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Question No. 2
Q: What is renal replacement therapy?
Answer No. 2
- Renal replacement therapy (RRT) describes techniques used to purify the blood and achieve the solute and fluid homeostasis usually produced by the kidney
- Continuous renal replacement therapy (CRRT) describes treatments which are applied for prolonged periods at a time (usually >24 hours)
Question No. 3
Q: How can modes of renal replacement be classified?
Answer No. 3
Modality of Fluid and Solute Removal
- Hemofiltration
- Haemodialysis
- Hemodiafiltration
Continuous vs. Intermittent Modality
- Intermittent:
- Intermittent haemodialysis (IHD)
- Slow low-efficiency dialysis (SLED)
- Extended daily dialysis (EDD)
- Continuous:
- Continuous venovenous hemofiltration (CVVH)
- Continuous venovenous haemodialysis (CVVHD)
- Continuous venovenous hemodiafiltration (CVVHDF)
- Slow continuous ultrafiltration
- Peritoneal dialysis (PD)
Source of Pressure Gradient
- Arteriovenous
- Venovenous - requires an external pump
Question No. 4
Q: What are the mechanisms used for fluid and solute removal in renal replacement, and how are they defined?
Answer No. 4
Definition
Diffusion
The transport of solute across a semi-permeable membrane, down a concentration gradient
Ultrafiltration
The passage of fluid across a semi-permeable membrane due to a hydrostatic pressure
Convection
The transport of a solute across a semi-permeable membrane along with solvent (by "solvent drag")
Question No. 5
Q: What are the modalities of RRT?
Answer No. 5
Mechanisms
Haemodialysis
- Clearance of solutes via diffusion down a concentration gradient
- A counter-current flow of a solution containing various electrolytes on the opposite side of membrane to blood allows diffusion to occur
- No fluid is added to the filtrate after diffusion
- Rate of solute clearance determined by:
- Concentration gradient between plasma and dialysate
- Particle size, ionic charge and protein binding
- Membrane pores, thickness and surface area
Haemofiltration
- Clearance of solutes via convection driven by hydrostatic pressure
- Filtrate removal is balanced by the addition of a solution to maintain volume
- Does not significantly change the concentration of serum electrolytes and waste products unless a replacement fluid is infused into the blood, effectively diluting out those solutes the physician wishes to remove
- Rate of filtrate and solute removal determined by:
- Blood flow
- Transmembrane pressure gradient
- Membrane coefficient (pore size/permeability)
Haemodiafiltration (Combined)
- Clearance of solute via convection and diffusion
- Countercurrent dialysate is used in addition to hydrostatic pressure
- Fluid replacement is required to maintain plasma volumes
Question No. 6
Q: How does the modality effect solute clearance?
Answer No. 6
Size
Examples
Removal
Small Molecules
<500 Da
- Urea
- Creatinine
- Potassium
- Oxalate
- Uric acid
- Better cleared by diffusion (Haemodialysis)
- Rate of clearance increased by:
- Increasing speed of dialysis fluid flow
Middle Molecules
500 - 60,000 Da
Albumin is 66 kDa (Not a middle molecule!)
Albumin is 66 kDa (Not a middle molecule!)
- Interleukins
- Cytokines
- B2 -microglobulin
- ANP
- TNF
- Light chains
- Better cleared by convection (Hemofiltration)
- Dialysis clearance increased by:
- Increased dialysis time
- Increased membrane pore size
- Increased membrane surface area
Protein-Bound Molecules
Variable
- Homocysteine
- Hippuric acid
- Phenol
- Difficult to remove via RRT
- Clearance improved by:
- Increased time and flow
- Absorbent technology
- Albumin dialysis
Question No. 7
Q: What are the advantages and disadvantages of different modalities?
Answer No. 7
Advantages
Disadvantages
Requirement
Intermittent Haemodialysis
- Efficient and intensive technique
- Allows for down-time for interventions
- Can be performed overnight
- Staff required for a shorter time
- Lower costs
- May cause haemodynamic instability with rapid fluid removal
- Relative need for anticoagulation
- Potential for disequilibrium
- Need for expensive machinery
- Need for personnel
- Venous access
- Anticoagulation
- Skilled staff
- Expensive equipment
Continuous RRT
- Provides better haemodynamic stability
- Efficient solute removal and electrolyte balance with continuous removal
- Round-the-clock maintenance of volume status
- Nutrition and medication given while volume status maintained
- User-friendly machines
- Patient immobilisation
- Need for prolonged anticoagulation
- Nursing staff intensive
- Expensive machinery
- Risk of hypothermia
- Round the clock skilled nursing staff
- Venous access
- Anticoagulation
- Complex equipment
Peritoneal Dialysis
- Better haemodynamic stability than haemodialysis
- No need for anticoagulation
- Slow correction of volume and electrolyte disorders
- Many nurses unfamiliar with methods
- Risks of leaks and peritoneal infection
- Peritoneal catheter
- Sterile peritoneal solutions
- Trained staff
Question No. 8
Q: Which factors influence the choice of modality that should be used?
Answer No. 8
Solutes to be Removed from the Plasma
- Hemofiltration is better at removing middle molecules
- Hemodialysis better at removing small molecules
Patient`s Cardiovascular and Neurological Status
- CRRT causes less rapid fluid shifts and is the preferred option if there is any degree of cardiovascular instability
- CRRT may be associated with better cerebral perfusion in patients with an acute brain injury or fulminant hepatic failure
Availability of Resources
- CRRT is more labour intensive and more expensive than IHD
- Availability of equipment may dictate the form of RRT.
Clinician`s Experience
- It is wise to use a form of RRT that is familiar to all the staff involved
Other Specific Clinical Considerations
- Convective modes of RRT may be beneficial if the patient has septic shock
- CRRT can aid feeding regimes by improving fluid management
Question No. 9
Q: Which modality is better for critically unwell patients?
Answer No. 9
- There is no evidence that the use of either continuous or intermittent therapies have a survival benefit in critical illness
- Continuous modalities have been Several benefits have been proposed from the use of continuous modalities:
- Improved haemodynamic stability and lower rates of therapy-induced hypotension – due to slower and more predictable rates of fluid removal and solute flux
- Increased clearance with continuous modes to aid with the resolution of uraemia and electrolyte imbalance
- Better tolerated in patients with raised intracranial pressure or hepatic encephalopathy due to less rapid shift in solute concentration and preservation of cerebral perfusion
- Better clearance of inflammatory mediators
- Given this international guidance favours the use of continuous therapies in critical illness
- KDIGO guidance recommends continuous therapies in ‘haemodynamically unstable patients’ or those with ‘raised ICP, brain injury or other forms of brain oedema.’
- Surviving sepsis guidance recommends continuous therapies in ‘haemodynamically unstable septic patients
Question No. 10
Q: What type of catheters should be used for RRT?
Answer No. 10
Catheters should be:
- Large diameter catheter (>11FG) - allowing blood flow in excess of 250 ml/min
- Polyurethane material
- Coaxial or lumen-in-lumen
- Uncuffed, non-tunnelled
Question No. 11
Q: At which site should dialysis catheters be inserted?
Answer No. 11
Anticoagulation should be used in all patients requiring RRT for AKI unless:
- There is an ↑ risk of bleeding
- They are already receiving systemic anticoagulation
Question No. 12
Q: Who should receive anticoagulation for RRT?
Answer No. 12
Anticoagulation should be used in all patients requiring RRT for AKI unless:
- There is an ↑ risk of bleeding
- They are already receiving systemic anticoagulation
Question No. 13
Q: What are the types of anticoagulation that can be used for RRT?
Answer No. 13
Mechanical
- Optimising CVP
- Pre-dilution replacement fluid
- High flowrates
- Reducing air-blood contact in the bubble trap
Regional
- Heparin
- Citrate
Systemic
- Heparin
- LMWH
- Prostacyclin
- Thrombin Inhibitors:
- Argatroban
- Lepirudin
- Fondaparinux
- Heparinoids
- Warfarin
Question No. 14
Q: What are the advantages and disadvantages of each mode of anticoagulation?
Answer No. 14
Advantages
Disadvantages
No / Mechanical Anticoagulation
- Reduced bleeding risk
- ↓ cost
- ↑ risk of filter clotting:
- Shorter filter lifespan
- Reduced adequacy of RRT
- Not suitable for patients with HIT who are pro-thrombotic
Unfractionated Heparin
- Easily titratable
- Easily monitored
- Can be reversed with protamine
- ↑ bleeding risk
- Risk of HIT
LMWH
- ↓ cost
- ↑ familiarity
- ↑ bleeding risk
- Not titratable
- No reversal agent
Prostacyclin
- Reduced bleeding risk
- Shorter filter life
- Causes systemic hypotension
Citrate
- Good regional anticoagulation with reversal by calcium
- Reduced bleeding risk
- Associated with metabolic complications (Hypernatraemia, hypocalcaemia, metabolic alkalosis)
- Special dialysate required
- Contraindicated in liver failure
- Labour intensive