Renal replacement therapy Flashcards
What are the components of a renal replacement circuit?
Extracorporeal circuit, including a semi-permeable membrane
Blood pumps to generate pressure gradient
Pressure sensors and air detectors and traps
Vascular access device
Anticoagulation to minimize coagulation and subsequent thrombus and circuit failure
Veno-venous
What is the basic principle behind RRT?
Relies on passage of blood through a semi-permeable membrane to achieve solute and fluid removal, which occurs by 2 standard methods - haemofiltration of haemodialysis (or a combination of both)
Describe haemofiltration
Operates on the basis of convection
A hydrostatic pressure gradient drives water across a semi-permeable membrane. Solvent drag carries low molecular weight solutes (<5000 Daltons) through the membrane in the same directionas the water, the resultant fluid is described as an ultrafiltrate
The ultrafiltrate is discarded, a buffered electrolyte fluid is administered post-filter to replace it
The volume of replace fluid can be adjusted - the volume of the replacement relative to the ultrafiltrate determines the net balance
Describe haemodialysis
Operates on the basis of diffusion (movement of sultes down a concentration gradient)
Blood and dialysate fluid are separated by a semi-permeable membrane and flow past each other in opposite directions (counter-current flow)
Dialysate is a fluid containing electrolytes and a buffer (normally bicarb or lactate)
Solutes move from blood to dialysate, buffer moves from dialysate to blood
Fluid removal can be achieved by ultrafiltration - by increasing the pressure across the membrane, with fluid moving from the blood into the dialysate compartment- no significant contribution to solute removal occurs
Describe haemodiafiltration
Combines the principles of filtration and dialysis
Differs from haemodialysis in that the filtration component is increased beyond the point required for fluid removal and therefore contibutes to solute clearance
What size particles do the different modes of RRT remove?
Dialysis tends to remove smaller solutes (<500 Daltons e.g. urea, creatinine, electrolytes and lithium)
Filtration is more effective at middle sized particles (500-5000 Daltons e.g. large drugs such as vancomycin) and large solutes (>5000 Daltons e.g. cytokines and complement)
Describe RRT membranes
Normally cellulose based or semi-synthetic
Cellulose base:
-have a low permeability to water (suitable for dialysis)
-activate infkammatory response
-thought to be less suitable for use in critically ill patients
Semi-synthetic
-high permeability to water and allow transfer of solutes < 20kDa (suitable for haemofiltration)
-Less likely to activate inflammatory response
-Can be used for haemodilaysis or haemodiafiltration
Discuss RRT access
Veno-venous RRT used in ICU require a dual lumen, wide-bore catheter at least 11F guage
IJV is the site of choice
SCV should be used with caution - can cause SCV stenosis which is an issue if pts need long-term dialysis as it interferes with fistula formation
Femoral route is ok but more prone to infection
What are the indications for RRT?
Conventional rescue interventions: Urea >36 Uraemic complications Hyperkaleamia Hypermagnaesaemia Significant acidosis Oligo-anuria Diuretic-resistant fluid overload Earlier initiation of RRT and initiation for more relative indications is now becoming more widespread Volume removal Immunomodulation in sepsis Drugs - OD Refractory ARDS Hypercatabolism Rapidly worsening AKI To allow adequate nutritional support
What are the different modalities of RRT
Continuous (CRRT) Intermittent haemodialysis (IHD) Slow low efficiency dialysis (SLED) Peritneal dialysis - limited use in ICU No high quality evidence suggesting superiority of any of the modalities
What are the different types of CRRT?
What blood flow rate is required
Most widely used modality in ICU
Includes CVVH, CVHD, CVVHDF
Typically blood flows of 100-200ml/min are required
CVVHDF has the theoretical advantage of increased solute clearance - however there is no good evidence that it improves survival
The requirement for dialysate provision in CVVHDF may relatively increase costs
What should the effluent rate of CRRT be?
20-25ml/kg.hr should be the target prescription - based on the RENAL and ATN trials
Rates above this may have no mortality benefit and increase harm and cost
High volume RRT has been suggested in severe sepsis but there is currently no evidence for this
What are the advantages of CRRT?
Less cardiovascular instability
Compatible with continuous admin of nutrition and drug infusions
Potentially a/w better cerebral perfusion in patients with TBI or hepatic failure
Drug dosing may be more predictable
Continuous nature improves solute clearance and limits peak solute concentrations
What are the disadvantages of CRRT?
Labour intensive Expensive in terms of comsubmables May impair mobility/rehab of patients in the recovery phase Hypothermia/masking of pyrexia Requirement for anticoagulation
What blood flow rates are used in IHD?
400ml/min
