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

2. Continuous Renal Replacement Therapy 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. Basis for CRRT Nomenclature
C VV H
Rate/Interval for Therapy
Blood Access
Method for Solute Removal

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. CRRT Schematic R
SCUF
CVVH UF D
CVVHD
CVVHDF

10. Rate Limitations of Volume RemovalBP
Extra-Vascular Compartment
Vascular Compartment

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

12. CRRT for Metabolic Control

13. Hollow Fiber Hemofilter

14. Hemofiltration Membranes
Capillary
Cross Section
Blood Side

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
Urea 60 D
Glucose 180 D
Vancomycin
~1,500 D
IL-6
~25,000 D
Albumin
~66,000 D

17. Effect of Pore Size on Membrane Selectivity
Creatinine 113 D
Urea 60 D
Glucose 180 D
Vancomycin
~1,500 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. Blood Flow and Dialyzer Have Major Impact on Intermittent HD Clearance
Dialyzer 2: Higher K0A
Dialyzer 1: Lower K0A
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. Small molecules diffuse easily Larger molecules diffuse slowly
Diffusion
Small molecules diffuse easily
Larger molecules diffuse slowly
Dialysate required
Concentration gradient
Faster dialysate flow increases mass transfer

26. Small/large molecules move equally Limit is cut-off size of membrane
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
Net Pressure
H2O
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. Klin Wochenschr 55:1121-2, 1977.

29. CRRT Machines

30. 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

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