Transcript Symons-Term-Physio pAKI - Pediatric Continuous Renal

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:
CVVH:
Slow Continuous Ultrafiltration
Continuous Veno-Venous
Hemofiltration
CVVHD: Continuous Veno-Venous
Hemodialysis
CVVHDF: Continuous Veno-Venous
Hemodiafiltration
Basis for CRRT Nomenclature
C VV H
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
CRRT Schematic
R
• SCUF
• CVVH
D
UF
• CVVHD
• CVVHDF
Rate Limitations of Volume
Removal
BP
Extra-Vascular
Compartment
Vascular
Compartment
Improved Volume Removal with
Slower Ultrafiltration Rates
BP
Stable
Extra-Vascular
Compartment
Vascular
Compartment
CRRT for Metabolic Control
120
BUN (mg/dL)
100
80
60
40
20
0
Time
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
IL-6
~25,000 D
Albumin
~66,000 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
QR 600ml/hr
QB 150ml/min
QD 600ml/hr
Effluent 1200ml/hr +
Solution/Effluent Flow Rate is
Limiting Factor in CRRT
QR 1000ml/hr
QB 150ml/min
QD 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
H2 O
Net Pressure
•
•
H2 O
H2 O
•
H2 O
•
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
First CAVH Circuit
Kramer, P, et al.
Arteriovenous
haemofiltration: A new
and simple method for
treatment of overhydrated patients
resistant to diuretics.
Klin Wochenschr
55:1121-2, 1977.
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 CRRT
Vicenza, 1984