Renal replacement therapy Flashcards
What are the 3 layers of kidney
cortex
medulla
renal pelvis
What enters to form the glomerulus? Leaves?
What does this then become?
afferent arteriole
efferent arteriole - becomes peritubular capillaries of nephron
Afferrent is thicker -> high pressure in glomerulus allowing small molecules to pass through into nephron
Name the parts of the nephron in order
(Bowmans capsule)
Proximal tubule
Loop of henle
- thin descending
- Thin ascending
- thick ascending
Distal convoluted tubule
collecting tubule
collecting ductAppox cardiac output to kidneys at rest
25%
Main function of short vs long nephrons in loop of henle
short - excretion / regulatory functions
- Acid base balance, electrolytes, waste,
Longer - concentrate / dilute urine - based on current physiological status
What happens in proximal tubule
reabsorb water, electrolytes, solutes
H+ ions passively secreted here
What happens in distal convoluted tubule
Regulate BP
Absorb / secrete solutes
ADH here
can resorb Bicarb / actively secrete H+ ions
Key waste products secreted by kidneys
creatine
uric acid
urea
Indications for CRRT
Critically ill with severe AKI (ie too unstable for dialysis) Fluid overload severe electrolyte imbalance Acid base imbalance sepsis + cytokine clearance rhabdomyolysis Heart failure
Usual flow rate for CRRT
150-300ML per minute
What 3 fluids in addition to blood in CRRT? When are they added
Pre blood pump
- Added just after blood out of body BEFORE the filter
Dialysate
- Present IN filter
- Not actually added to blood but on the other side of the filter membrane
Post filter pump
-Added AFTER filter, but before the de-aeration chamber
Which pressures are measured in CRRT
Access pressure - (pre-filter)
-Always NEGATIVE
Pre-filter (after blood pump)
-Always POSITIVE
Return pressure (after the filter) -Always POSITIVE
Effluent pressure (pressure to pull plasma water from blood) -Usually negative (occasionally positive)
4 main principles of CRRT
Diffusion
Ultrafiltration
Convection
Adsorbtion
Direction of blood / diasylate in filter
Blood bottom-> top
dialysate top->bottom
Ensures an adequate diffusion concentration gradient throughout
What size molecues can be filtered with diffusion
Depends on pore thickness
Small molecules only
DIFFUSION in filtration
Which electrolytes are usually started at isotonic levels in dialysate?
Which are not isotonic?
What is not in dialysate?
Mg, Cl, Na
K - usually at lower levels because people tend to initially be hyperkalemic
->once normalised have to alter dialysate
Bicarb - Usually higher in dialysate initially
(renal failure tends to have low levels)
No urea/creatinine
What 3 pressures can cause liquid to pass through a membrane
Osmotic
Oncotic
Hydrostatic
Eg of hydrostatic pressure
Glomerulus
- Afferent bigger than efferent
- Able to control this with contraction/relaxation of efferent arteriole
ULTRAFILTRATION
What does ultrafiltration do
Uses hydrostatic pressure to mimic glomerulus
-> Removes fluid from patients blood
What is convection
Movement of solutes through a semi permeable membrane with the flow of water
Also called SOLUTE DRAG
Ultrafiltration and convection mean large volume of fluid is taken from patients blood - what must you then do?
Replace it - can use isotonic solution to do this
When can replacement fluid be added to blood?
Advantage / disadvantage
BEFORE filter
-Dilutes blood -> less solute clearance
+Reduces clotting of blood in filter -> prolongs the life of the filter
AGFTER filter
+More conencreated blood in filter -> better solute clearance
-Increased clotting in filter -> filter shorter lifespan
Often use a MIX of both
What is Adsorption? Key use?
Adherence of solutes/biological mater to the surface of the filtration membrane
->Eventually ‘clogs’ filter
Some filters are made to specifically adsorb cytokine for use in a cytokine storm.
What is SCUF ? What is it? When is it used? Dialysate / replacement solution? ph?
Slow continuous ultrafiltration
Slow filtration of fluid
Don’t get removal of solutes as slow ->less convection
pH NOT AFFECTED
In fluid overload
No dialysate / replacement fluid
What is CVVH? What does it do? When is it used? Dialysate / replacement solution? pH
Continuous veno-veno haemofiltration
Removes fluid and solutes through ULTRAFILTRATION + CONVECTION
->Requires replacement solution (no dialysate)
Uraemia
Severe acid/base disturbance
Large molecule clearance
pH affected
What is CVVHD? What does it do? When is it used? Dialysate / replacement solution? pH
Continuous veno-veno haemodyalysis
Removes waste products and solutes through DIFFUSION
- Small / medium molecules
Uraemia
Severe acid/base disturbance
->pH affected
Dialysate but not replacement fluid
What is CVVHDF
What does it do?
When is it used?
Dialysate / replacement solution?
Continuous veno-veno haemoDiaFiltration
Combination of Diffusion, Ultrafiltration + convection
MOST common as used for everything and can turn down flow of various things as required without having to reset whole system
Uses dialysate and replacement solution
ureamia Electrolye imbalance severe acid base disturbance removal of small-large molecules Fluid removal
When would you not anticoagulate patients on crrt
trauma / surgery
Bleed on the brain
Liver failure
sepsis with coagulopathy
What 2 main issues with filter clotting
Increased downtime without filtration
May lose up to 1/2 unit of blood in filter
-> if frequent occurs will require blood transfusion
Usual drug for systemic anticoagulation in CRRT?
When is a systemic method used?
Heparin
Used when need for systemic anticoagulation
Eg DVT/PE, mechanical heart valve etc
Reversal agent of heparin
protamine
What do you need to monitor for patients on heparin infusion
HIIT - heparin induced idiopathic thrombocytopenia
If platelets falling->may need to change agent
Bar heparin what other agents can be used for systemic anticoagulation on crrt ?
Reversal?
Direct thrombin inhibitors
No reversal -> use FFP / cryo / clotting factors
More common to use systemic or regional anticoagulation in CRRT? Why? What are the drawbacks
Regional
Decreased risk of patient bleeding whilst increasing filter life
Requires closer monitoring
Most common drug combo in regional anticoagulation
Heparin - pre-filter
Protamine - post filter
Approx dose of heparin: protamine
100units heparin: 1g protamine
An alternative drug combination of anticoagulation that is not heparin/protamine for regional anticoagulation
Citrate (pre-filter) / calcium (post filter)
How does citrate prevent clotting?
What happens to citrate during filtration?
What do you need to ensure when using citrate
binds to calcium (which is used in almost all parts of the clotting cascade )
Once bound to calcium becomes inactive
Most citrate filtered out
->Need to give calcium to ensure adequate systemic levels returned
What do you monitor in when using citrate as an anticoagulant in CRRT? Values?
Calcium levels
Post filter - Very low (if not then using insufficient citrate for filtration)
Patient - Normal levels
As filters get clogged what changes ? What is this?
TMP (trans membrane pressure) goes up
Pressure between blood and dialysate (hydrostatic pressure)
-This is driving pressure for convection
What is pressure drop
The pressure of blood going through the filter
Ie pressure before - pressure after
What causes pressure drop to increase
Clotting - as the membranes clot off need more pressure to get blood through the filter
