Pediatric CRRT: Terminology and Physiology

Jordan M. Symons, MD

University of Washington School of Medicine

Seattle Children’s Hospital

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

Current Nomenclature for CRRT

SCUF: Slow Continuous Ultrafiltration

CVVH: Continuous Veno-Venous Hemofiltration

CVVHD: Continuous Veno-Venous Hemodialysis

CVVHDF: Continuous Veno-Venous Hemodiafiltration

C VV H

Basis for CRRT Nomenclature

Rate/Interval for Therapy

Blood Access

Method for Solute Removal

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

Diffusion

Convection

Clearance: Convection vs. Diffusion

• SCUF

• CVVH

• CVVHD

• CVVHDF

UF

D

R

CRRT Schematic

Rate Limitations of Volume Removal

Vascular Compartment

Extra-Vascular Compartment

BP

Improved Volume Removal with Slower Ultrafiltration Rates

Vascular Compartment

Extra-Vascular Compartment

BP Stable

CRRT for Metabolic Control

0

20

40

60

80

100

120

Time

BU

N (

mg

/dL

)

IHD CRRT

Hollow Fiber Hemofilter

Hemofiltration Membranes

Capillary Cross Section Blood Side

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

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

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

--

--

--

--

--

-

-

-

-

-

-

-

-

Other Membrane Characteristics: e.g., Charge Negative charge

on membrane:• Negatively charged

particles may be repelled, limiting filtration

--

--

--

--

--

+

+

+

+

+

+

+

+

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

--

--

--

--

--

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

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

Solution/Effluent Flow Rate is Limiting Factor in CRRT

QB 150ml/min

QD 600ml/hr

QR 600ml/hr

Effluent 1200ml/hr +

Solution/Effluent Flow Rate is Limiting Factor in CRRT

QB 150ml/min

QD 1000ml/hr

QR 1000ml/hr

Effluent 2000ml/hr +

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

Diffusion• Small molecules

diffuse easily• Larger molecules

diffuse slowly• Dialysate required

– Concentration gradient– Faster dialysate flow

increases mass transfer

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

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

CRRT Machines

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

One of the first infants to receive CRRTVicenza, 1984