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Renal replacement therapy in critical care (Initiating and maintaining RRT…
Renal replacement therapy in critical care
Principles of haemodialysis and haemofiltration
Diffusion
rate of diffusion proportional to concentration gradient
diffusion of solute across membrane is called dialysis
random motion
determined by
dialysis membrane (type, porosity, thickness, surface area)
rate of delivery of solute
solute (size- < 20kDa, charge, protein binding)
concentration of dialysate
principle process (Haemodialysis)
Convection
porosity of membrane-> determine which solutes are to be removed
UF-> remove fluid-> correct overload
independent of solute concentration across the membrane
Relationship between transmembrane pressure and oncotic pressure determines filtration fraction
ultrafiltrate is produced by creating a positive pressure in the blood compartment of the dialyser and is facilitated by creating a negative pressure in the dialysate compartment; -> result in transmembrane pressure, driving force for UF (analogous to glomerulus of kidney)
direction and force of transmembrane fluid flux determine amount of convective transport
The optimal filtration fraction for HCT ~30% is 20-25%
aka solvent drag; occurs when a solute molecule is swept through a membrane by a moving stream of ultrafiltrate. -> Haemofiltrate
RRT membrane
2 types
cellulose-based
used for dialysis
activates inflammatory cascade and less suitable for critical care
low permeability coefficient to water (low flux)
synthetic
allow transfer of solutes with a mass < 20kDa eg urea, creatinine, ions, IL-6, endotoxin, heparin, pesticides, ammonia and most drugs unbound to plasma proteins
causes less damage to platelets and white cells
high permeability coefficient to water (high flux), high sieving coefficients to solutes in a wide range of molecular weights; much more appropriate to convective treatment (haemofiltration)
Vascular access
arterio-venous
disadvantages
greater vascular morbidity
depends on adequate arterial blood pressure to drive blood flow
venous-venous
right or left IJC is usually preferred; subclavian vein- stenosis upon chronic use and incompressible in event of haemorrhage
coagulopathic patient-> femoral catheter-> easily compressed
a single large vein is cannulated percutaneously with a double lumen cathether via seldinger technique
modern double lumen catheter
10-14 Fr
arterial outflow limb circuit, 'arterial port'; venous inflow limb circuit, 'venous port'
Anticoagulation
Heparin-most commonly used
target aPTT 1.5-2 x the control
to prolong filter life, while minimizing systemic anticoagulation
for patients platelet < 80, use prostacyclin (Flolan) at 2.5-5 ng/kg/min
RRT circuits
Haemodialysis
Extracorporeal blood circuit unit, a filtrate circuit
diffusive solute clearance, due to counter current dialysate flow through haemofilter
No replacement fluid is required
CVVH
Extracorporeal blood circuit unit, a filtrate circuit, + a replacement fluid circuit
transmembrane pressure
gradient across the haemofilter results in convective clearance
of solute with filtered fluid.
The resulting filtration rate varies
between 0 and 30% of the blood flow rate
Replacement fluids
Lactate is an acceptable bicarbonate sustitute
provided that it is efficiently metabolised to CO2 and water, generating new bicarbonate ions via TCA cycle in liver
in critically ill: may not happen because of hepatic dysfunction or high lactate load due to lactic acidosis
bicarbonate buffered replacement fluid or lactate free replacement fluid
normal healthy liver process lactate 100 mmol/hour
bicarbonate ions are not present in most replacement fluids for 2 main reasons
short shelf life;
bicarbonates are converted to carbonate which dissociates, with resultant loss of CO2, by diffusion through the container wall
bicarbonate+calcium -> precipitation of calcium carbonate
Bicarbonates are freely filtered and metabolic acidosis occurs unless these ions are replaced or regenerated
Initiating and maintaining RRT
early RRT in polytrauma patients with ARF has 50% reduction in mortality;
higher intensity of RRT in critical care patients has lower mortality rates
aim: urea <25-30 mmol/L
conservative measures (diuretics, bicarbonates administration or ROF) or early CRRT
UF rate 35 cc/kg/hr
no optimum starting point (except emergency indications)
Potential use in sepsis
HVHF (High Volume haemofiltration), produces 75 L/day -> maybe required to produce significant reduction in plasma mediators (high generation rate)
Mediators involved in SIRS: water-soluble middle size molecular weight compunds, eg tumour necrosis factor (TNF), interleukins (ILN), platelet activating factors (PAF) and complemet
IHD vs CRRT
see microsoft word doc