Pediatric CRRT: Terminology and Physiology
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Transcript of Pediatric CRRT: Terminology and Physiology
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Pediatric CRRT: Terminology and Physiology
Jordan M. Symons, MD
University of Washington School of Medicine
Seattle Children’s Hospital
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CRRT: What is it?
Continuous
Renal
Replacement
Therapy
•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
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Current Nomenclature for CRRT
SCUF: Slow Continuous Ultrafiltration
CVVH: Continuous Veno-Venous Hemofiltration
CVVHD: Continuous Veno-Venous Hemodialysis
CVVHDF: Continuous Veno-Venous Hemodiafiltration
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C VV H
Basis for CRRT Nomenclature
Rate/Interval for Therapy
Blood Access
Method for Solute Removal
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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
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Diffusion
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Convection
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Clearance: Convection vs. Diffusion
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• SCUF
• CVVH
• CVVHD
• CVVHDF
UF
D
R
CRRT Schematic
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Rate Limitations of Volume Removal
Vascular Compartment
Extra-Vascular Compartment
BP
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Improved Volume Removal with Slower Ultrafiltration Rates
Vascular Compartment
Extra-Vascular Compartment
BP Stable
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CRRT for Metabolic Control
0
20
40
60
80
100
120
Time
BU
N (
mg
/dL
)
IHD CRRT
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Hollow Fiber Hemofilter
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Hemofiltration Membranes
Capillary Cross Section Blood Side
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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
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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
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Effect of Pore Size on Membrane Selectivity
Creatinine 113 D
Urea 60 D
Glucose 180 D
Vancomycin~1,500 D
Albumin~66,000 D
IL-6~25,000 D
These effects are maximized in convection
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Other Membrane Characteristics: e.g., Charge Negative charge
on membrane:• Negatively charged
particles may be repelled, limiting filtration
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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
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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
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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
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Solution/Effluent Flow Rate is Limiting Factor in CRRT
QB 150ml/min
QD 600ml/hr
QR 600ml/hr
Effluent 1200ml/hr +
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Solution/Effluent Flow Rate is Limiting Factor in CRRT
QB 150ml/min
QD 1000ml/hr
QR 1000ml/hr
Effluent 2000ml/hr +
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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
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Diffusion• Small molecules
diffuse easily• Larger molecules
diffuse slowly• Dialysate required
– Concentration gradient– Faster dialysate flow
increases mass transfer
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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
H2O
Net Pressure
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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.
First CAVH Circuit
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CRRT Machines
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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
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One of the first infants to receive CRRTVicenza, 1984