Extracorporeal therapies Flashcards
4 modalities of CRRT and what is the main method of clearance of each method?
- Continuous ultrafiltration (ultrafiltration)
- Continuous venovenous hemodialysis (diffusion)
- Continuous venovenous hemofiltration (convection)
- Continuous venovenous hemodiafiltration (convection + diffusion)
What are the different renal replacement therapies? Which one(s) is/are extracorporeal therapy(ies)?
- Intermittent hemodialysis (IHD)
- Continuous renal replacement therapy (CRRT)
- Prolonged intermittent renal replacement therapy (PIRRT)
- Peritoneal dialysis (PD)
All extracorporeal except for PD
What are the 3 mechanisms of solute and fluid movement in RRT and what are their determinants
- Diffusion = movement of solutes following their concentration gradient
Determinants:
- Molecular weight of solutes
- Charge of solutes
- Membrane surface area
- Membrane permeability (including thickness and pores size)
- Concentration gradient between the 2 compartments - Convection = movement of solutes with water following an osmotic or hydrostatic gradient (solvent drag)
Determinants:
- Water movement across membrane (= osmotic +/- hydrostatic pressure gradient)
- Membrane pore size and solute molecular weight
- Membrane surface area - Ultrafiltration = convective process referring to the removal of plasma water from the intravascular compartment
Same determinants as convection (same thing except it refers to fluid removal instead of solute removal)
Name parameters that need to be monitored closely during RRT
- HR, BP
- RR
- Temperature
- Blood volume change (or dialysate input vs output for PD)
- Neurological signs, nausea, restlessness
- ACT (heparin) or iCa (citrate) + signs of hemorrhage
- SvO2
Name complications of RRT
- IHD / CRRT
- Hypotension
- Dialysis disequilibrium syndrome
- Hemorrhage (with heparin)
- Hypocalcemia and alkalosis (with citrate)
- Air embolism
- Dialyzer membrane reaction
- Clotting of the circuit (and subsequent blood loss)
- Catheter occlusion
- Thrombosis - PD:
- Septic peritonitis
- Dialysate leakage
- Hyperglycemia
- Hypoalbuminemia
- Dyspnea (from increased intra-abdominal pressure)
What is the survival rate following hemodialysis / peritoneal dialysis?
About 50% survival to discharge depending on studies (up to 80% for lepto). 1 year survival 30-40%.
Little data on PD but does not seem to be less.
2 methods of apheresis
- Centrifugal
- Filtration
What determines the efficiency of substance removal with TPE
- Substance volume of distribution (the lower the better)
- Rapidity of equilibration of the substance between body compartments
- Number of plasma volumes exchanged
What characteristics of a substance should be considered when choosing an extracorporeal therapy for blood purification
- Volume of distribution (needs to be <1-2L/kg for all, <0.5-1L/kg for TPE)
- Protein binding (>95%: TPE, 80-95%: hemoperfusion, <80%: based on molecular weight)
- Molecular weight (<500-1000Da: HD, 1000-10000Da: ultrafiltration / convection, 10000-50000Da: hemoperfusion, >50000Da: TPE)
- Water solubility
What are the different methods of clearance for extracorporeal blood purification
- Diffusion
- Convection
- Adsorption
- Centrifugation
- Filtration
Name some drugs / toxins that can be removed with extracorporeal therapies
- NSAIDs
- Acetaminophen
- Ethylene glycol, propylene glycol
- Baribiturates
- Aminoglycosides
- Methotrexate
- Baclofen
- Cyclosporine
- Amanita mushrooms
Indicate what solute removal mechanism is appropriate for the following solutes: urea, creatinine, K, oxalic acid, bilirubin
- Urea (60 Da), creat (113 Da), K (39 Da), oxalic acid: diffusion
- Bilirubin (584 Da): convection
What are mechanisms of dialysis disequilibrium syndrome? How to treat it / prevent it?
- Mechanisms:
- Rapid decrease in urea concentration in the intravascular space: urea remains higher in neurons due to time for equilibration which causes cerebral edema (“reverse urea effect”) + neurons might have accumulated idiogenic osmoles contributing to the edema (“idiogenic osmole theory”)
- Paradoxical acidosis in the brain due to administration of bicarbonates with dialysis might contribute too - Treatment: osmolar therapy (mannitol), slow down or stop dialysis session, diazepam in case of seizures
- Prevention: using slow urea reduction especially for first sessions +/- sodium modelling (giving more Na as urea decreases)
What is the urea reduction rate (URR)
[(BUNpre - BUNpost)/BUNpre]*100
What is typically the volume of dialysate infused in PD
30-40 mL/kg (but can start with 10-20 mL/kg)
When should RRT be initiated (indications and timing)
- Indications:
In a context of AKI with oligo-anuria:
- Refractory hyperK (> 6.5)
- Refractory metabolic acidosis (< 7.15)
- Refractory fluid overload
- Severe azotemia with clinical signs of uremia
In a context of ingestion of a toxin:
- If the toxin has potential for severe toxicity
- If exogenous clearance is thought to be more efficient than endogenous clearance
- If an antidote / treatment is not available
- Timing: No evidence of improved survival when initiated earlier in the course of disease (and might be able to avoid it if initiate later) so can wait for indications to be met. (For toxin exposure, do it ASAP).
- Ensure patient has been adequately rehydrated
- Hypovolemia/hypotension has been corrected
- Oligoanuria challenged with diuretic?
For a given type of dialyzer, what parameters of the prescription determine urea clearance in IHD / CRRT / PD?
- IHD: blood flow rate
- CRRT: effluent rate (dialysate rate and replacement rate) - but blood flow rate must be 3 times dialysate rate for it to be saturated
- PD: dialysate composition (mostly dextrose content determining convection), dwell time, frequency of exchange
What clinical scoring system has been developed for dogs and cats with AKI
Model by Segev et al. 3 models based on several parameters with different Se and Sp, seems to be right in about 80% of cases.
What is the fractional clearance of urea
Kt/V = -ln(1-URR)
K = urea clearance
t = time (duration of treatment)
V = volume of distribution of urea (60% of bodyweight but needs to be adjusted for overhydration)
URR = urea reduction ratio (decided by operator)
What is the maximum extracorporeal circuit volume after which a blood prime is required
~10-15% of total blood volume
What is the risk with dialysis circuits made of membrane AN69 that are not surface treated with polyethylene imine (PEI)? What is a factor of increased risk?
Lead to bradykinin release when in contact with blood -> anaphylaxis, hypotension
Risk is increased if patient is receiving ACE inhibitors (prevent degradation of bradykinin)
Bradykinin release is worse with acidic products -> these circuits need addition of bicarbonate in blood primes or recirculation of blood primes
In PIRRT, what are the limiting factors for choosing the dialysate rate / replacement rate
- Dialysate rate: must be < 1/3 of blood flow rate (to allow full saturation)
- Replacement rate: filtration fraction must be < 20-25%
Filtration fraction = total convective clearance / (plasma flow rate + pre blood pump + pre-filter replacement rate)
Total convective clearance = patient fluid removal + replacement + pre-blood pump
What is the maximum patient fluid removal rate
10 mL/kg/h (risk of hypotension beyond that)
How to assess the efficiency of a dialysis session
Measure URR (urea reduction ratio) = (BUNpre-BUNpost)/BUNpre * 100 and compare it with the prescribed URR. (Can also calculate Kt/V)
BUNpost should be sampled about 1h after the end of the treatment
What are common causes of loss of efficiency in dialysis / CRRT
- Machine “down time” (alarms, patient disconnection, etc.)
- Clotting of the dialyzer (reduces membrane surface area available for exchange)
- Wrong estimation of hydration and distribution volume of urea
- Access recirculation (if lines reversed, recirculation is 15-25%)
What are main differences between short term and long term dialysis catheters
- Short term: placed percutaneously with Seldinger technique, not cuffed, not tunnelled. Should be used for max 1 month.
- Long term: catheter is tunnelled under the skin and has a cuff inflated in SQ tissues. Needs to be placed surgically. Can be used long term (up to 2 years).
What are causes of dialysis catheter dysfunction
- Intra-luminal thrombosis
- Extra-luminal thrombosis
- Vessel stenosis
- Fibrin sheath
True or false: A dialysis catheter should always be removed in case of a new fever with high suspicion of catheter-associated infection
False. If fever resolves within 48h of antibiotics the catheter can in theory continue to be used.
What are risk factors of dialysis circuit clotting
- Slow blood flow
- Blood flow interruptions
- Hypercoagulable patient
- Administration of blood products
- High ultrafiltration rate
What is the target ACT for cats and dogs during extracorporeal therapy with heparinization? The iCa for citrate?
- ACT (heparinization)
Cats: 200-250 sec
Dogs: 160-200 sec
* during blood transfusion, target 300 sec (blood transfusion is procoagulable) - iCa (citrate)
1.0-1.4 mmol/L in patient, <0.2-0.3 mmol/L in circuit
Dose for protamine
1 mg for 100 U of heparin
How is the patient’s Ca in case of citrate toxicity
iCa is low, tCa is elevated (too much citrate not metabolized -> complexes Ca ->decreased iCa -> the rate of Ca CRI is increased but only increases tCa)
- low iCa refractory to correction with Ca infusion –> must decrease citrate and allow liver to metabolize
What is the cause for dialysis encephalopathy (not DDS)
Aluminum toxicity if high levels in water
What are causes of dialyzer reactions (with associated risk factors)
- Reaction to ethylene oxide (EtO), mostly if too much delay between priming and treatment
- Reaction to AN69, mostly if dialyzer not surface treated and patient on ACE inhibitors
- Bacterial contamination
What blood flow rates need to be achieved with a dialysis catheter
15-20 mL/kg/min
What is the plasma volume usually exchanged in TPE/ Where does this number come from?
1.0-1.5 times plasma volume
Based on predicted clearance of various substances –> results in removal of approximately 60-70% of substance in intravascular space.
> 1-1.5 would decrease the efficiency of removal of the substance.
Name complications of charcoal hemoperfusion
Hypoglycemia, thrombocytopenia, hypokalemia
For how long should procedures be avoided after heparinization
6-8h
Name contraindications (or considerations) of RRT and PD
RRT
- Hypotension
- Severe coagulopathy
- Small size?
PD
- Peritonitis
- Recent abdominal or thoracic surgery
- Hypoalbuminemia
- Severe ypercatabolic states
What percentage of dogs & cats receive blood products when undergoing dialysis?
87% of cats, 32% of dogs
Indications for TPE in IMHA (from SACCM)
- Severe IMHA requiring one or more blood transfusions
- Dogs with crossmatch incompatibility to donors
- Dogs that have not yet responded to 4 or more days of appropriate immunosuppressive therapy
Name a few advantages and disadvantages of continuous renal replacement therapy (vs intermittent)
Advantages:
- Better hemodynamic stability
- Reduced risk for dialysis disequilibrium
- Better control of fluid balance
Disadvantages:
- Limited patient mobility
- Need for continuous anticoagulation (prevents procedures)
- Higher costs
- Requires more personnel
What should be considered when selecting the potassium concentration of dialysate?
Large gradients and rapid changes in K+ can increase risk of ventricular arrhythmias
Rule of 7 –> Patient K+ + dialysate K+ = 7
Differences between pre and post filter replacement location
Pre-filter:
- less efficient (dilution of blood)
- reduced risk of clotting
- 1ml effluent = <1ml of plasma clearance
Post-filter:
- more efficient
- more prone to clotting
- 1ml effluent = 1ml plasma clearance
What are the components of a PIRRT prescription?
- Target URR outcome (%)
- Treatment time (hrs)
- Target hourly clearance (K - ml/h)
- Diffusive: convective proportion
- Dialysate rate (Qd - ml/h)
- Convective rate (Quf - ml/h)
- Patient fluid removal rate (Qpfr - ml/h)
- PBP rate (Qpbp - ml/h)
- Replacement rate (Qr - ml/h)
- Blood flow rate (Qb - ml/h) - start slow
Checklist for prescription?
- Safe URR? –> DDS
- Blood prime? –> anemic/hypovolemic patients
- Pre vs post-filter replacement?
- Filtration fraction < 20-25%? –> Increased risk of clotting
- 100% dialysate saturation?
- Practical/economical?
- Safe PFFR < 10ml/kg/h? –> hypovolemia/hypotension
List complications of citrate toxicity
- Hypocalcemia (iCa)
- Hypercalcemia (tCa)
- Hypernatremia (trisodium citrate)
- Metabolic alkalosis
- Citrate toxicity –> metabolic acidosis
What are treatment specific causes of hypotension during RRT?
- Antihypertensive drugs
- Rapid reduction in plasma osmolarity
- Warm dialysate
- Low sodium dialysate
- Low dialysate osmolarity
- Acetate as buffer
- High UFR
What are the main determinants of TMP (transmembrane pressure) and filter pressure drop (delta P)?
TMP: Pressure gradient across the filter membrane
- Membrane pore permeability
- Hydrostatic/oncotic pressure gradients across filter
- Rate of replacement solutions
- Blood flow rate
Delta P: Difference between Return and Filter pressures
- Number of patent capillaries in the filter
- Patient blood viscosity
