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BASICS IN CRRT
KERRIANNE HUYNH
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WHAT IT DOES
Replaces the function of the kidneys
Acid base homeostasis
Osmolality homeostasis
Electrolyte balance
Water balance
Excretion of wastes
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INDICATIONS: AEIOU
Acidosis
Electrolyte abnormalities (hyperkalaemia)
Ingestion (drugs that can be dialysed)
Overload (fluid)
Uraemic encephalopathy
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THE MODES
Continuous venovenous haemofiltration CVVH
Continuous venovenous haemodialysis CVVHD
Continuous venovenous haemodiafiltration CVVHDF
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PRINCIPLES OF CRRT
Diffusion
Convection
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DIFFUSION
The movement of molecules down their
concentration gradient
Across a semipermeable membrane
From high to low concentration
Important for small molecules
Fick’s law
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CONVECTION
Movement of molecules from one
region of fluid to another by virtue
of current of fluid movement
Water is pushed across the
membrane by a hydrostatic
pressure gradient
Most important in the removal of
middle sized to large molecules
Water movement + solvent drag
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CONTINUOUS VENOVENOUS METHODS
DIFFUSION = DIALYSIS = SOLUTE REMOVAL (+ small molecules) = CVVHD
CONVECTION = FILTRATION = FLUID REMOVAL (+ middle and large molecules) = CVVH
DIFFUSION + CONVECTION = DIAFILTRATION = BOTH = CVVHDF
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CVVHD – BASED ON DIFFUSION
A dual pump system that has a dialysate
and blood flowing countercurrrent to
each other
There is a constant concentration
gradient; maximal solute exchange
occurs
Creates less ultrafiltrate than CVVH
No replacement fluid, no fluid removal
Favours small molecules
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CVVH – PURELY CONVECTIVE
Pump system generates UF rates ~4L/hr
No dialysate
Large volumes of ultrafiltrate replaced
with replacement fluid
Solute clearance and volume removal
adjusted by altering UF rate and rate of
RF
Replacement fluid = can be post or pre
filter
Favours middle to large molecules
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CVVHDF – CONVECTION + DIFFUSION
Dialysate is infused at 1-2L/hr to boost
convective clearance generated by high
filtration rates
Replacement fluid is required to offset the
high rate of UF
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IHD VS CRRT
Intermittent Haemodialysis Continuous RRT
Advantages Greater volume removal in a
short period
Slow volume removal
Good cardiovascular stability
Good solute control
Disadvantages High rates of ultrafiltration are
NOT well tolerated
Periodic solute and fluid
control problematic
Costly, complex
Frequent interruptions
Continuous anticoagulation
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IN CONCLUSION
Indications for CRRT
Different modes
Different circuits