Vancomycin ~1,500 D Urea 60 D Glucose 180 D IL-6 ~25,000 D These effects are maximized in convection Albumin ~66,000 D

  18. Other Membrane Characteristics: e.g., Charge - - - - - - - - - - • Negative charge on membrane: • Negatively charged particles may be repelled, limiting filtration - - - - - - - -

  19. Other Membrane Characteristics: e.g., Charge - - - - - - - - - - • Negative charge on membrane: • Negatively charged particles may be repelled, limiting filtration • Positively charged particles may have increased sieving + + + + + + + +

  20. Other Membrane Characteristics: e.g., Charge - - - - - - - - - - • Negative charge on membrane: • Negatively charged particles may be repelled, limiting filtration • Positively charged particles may have increased sieving • Charge may change adsorption

  21. Dialyzer 2: Higher K0A Dialyzer 1: Lower K0A Blood Flow and Dialyzer Have Major Impact on Intermittent HD Clearance Dialysate flow rate (QD) always exceeds QB

  22. Solution/Effluent Flow Rate is Limiting Factor in CRRT QR 600ml/hr QB 150ml/min QD 600ml/hr Effluent 1200ml/hr +

  23. Solution/Effluent Flow Rate is Limiting Factor in CRRT QR 1000ml/hr QB 150ml/min QD 1000ml/hr Effluent 2000ml/hr +

  24. Patient’s Chemical Balance on CRRT Approximates Delivered Fluids • Diffusion: blood equilibrates to dialysate • Convection: loss is isotonic; volume is “replaced” • Consider large volumes for other fluids (IVF, feeds, meds, etc.) • Watch for deficits of solutes not in fluids

  25. Diffusion • Small molecules diffuse easily • Larger molecules diffuse slowly • Dialysate required • Concentration gradient • Faster dialysate flow increases mass transfer

  26. Net Pressure H2O Convection • Small/large molecules move equally • Limit is cut-off size of membrane • Higher UF rate yields higher convection but risk of hypotension • May need to Replace excess UF volume H2O H2O H2O

  27. First CAVH Circuit Kramer, P, et al. Arteriovenous haemofiltration: A new and simple method for treatment of over-hydrated patients resistant to diuretics. KlinWochenschr 55:1121-2, 1977.

  28. CRRT Machines

  29. Pediatric CRRT Terminology and Physiology: Summary • CRRT comes in several flavors • SCUF, CVVH, CVVHD, CVVHDF • Solute transport: diffusion/convection • UF approximates 1-compartment model • Membrane characteristics affect therapy • Fluid composition, rates drive clearance • Advancing technology provides more options

  30. One of the first infants to receive CRRT Vicenza, 1984

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